Vitamins - Integration of Metabolism - Lippincott’s Illustrated Reviews: Biochemistr, Sixth Edition (2014)

Lippincott’s Illustrated Reviews: Biochemistr, Sixth Edition (2014)

UNIT V: Integration of Metabolism

Chapter 28. Vitamins

I. OVERVIEW

Vitamins are chemically unrelated organic compounds that cannot be synthesized in adequate quantities by humans and, therefore, must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water soluble. Because they are readily excreted in the urine, toxicity is rare. However, deficiencies can occur quickly. Four vitamins (A, D, K, and E) are termed fat soluble (Figure 28.1). They are released, absorbed, and transported (in chylomicrons) with dietary fat. They are not readily excreted, and significant quantities are stored in the liver and adipose tissue. In fact, consumption of vitamins A and D in excess of the Dietary Reference Intakes can lead to accumulation of toxic quantities of these compounds. Vitamins are required to perform specific cellular functions. For example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat-soluble vitamin (vitamin K) has a coenzyme function.

Figure 28.1 Classification of the vitamins.

II. FOLIC ACID

Folic acid (or folate), which plays a key role in one-carbon metabolism, is essential for the biosynthesis of several compounds. Folic acid deficiency is probably the most common vitamin deficiency in the United States, particularly among pregnant women and alcoholics. [Note: Leafy, dark green vegetables are a good source of folic acid.]

Figure 28.2 Classification of nutritional anemias by cell size. The normal mean corpuscular volume (MCV) for people older than age 18 is between 80 and 100 μ;m3. [Note: Microcytic anemia is also seen with lead poisoning.]

A. Function of folic acid

Tetrahydrofolate (THF), the reduced, coenzyme form of folate, receives one-carbon fragments from donors such as serine, glycine, and histidine and transfers them to intermediates in the synthesis of amino acids, purines, and thymidine monophosphate (TMP), a pyrimidine nucleotide found in DNA (Figure 28.3).

B. Nutritional anemias

Anemia is a condition in which the blood has a lower than normal concentration of hemoglobin, which results in a reduced ability to transport oxygen. Nutritional anemias (that is, those caused by inadequate intake of one or more essential nutrients) can be classified according to the size of the red blood cells (RBCs) or mean corpuscular volume (MCV) observed in the individual (Figure 28.2). Microcytic anemia (MCV below normal), caused by lack of iron, is the most common form of nutritional anemia. The second major category of nutritional anemia, macrocytic (MCV above normal), results from a deficiency in folic acid, or vitamin B12. [Note: These macrocytic anemias are commonly called megaloblastic because a deficiency of either vitamin (or both) causes accumulation of large, immature RBC precursors, known as megaloblasts, in the bone marrow and the blood.]

1. Folate and anemia: Inadequate serum levels of folate can be caused by increased demand (for example, pregnancy and lactation), poor absorption caused by pathology of the small intestine, alcoholism, or treatment with drugs (for example, methotrexate) that are dihydrofolate reductase inhibitors (see Figure 28.3). A folate-free diet can cause a deficiency within a few weeks. A primary result of folic acid deficiency is megaloblastic anemia (Figure 28.4), caused by diminished synthesis of purines and TMP, which leads to an inability of cells (including RBC precursors) to make DNA and, therefore, an inability to divide.

Figure 28.3 Production and use of tetrahydrofolate. NADP(H) = nicotinamide adenine dinucleotide phosphate.

2. Folate and neural tube defects in the fetus: Spina bifida and anencephaly, the most common neural tube defects (NTDs), affect approximately 3,000 pregnancies in the United State annually. Folic acid supplementation before conception and during the first trimester has been shown to significantly reduce NTDs. Therefore, all women of childbearing age are advised to consume 0.4 mg/day of folic acid to reduce the risk of having a pregnancy affected by NTDs and ten times that amount if a previous pregnancy was affected. Adequate folate nutrition must occur at the time of conception because critical folate-dependent development occurs in the first weeks of fetal life, at a time when many women are not yet aware of their pregnancy. In 1998, the U.S. Food and Drug Administration authorized the addition of folic acid to enriched grain products, resulting in a dietary supplementation of about 0.1 mg/day. It is estimated that this supplementation allows approximately 50% of all reproductive-aged women to receive 0.4 mg of folate from all sources. [Note: High-dose folate supplementation can mask the symptoms of vitamin B12 deficiency (see blue box below) and is not recommended for most adults.]

Figure 28.4 Bone marrow histology in normal and folate-deficient individuals.

III. COBALAMIN (VITAMIN B12)

Vitamin B12 is required in humans for two essential enzymatic reactions: the remethylation of homocysteine (Hcy) to methionine and the isomerization of methylmalonyl coenzyme A (CoA), which is produced during the degradation of some amino acids (isoleucine, valine, threonine, and methionine) and fatty acids (FAs) with odd numbers of carbon atoms (Figure 28.5). When cobalamin is deficient, unusual (branched) FAs accumulate and become incorporated into cell membranes, including those of the central nervous system (CNS). This may account for some of the neurologic manifestations of vitamin B12 deficiency. [Note: Folic acid (as N5-methyl THF) is also required in the remethylation of Hcy. Therefore, deficiency of B12 or folate results in elevated Hcy levels.]

A. Structure of cobalamin and its coenzyme forms

Cobalamin contains a corrin ring system that resembles the porphyrin ring of heme (see p. 280), and but differs in that two of the pyrrole rings are linked directly rather than through a methene bridge. Cobalt is held in the center of the corrin ring by four coordination bonds with the nitrogens of the pyrrole groups. The remaining coordination bonds of the cobalt are with the nitrogen of 5,6-dimethylbenzimidazole and with cyanide in commercial preparations of the vitamin in the form of cyanocobalamin (Figure 28.6). The physiologic coenzyme forms of cobalamin are 5ʹ-deoxyadenosylcobalamin and methylcobalamin, in which cyanide is replaced with 5-deoxyadenosine or a methyl group, respectively (see Figure 28.6).

Figure 28.5 Reactions requiring coenzyme forms of vitamin B12. CoA = coenzyme A.

Figure 28.6 Structure of vitamin B12 (cyanocobalamin) and its coenzyme forms (methylcobalamin and 5ʹ-deoxyadenosylcobalamin).

B. Distribution of cobalamin

Vitamin B12 is synthesized only by microorganisms, and it is not present in plants. Animals obtain the vitamin preformed from their natural bacterial flora or by eating foods derived from other animals. Cobalamin is present in appreciable amounts in liver, red meat, fish, eggs, dairy products, and fortified cereals.

C. Folate trap hypothesis

The effects of cobalamin deficiency are most pronounced in rapidly dividing cells, such as the erythropoietic tissue of bone marrow and the mucosal cells of the intestine. Such tissues need both the N5,N10-methylene and N10-formyl forms of THF for the synthesis of nucleotides required for DNA replication (see pp. 293 and 303). However, in vitamin B12 deficiency, the utilization of the N5-methyl form of THF in the B12-dependent methylation of homocysteine to methionine is impaired. Because the methylated form cannot be converted directly to other forms of THF, folate is trapped in the N5-methyl form, which accumulates. The levels of the other forms decrease. Thus, cobalamin deficiency leads to a deficiency of the THF forms needed in purine and TMP synthesis, resulting in the symptoms of megaloblastic anemia.

D. Clinical indications for vitamin B12

In contrast to other water-soluble vitamins, significant amounts (2–5 mg) of vitamin B12 are stored in the body. As a result, it may take several years for the clinical symptoms of B12 deficiency to develop as a result of decreased intake of the vitamin. [Note: Deficiency happens much more quickly if absorption is impaired (see below).] B12 deficiency can be determined by the level of methylmalonic acid in blood, which is elevated in individuals with low intake or decreased absorption of the vitamin.

Figure 28.7 Absorption of vitamin B12. IF = intrinsic factor.

1. Pernicious anemia: Vitamin B12 deficiency is most commonly seen in patients who fail to absorb the vitamin from the intestine. B12 is released from food in the acidic environment of the stomach. [Note: Malabsorption of cobalamin in the elderly is most often due to reduced secretion of gastric acid (achlorhydria).] Free B12 then binds a glycoprotein (R-protein), and the complex moves into the intestine. B12 is released from the R-protein by pancreatic enzymes and binds another glycoprotein, intrinsic factor (IF). The cobalamin–IF complex travels through the intestine and binds to specific receptors on the surface of mucosal cells in the ileum. The cobalamin is transported into the mucosal cell and, subsequently, into the general circulation, where it is carried by its binding protein (transcobalamin). B12 is taken up and stored in the liver, primarily. It is released into bile and efficiently reabsorbed in the ileum. Severe malabsorption of vitamin B12 leads to pernicious anemia. This disease is most commonly a result of an autoimmune destruction of the gastric parietal cells that are responsible for the synthesis of IF (lack of IF prevents B12 absorption). [Note: Patients who have had a partial or total gastrectomy become IF deficient and, therefore, B12 deficient.] Individuals with cobalamin deficiency are usually anemic, and they show neuropsychiatric symptoms later, as the disease develops. The CNS effects are irreversible and occur by mechanisms that appear to be different from those described for megaloblastic anemia. Pernicious anemia requires life-long treatment with either high-dose oral B12 or intramuscular injection of cyanocobalamin. [Note: Supplementation works even in the absence of IF because approximately 1% of B12 uptake is by IF-independent diffusion.]

Figure 28.8 Structure of ascorbic acid.

Folic acid supplementation can partially reverse the hematologic abnormalities of B12 deficiency and, therefore, can mask a cobalamin deficiency. Thus, to prevent the CNS effects of B12 deficiency, therapy for megaloblastic anemia is initiated with both vitamin B12 and folic acid until the cause of the anemia can be determined.

Figure 28.9 Structures of vitamin B6 and the antituberculosis drug isoniazid.

IV. ASCORBIC ACID (VITAMIN C)

The active form of vitamin C is ascorbic acid (Figure 28.8). The main function of ascorbate is as a reducing agent in several different reactions. Vitamin C has a well-documented role as a coenzyme in hydroxylation reactions (for example, hydroxylation of prolyl and lysyl residues of collagen; see p. 47). Vitamin C is, therefore, required for the maintenance of normal connective tissue as well as for wound healing. Vitamin C also reduces ferric iron to the ferrous form, thereby facilitating the absorption of dietary iron from the intestine.

A. Deficiency of ascorbic acid

A deficiency of ascorbic acid results in scurvy, a disease characterized by sore and spongy gums, loose teeth, fragile blood vessels, swollen joints, fatigue, and a microcytic anemia caused by decreased absorption of iron (Figure 28.9). Many of the deficiency symptoms can be explained by a deficiency in the hydroxylation of collagen, resulting in defective connective tissue.

Figure 28.10 Structures of vitamin B6 and the antituberculosis drug isoniazid.

B. Prevention of chronic disease

Vitamin C is one of a group of nutrients that includes vitamin E (see p. 391) and β-carotene (see p. 382), which are known as antioxidants. [Note: Ascorbate regenerates the functional, reduced form of vitamin E.] Consumption of diets rich in these compounds is associated with a decreased incidence of some chronic diseases, such as coronary heart disease and certain cancers. However, clinical trials involving supplementation with the isolated antioxidants have failed to demonstrate any convincing beneficial effects.

Figure 28.11 A. Structure of thiamine and its coenzyme form, thiamine pyrophosphate. B. Structure of intermediate formed in the reaction catalyzed by pyruvate dehydrogenase. C. Structure of intermediate formed in the reaction catalyzed by α-ketoglutarate dehydrogenase. AMP = adenosine monophosphate.

V. PYRIDOXINE (VITAMIN B6)

Vitamin B6 is a collective term for pyridoxine, pyridoxal, and pyridoxamine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring (Figure 28.10). Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals. All three compounds can serve as precursors of the biologically active coenzyme, pyridoxal phosphate (PLP). PLP functions as a coenzyme for a large number of enzymes, particularly those that catalyze reactions involving amino acids, for example, in the synthesis of cysteine from Hcy (see p. 264). [Note: PLP is also required by glycogen phosphorylase (see p. 128).]

Reaction type

Example

Transamination

Oxaloacetate + glutamate aspartate + α-ketoglutarate

Deamination

Serine → pyruvate + NH3

Decarboxylation

Histidine → histamine + CO2

Condensation

Glycine + succinyl CoA → δ-aminolevulinic acid

A. Clinical indications for pyridoxine

Isoniazid, a drug commonly used to treat tuberculosis, can induce a vitamin B6 deficiency by forming an inactive derivative with PLP. Dietary supplementation with B6 is, thus, an adjunct to isoniazid treatment. Otherwise, dietary deficiencies in pyridoxine are rare but have been observed in newborn infants fed formulas low in B6, in women taking oral contraceptives, and in alcoholics.

B. Toxicity of pyridoxine

Pyridoxine is the only water-soluble vitamin with significant toxicity. Neurologic symptoms (sensory neuropathy) occur at intakes above 500 mg/day, an amount nearly 400 times the Recommended Dietary Allowance (RDA) and over 5 times the Tolerable Upper Limit (UL). Substantial improvement, but not complete recovery, occurs when the vitamin is discontinued.

VI. THIAMINE (VITAMIN B1)

Thiamine pyrophosphate (TPP) is the biologically active form of the vitamin, formed by the transfer of a pyrophosphate group from adenosine triphosphate (ATP) to thiamine (Figure 28.11). TPP serves as a coenzyme in the formation or degradation of α-ketols by transketolase (Figure 28.12A), and in the oxidative decarboxylation of α-keto acids (Figure 28.12B).

A. Clinical indications for thiamine

The oxidative decarboxylation of pyruvate and α-ketoglutarate, which plays a key role in energy metabolism of most cells, is particularly important in tissues of the CNS. In thiamine deficiency, the activity of these two dehydrogenase-catalyzed reactions is decreased, resulting in decreased production of ATP and, therefore, impaired cellular function. TPP is also required by branched-chain α-keto acid dehydrogenase of muscle. [Note: It is the decarboxylase of each of these α-keto acid dehydrogenase multienzyme complexes that requires TPP.] Thiamine deficiency is diagnosed by an increase in erythrocyte transketolase activity observed on addition of TPP.]

1. Beriberi: This is a severe thiamine-deficiency syndrome found in areas where polished rice is the major component of the diet. Adult beriberi is classified as dry (characterized by peripheral neurologic deficits) or wet (characterized by edema due to cardiac dysfunction). Infantile beriberi is seen in nursing infants whose mothers are deficient in thiamine.

2. Wernicke-Korsakoff syndrome: In the United States, thiamine deficiency, which is seen primarily in association with chronic alcoholism, is due to dietary insufficiency or impaired intestinal absorption of the vitamin. Some alcoholics develop Wernicke-Korsakoff syndrome, a thiamine deficiency state characterized by confusion, ataxia, and a rhythmic to-and-fro motion of the eyeballs (nystagmus) with Wernicke encephalopathy as well as memory problems and hallucinations with Korsakoff dementia. The syndrome is treatable with thiamine supplementation, but recovery of memory is typically incomplete.

Figure 28.12 Reactions that use thiamine pyrophosphate (TPP) as coenzyme. A. Transketolase. B. Pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Note that TPP is also used by branched-chain α-keto acid dehydrogenase. P = phosphate; CoA = coenzyme A.

VII. NIACIN

Niacin, or nicotinic acid, is a substituted pyridine derivative. The biologically active coenzyme forms are nicotinamide adenine dinucleotide (NAD+) and its phosphorylated derivative, nicotinamide adenine dinucleotide phosphate (NADP+) as shown in Figure 28.13. Nicotinamide, a derivative of nicotinic acid that contains an amide instead of a carboxyl group, also occurs in the diet. Nicotinamide is readily deaminated in the body and, therefore, is nutritionally equivalent to nicotinic acid. NAD+ and NADP+ serve as coenzymes in oxidation-reduction reactions in which the coenzyme undergoes reduction of the pyridine ring by accepting a hydride ion (hydrogen atom plus one electron) as shown in Figure 28.14. The reduced forms of NAD+ and NADP+ are NADH and NADPH, respectively. [Note: A metabolite of tryptophan, quinolinate, can be converted to NAD(P). In comparison, 60 mg of tryptophan = 1 mg of niacin.]

Figure 28.13 Structure and biosynthesis of oxidized nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+). ADP = adenosine diphosphate.

A. Distribution of niacin

Niacin is found in unrefined and enriched grains and cereal; milk; and lean meats, especially liver.

Figure 28.14 Reduction of oxidized nicotinamide adenine dinucleotide (NAD+) to NADH. = phosphate.

B. Clinical indications for niacin

1. Deficiency of niacin: A deficiency of niacin causes pellagra, a disease involving the skin, gastrointestinal tract, and CNS. The symptoms of pellagra progress through the three Ds: dermatitis; diarrhea; dementia; and, if untreated, death. Hartnup disorder, characterized by defective absorption of tryptophan, can result in pellagra-like symptoms. [Note: Corn is low in both niacin and tryptophan. Corn-based diets can cause pellagra.]

2. Treatment of hyperlipidemia: Niacin at doses of 1.5 g/day, or 100 times the RDA, strongly inhibits lipolysis in adipose tissue, the primary producer of circulating free fatty acids (FFAs). The liver normally uses these circulating FFAs as a major precursor for triacylglycerol (TAG) synthesis. Thus, niacin causes a decrease in liver TAG synthesis, which is required for very-low-density lipoprotein ([VLDL] see p. 231) production. Low-density lipoprotein (LDL, the cholesterol-rich lipoprotein) is derived from VLDL in the plasma. Thus, both plasma TAG (in VLDL) and cholesterol (in LDL) are lowered. Therefore, niacin is particularly useful in the treatment of type IIb hyperlipoproteinemia, in which both VLDL and LDL are elevated. The high doses of niacin required can cause acute, prostaglandin-mediated flushing. Aspirin can reduce this side effect by inhibiting prostaglandin synthesis (see p. 214). [Note: Niacin raises high-density lipoprotein levels.]

Figure 28.15 Structure and biosynthesis of the oxidized forms of flavin mononucleotide and flavin adenine dinucleotide. ADP = adenosine diphosphate; PPi = pyrophosphate.

VIII. RIBOFLAVIN (VITAMIN B2)

The two biologically active forms of B2 are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), formed by the transfer of an adenosine monophosphate moiety from ATP to FMN (Figure 28.15). FMN and FAD are each capable of reversibly accepting two hydrogen atoms, forming FMNH2 or FADH2. FMN and FAD are bound tightly, sometimes covalently, to flavoenzymes (for example, NADH dehydrogenase [FMN] and succinate dehydrogenase [FAD]) that catalyze the oxidation or reduction of a substrate. Riboflavin deficiency is not associated with a major human disease, although it frequently accompanies other vitamin deficiencies. Deficiency symptoms include dermatitis, cheilosis (fissuring at the corners of the mouth), and glossitis (the tongue appearing smooth and dark).

Figure 28.16 A. Structure of biotin. B. Biotin covalently bound to a lysyl residue of a biotin-dependent enzyme.

IX. BIOTIN

Biotin is a coenzyme in carboxylation reactions, in which it serves as a carrier of activated carbon dioxide (see Figure 10.3, p. 119, for the mechanism of biotin-dependent carboxylations). Biotin is covalently bound to the ε-amino group of lysine residues in biotin-dependent enzymes (Figure 28.16). Biotin deficiency does not occur naturally because the vitamin is widely distributed in food. Also, a large percentage of the biotin requirement in humans is supplied by intestinal bacteria. However, the addition of raw egg white to the diet as a source of protein induces symptoms of biotin deficiency, namely, dermatitis, glossitis, loss of appetite, and nausea. Raw egg white contains a glycoprotein, avidin, which tightly binds biotin and prevents its absorption from the intestine. With a normal diet, however, it has been estimated that 20 eggs/day would be required to induce a deficiency syndrome. Thus, inclusion of an occasional raw egg in the diet does not lead to biotin deficiency, although eating raw eggs is generally not recommended due to the possibility of salmonella infection.

Multiple carboxylase deficiency results from a defect in the ability to add biotin to carboxylases during their synthesis or to remove it from carboxylases during their degradation. Treatment is biotin supplementation.

Figure 28.17 Structure of coenzyme A.

X. PANTOTHENIC ACID

Pantothenic acid is a component of CoA, which functions in the transfer of acyl groups (Figure 28.17). CoA contains a thiol group that carries acyl compounds as activated thiol esters. Examples of such structures are succinyl CoA, fatty acyl CoA, and acetyl CoA. Pantothenic acid is also a component of the acyl carrier protein domain of fatty acid synthase (see p. 184). Eggs, liver, and yeast are the most important sources of pantothenic acid, although the vitamin is widely distributed. Pantothenic acid deficiency is not well characterized in humans, and no RDA has been established.

XI. VITAMIN A

The retinoids, a family of molecules that are related to dietary retinol (vitamin A), are essential for vision, reproduction, growth, and maintenance of epithelial tissues. They also play a role in immune function. Retinoic acid, derived from oxidation of retinol, mediates most of the actions of the retinoids, except for vision, which depends on retinal, the aldehyde derivative of retinol.

Figure 28.18 Structure of the retinoids.

A. Structure of vitamin A

Vitamin A is often used as a collective term for several related biologically active molecules (Figure 28.18). The term retinoids includes both natural and synthetic forms of vitamin A that may or may not show vitamin A activity.

1. Retinol: A primary alcohol containing a β-ionone ring with an unsaturated side chain, retinol is found in animal tissues as a retinyl ester with long-chain FAs.

2. Retinal: This is the aldehyde derived from the oxidation of retinol. Retinal and retinol can readily be interconverted.

3. Retinoic acid: This is the acid derived from the oxidation of retinal. Retinoic acid cannot be reduced in the body, and, therefore, cannot give rise to either retinal or retinol.

4. β-Carotene: Plant foods contain β-carotene, which can be oxidatively cleaved in the intestine to yield two molecules of retinal. In humans, the conversion is inefficient, and the vitamin A activity of β-carotene is only about 1/12 that of retinol.

B. Absorption and transport of vitamin A

1. Transport to the liver: Retinyl esters present in the diet are hydrolyzed in the intestinal mucosa, releasing retinol and FFAs (Figure 28.19). Retinol derived from esters and from the cleavage and reduction of carotenes is re-esterified to long-chain FAs in the intestinal mucosa and secreted as a component of chylomicrons into the lymphatic system (see Figure 28.19). Retinyl esters contained in chylomicron remnants are taken up by, and stored in, the liver.

2. Release from the liver: When needed, retinol is released from the liver and transported to extrahepatic tissues by the plasma retinol-binding protein (RBP). The retinol–RBP complex binds to a transport protein on the surface of the cells of peripheral tissues, permitting retinol to enter. Many tissues contain a cellular retinol-binding protein that carries retinol to sites in the nucleus where the vitamin acts in a manner analogous to that of steroid hormones.

C. Mechanism of action of vitamin A

Retinol is oxidized to retinoic acid. Retinoic acid binds with high affinity to specific receptor proteins (retinoic acid receptors [RARs]) present in the nucleus of target tissues such as epithelial cells (Figure 28.20). The activated retinoic acid–RAR complex binds to response elements on DNA and recruits activators or repressors to regulate retinoid-specific RNA synthesis, resulting in control of the production of specific proteins that mediate several physiologic functions. For example, retinoids control the expression of the gene for keratin in most epithelial tissues of the body. The RAR proteins are part of the superfamily of transcriptional regulators that includes the nuclear receptors for steroid and thyroid hormones and 1,25-dihydroxycholecalciferol, all of which function in a similar way (see p. 240).

Figure 28.19 Absorption, transport, and storage of vitamin A and its derivatives. RBP = retinol-binding protein; CoA = coenzyme A; mRNA = messenger RNA.

D. Functions of vitamin A

1. Visual cycle: Vitamin A is a component of the visual pigments of rod and cone cells. Rhodopsin, the visual pigment of the rod cells in the retina, consists of 11-cis retinal specifically bound to the protein opsin. When rhodopsin is exposed to light, a series of photochemical isomerizations occurs, which results in the bleaching of the visual pigment and release of all-trans retinal and opsin. This process triggers a nerve impulse that is transmitted by the optic nerve to the brain. Regeneration of rhodopsin requires isomerization of all-trans retinal back to 11-cis retinal. All-trans retinal, after being released from rhodopsin, is reduced to all-trans retinol, esterfied, and isomerized to 11-cis retinol that is oxidized to 11-cis retinal. The latter combines with opsin to form rhodopsin, thus completing the cycle. Similar reactions are responsible for color vision in the cone cells.

2. Maintenance of epithelial cells: Vitamin A is essential for normal differentiation of epithelial tissues and mucus secretion, and thus, supports the body’s barrier-based defense against pathogens.

3. Reproduction: Retinol and retinal are essential for normal reproduction, supporting spermatogenesis in the male and preventing fetal resorption in the female. Retinoic acid is inactive in maintaining reproduction and in the visual cycle but promotes growth and differentiation of epithelial cells. Therefore, animals given vitamin A only as retinoic acid from birth are blind and sterile.

Figure 28.20 Action of the retinoids. [Note: Retinoic acid-receptor complex forms a dimer, but is shown as monomer for simplicity.] RBP = retinol-binding protein; mRNA = messenger RNA.

E. Distribution of vitamin A

Liver, kidney, cream, butter, and egg yolk are good sources of preformed vitamin A. Yellow, orange, and dark green vegetables and fruits are good dietary sources of the carotenes, which serve as precursors of vitamin A.

F. Requirement for vitamin A

The RDA for adults is 900 retinol activity equivalents (RAEs) for males and 700 RAE for females. In comparison, 1 RAE = 1 mg of retinol, 12 mg of β-carotene, or 24 mg of other carotenoids.

G. Clinical indications

Although chemically related, retinoic acid and retinol have distinctly different therapeutic applications. Retinol and its carotenoid precursor are used as dietary supplements, whereas various forms of retinoic acid are useful in dermatology.

1. Dietary deficiency: Vitamin A, administered as retinol or retinyl esters, is used to treat patients who are deficient in the vitamin (Figure 28.21). Night blindness is one of the earliest signs of vitamin A deficiency. The visual threshold is increased, making it difficult to see in dim light. Prolonged deficiency leads to an irreversible loss in the number of visual cells. Severe vitamin A deficiency leads to xerophthalmia, a pathologic dryness of the conjunctiva and cornea, caused, in part, by increased keratin synthesis. If untreated, xerophthalmia results in corneal ulceration and, ultimately, in blindness because of the formation of opaque scar tissue. The condition is most commonly seen in children in developing tropical countries. Over 500,000 children worldwide are blinded each year by xerophthalmia caused by insufficient vitamin A in the diet.

Figure 28.21 Summary of actions of retinoids. Compounds in are available as dietary components or as pharmacologic agents.

2. Acne and psoriasis: Dermatologic problems such as acne and psoriasis are effectively treated with retinoic acid or its derivatives (see Figure 28.21). Mild cases of acne, Darier disease (keratosis follicularis), and skin aging are treated with topical application of tretinoin (all-trans retinoic acid), as well as benzoyl peroxide and antibiotics. [Note: Tretinoin is too toxic for systemic administration and is confined to topical application.] In patients with severe cystic acne unresponsive to conventional therapies, the drug of choice is isotretinoin (13-cis retinoic acid) administered orally. Retinoic acid is also used in the treatment of acute promyelocytic leukemia.

H. Toxicity of retinoids

1. Vitamin A: Excessive intake of vitamin A produces a toxic syndrome called hypervitaminosis A. Amounts exceeding 7.5 mg/day of retinol should be avoided. Early signs of chronic hypervitaminosis A are reflected in the skin, which becomes dry and pruritic (due to decreased keratin synthesis); the liver, which becomes enlarged and can become cirrhotic; and in the CNS, where a rise in intracranial pressure may mimic the symptoms of a brain tumor. Pregnant women particularly should not ingest excessive quantities of vitamin A because of its potential for teratogenesis (causing congenital malformations in the developing fetus). UL is 3,000 mg/day. [Note: Vitamin A promotes bone growth. In excess, however, it is associated with decreased bone mineral density and increased risk of fractures.]

2. Isotretinoin: The drug, an isomer of retinoic acid, is teratogenic and absolutely contraindicated in women with childbearing potential unless they have severe, disfiguring cystic acne that is unresponsive to standard therapies. Pregnancy must be excluded before initiation of treatment, and adequate birth control must be used. Prolonged treatment with isotretinoin leads to hyperlipidemia with an increase in TAGs and cholesterol, providing some concern for an increased risk of cardiovascular disease.

Figure 28.22 Sources of vitamin D.

XII. VITAMIN D

The D vitamins are a group of sterols that have a hormone-like function. The active molecule, 1,25-dihydroxycholecalciferol ([1,25-diOH-D3] calcitriol), binds to intracellular receptor proteins. The 1,25-diOH-D3–receptor complex interacts with DNA in the nucleus of target cells in a manner similar to that of vitamin A (see Figure 28.20) and either selectively stimulates or represses gene transcription. The most prominent actions of 1,25-diOH-D3 are to regulate the plasma levels of calcium and phosphorus.

A. Distribution of vitamin D

1. Endogenous vitamin precursor: 7-Dehydrocholesterol, an intermediate in cholesterol synthesis, is converted to cholecalciferol in the dermis and epidermis of humans exposed to sunlight and transported to liver bound to vitamin D–binding protein.

2. Diet: Ergocalciferol (vitamin D2), found in plants, and cholecalciferol (vitamin D3), found in animal tissues, are sources of preformed vitamin D activity (Figure 28.22). Ergocalciferol and cholecalciferol differ chemically only in the presence of an additional double bond and methyl group in the plant sterol. Dietary vitamin D is packaged into chylomicrons. [Note: Preformed vitamin D is a dietary requirement only in individuals with limited exposure to sunlight.]

B. Metabolism of vitamin D

1. Formation of 1,25-dihydroxycholecalciferol: Vitamins D2 and D3 are not biologically active but are converted in vivo to the active form of the D vitamin by two sequential hydroxylation reactions (Figure 28.23). The first hydroxylation occurs at the 25 position and is catalyzed by a specific 25-hydroxylase in the liver. The product of the reaction, 25-hydroxycholecalciferol ([25-OH-D3], calcidiol), is the predominant form of vitamin D in the plasma and the major storage form of the vitamin. 25-OH-D3 is further hydroxylated at the 1 position by 25-hydroxycholecalciferol 1-hydroxylase found primarily in the kidney, resulting in the formation of 1,25-diOH-D3(calcitriol). [Note: This 1-hydroxylase, as well as the liver 25-hydroxylase, are cytochrome P450 (CYP) proteins (see p. 149).]

2. Regulation of 25-hydroxycholecalciferol 1-hydroxylase: 1,25-diOH-D3 is the most potent vitamin D metabolite. Its formation is tightly regulated by the level of plasma phosphate and calcium ions (Figure 28.24). 25-Hydroxycholecalciferol 1-hydroxylase activity is increased directly by low plasma phosphate or indirectly by low plasma calcium, which triggers the secretion of parathyroid hormone (PTH) from the chief cells of the parathyroid gland. PTH upregulates the 1-hydroxylase. Thus, hypocalcemia caused by insufficient dietary calcium results in elevated levels of plasma 1,25-diOH-D3. [Note: 1,25-diOH-D3 inhibits synthesis of PTH, forming a negative feedback loop. It also inhibits activity of the 1-hydroxylase.]

Figure 28.23 Metabolism and actions of vitamin D. [Note: Calcitonin, a thyroid hormone, decreases blood calcium by inhibiting mobilization from bone, absorption from the intestine, and reabsorption by the kidney. It opposes the actions of PTH.] mRNA = messenger RNA; 25-OH-D3 = 25-hydroxycholecalciferol; 1,25-diOH-D3 = 1, 25-dihydroxycholecalciferol.

C. Function of vitamin D

The overall function of 1,25-diOH-D3 is to maintain adequate plasma levels of calcium. It performs this function by: 1) increasing uptake of calcium by the intestine, 2) minimizing loss of calcium by the kidney by increasing reabsorption, and 3) stimulating resorption (demineralization) of bone when blood calcium is low (see Figure 28.23).

1. Effect of vitamin D on the intestine: 1,25-diOH-D3 stimulates intestinal absorption of calcium. 1,25-diOH-D3 enters the intestinal cell and binds to a cytosolic receptor. The 1,25-diOH-D3–receptor complex then moves to the nucleus where it selectively interacts with response elements on the DNA. As a result, calcium uptake is enhanced by an increased synthesis of a specific calcium-binding protein, calbindin. Thus, the mechanism of action of 1,25-diOH-D3 is typical of steroid hormones (see p. 240).

2. Effect of vitamin D on bone: 1,25-diOH-D3 stimulates the mobilization of calcium from bone by a process that requires protein synthesis and the presence of PTH. The result is an increase in plasma calcium and phosphate. Therefore, bone is an important reservoir of calcium that can be mobilized to maintain plasma levels. [Note: PTH and calcitriol also work together to prevent renal loss of calcium.]

Figure 28.24 Response to low plasma calcium. 1,25-diOH-D3 = 1,25-dihydroxycholecalciferol.

D. Distribution and requirement of vitamin D

Vitamin D occurs naturally in fatty fish, liver, and egg yolk. Milk, unless it is artificially fortified, is not a good source of the vitamin. The RDA for individuals ages 1 to 70 years is 15 mg/day and 20 mg/day if over age 70 years. Experts disagree, however, on the optimal level of vitamin D needed to maintain health. [Note: 1 mg = 40 international units (IUs).] Because breast milk is a poor source of vitamin D, supplementation is recommended for breastfed babies.

E. Clinical indications

1. Nutritional rickets: Vitamin D deficiency causes a net demineralization of bone, resulting in rickets in children and osteomalacia in adults (Figure 28.25). Rickets is characterized by the continued formation of the collagen matrix of bone, but incomplete mineralization results in soft, pliable bones. In osteomalacia, demineralization of pre-existing bones increases their susceptibility to fracture. Insufficient exposure to daylight and/or deficiencies in vitamin D consumption occur predominantly in infants and the elderly. Vitamin D deficiency is more common in the northern latitudes, because less vitamin D synthesis occurs in the skin as a result of reduced exposure to ultraviolet light.

2. Renal osteodystrophy: Chronic kidney disease causes decreased ability to form active vitamin D as well as increased retention of phosphate, resulting in hyperphosphatemia and hypocalcemia. The low blood calcium causes a rise in PTH and associated bone demineralization with release of calcium and phosphate. Supplementation with calcitriol is an effective therapy. However, supplementation must be accompanied by phosphate reduction therapy to prevent further bone loss and precipitation of calcium phosphate crystals.

3. Hypoparathyroidism: Lack of PTH causes hypocalcemia and hyperphosphatemia. These patients may be treated with calcitriol and calcium supplementation.

Figure 28.25 Bowed legs of middle-aged man with osteomalacia, a nutritional vitamin D deficiency that results in demineralization of the skeleton.

F. Toxicity of vitamin D

Like all fat-soluble vitamins, vitamin D can be stored in the body and is only slowly metabolized. High doses (100,000 IUs for weeks or months) can cause loss of appetite, nausea, thirst, and stupor. Enhanced calcium absorption and bone resorption results in hypercalcemia, which can lead to deposition of calcium in many organs, particularly the arteries and kidneys. The UL is 100 mg/day (4,000 IU/day) for individuals ages 9 years or older, with a lower level for those under age 9 years. [Note: Toxicity is only seen with use of supplements. Excess vitamin D produced in the skin is converted to inactive forms.]

XIII. VITAMIN K

The principal role of vitamin K is in the posttranslational modification of a number of proteins (most of which are involved with blood clotting), in which it serves as a coenzyme in the carboxylation of certain glutamic acid residues present in these proteins. Vitamin K exists in several forms, for example, in plants as phylloquinone (or vitamin K1), and in intestinal bacterial flora as menaquinone (or vitamin K2). A synthetic form of vitamin K, menadione, is able to be converted to K2.

A. Function of vitamin K

1. Formation of γ-carboxyglutamate: Vitamin K is required in the hepatic synthesis of the blood clotting proteins, prothrombin (factor II) and factors VII, IX, and X. (See online Chapter 34.) Formation of the functional clotting factors requires the vitamin K–dependent carboxylation of several glutamic acid residues to γ-carboxyglutamate (Gla) residues (Figure 28.26). The carboxylation reaction requires γ-glutamyl carboxylase, O2, CO2, and the hydroquinone form of vitamin K (which gets oxidized to the epoxide form). The formation of Gla residues is sensitive to inhibition by warfarin, a synthetic analog of vitamin K that inhibits vitamin K epoxide reductase(VKOR), the enzyme required to regenerate the functional hydroquinone form of vitamin K.

2. Interaction of prothrombin with membranes: The Gla residues are good chelators of positively charged calcium ions, because of their two adjacent, negatively charged carboxylate groups. With prothrombin, for example, the prothrombin–calcium complex is able to bind to negatively charged membrane phospholipids on the surface of damaged endothelium and platelets. Attachment to membrane increases the rate at which the proteolytic conversion of prothrombin to thrombin can occur (Figure 28.27).

Figure 28.26 Carboxylation of glutamate to form γ-carboxyglutamate.

3. γ-Carboxyglutamate residues in other proteins: Gla residues are also present in proteins other than those involved in forming a blood clot. For example, osteocalcin of bone and proteins C and S (involved in limiting the formation of blood clots) also undergo γ-carboxylation.

Figure 28.27 Role of vitamin K in blood coagulation.

B. Distribution and requirement of vitamin K

Vitamin K is found in cabbage, kale, spinach, egg yolk, and liver. There is also extensive synthesis of the vitamin by the bacteria in the gut. The adequate intake for vitamin K is 120 mg/day for adult males and 90 mg for adult females.

C. Clinical indications

1. Deficiency of vitamin K: A true vitamin K deficiency is unusual because adequate amounts are generally produced by intestinal bacteria or obtained from the diet. If the bacterial population in the gut is decreased (for example, by antibiotics), the amount of endogenously formed vitamin is depressed, and this can lead to hypoprothrombinemia in the marginally malnourished individual (for example, a debilitated geriatric patient). This condition may require supplementation with vitamin K to correct the bleeding tendency. In addition, certain second-generation cephalosporin antibiotics (for example, cefamandole) cause hypoprothrombinemia, apparently by a warfarin-like mechanism that inhibits VKOR. Consequently, their use in treatment is usually supplemented with vitamin K.

2. Deficiency of vitamin K in the newborn: Newborns have sterile intestines and, so, initially lack the bacteria that synthesize vitamin K. Because human milk provides only about one fifth of the daily requirement for vitamin K, it is recommended that all newborns receive a single intramuscular dose of vitamin K as prophylaxis against hemorrhagic disease.

D. Toxicity of vitamin K

Prolonged administration of large doses of synthetic vitamin K (menadione) can produce hemolytic anemia and jaundice in the infant, due to toxic effects on the membrane of RBCs. Therefore, it is no longer used to treat vitamin K deficiency. No UL has been set for vitamin K.

Figure 28.28 Structure of vitamin E.

XIV. VITAMIN E

The E vitamins consist of eight naturally occurring tocopherols, of which α-tocopherol is the most active (Figure 28.28). The primary function of vitamin E is as an antioxidant in prevention of the nonenzymic oxidation of cell components (for example, peroxidation of polyunsaturated FAs by molecular oxygen and free radicals).

A. Distribution and requirements of vitamin E

Vegetable oils are rich sources of vitamin E, whereas liver and eggs contain moderate amounts. The RDA for α-tocopherol is 15 mg/day for adults. The vitamin E requirement increases as the intake of polyunsaturated FA increases to limit FA peroxidation.

B. Deficiency of vitamin E

Newborns have low reserves of vitamin E, but breast milk (and formulas) contain the vitamin. Very-low-birth-weight infants may be given supplements to prevent the hemolysis and retinopathy associated with deficiency of vitamin E. When observed in adults, deficiency is usually associated with defective lipid absorption or transport. [Note: Abetalipoproteinemia, caused by a defect in the formation of chylomicrons (and VLDL), results in vitamin E deficiency (see p. 231).]

C. Clinical indications

Vitamin E is not recommended for the prevention of chronic disease, such as coronary heart disease or cancer. Clinical trials using vitamin E supplementation have been uniformly disappointing. For example, subjects in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study trial who received high doses of vitamin E not only lacked cardiovascular benefit but also had an increased incidence of stroke.

D. Toxicity of vitamin E

Vitamin E is the least toxic of the fat-soluble vitamins, and no toxicity has been observed at doses of 300 mg/day (UL = 1,000 mg/day).

Populations consuming diets high in fruits and vegetables show decreased incidence of some chronic diseases. However, clinical trials have failed to show a definitive benefit from supplements of vitamins A, C, or E; multivitamins with folic acid; or antioxidant combinations for the prevention of cancer or cardiovascular disease.

The vitamins are summarized in Figure 28.29.

Figure 28.29 Summary of vitamins. P = phosphate; NAD(P) = nicotinamide adenine dinucleotide (phosphate); FMN = flavin mononucleotide; FAD = flavin adenine dinucleotide; CoA = coenzyme A. Summary of vitamins.

Study Questions

Choose the ONE best answer.

For Questions 28.1–28.5, match the vitamin deficiency to the clinical consequence.

A.

Folic acid

E.

Vitamin C

B.

Niacin

F.

Vitamin D

C.

Vitamin A

G.

Vitamin E

D.

Vitamin B12

H.

Vitamin K

28.1 Bleeding

28.2 Diarrhea and dermatitis

28.3 Neural tube defects

28.4 Night blindness

28.5 Sore, spongy gums and loose teeth

Correct answers = H, B, A, C, E. Vitamin K is required for formation of the γ-carboxyglutamate residues in several proteins required for blood clotting. Consequently, a deficiency of vitamin K results in a tendency to bleed. Niacin deficiency is characterized by the three Ds: diarrhea, dermatitis, and dementia (and death if untreated). Folic acid deficiency can result in neural tube defects in the developing fetus. Night blindness is one of the first signs of vitamin A deficiency. Rod cells in the retina detect white and black images and work best in low light, for example, at night. Rhodopsin, the visual pigment of the rod cells, consists of 11-cis retinal bound to the protein opsin. Vitamin C is required for the hydroxylation of proline and lysine during collagen synthesis. Severe vitamin C deficiency (scurvy) results in defective connective tissue, characterized by sore and spongy gums, loose teeth, capillary fragility, anemia, and fatigue.

28.6 A 52-year-old woman presents with fatigue of several months’ duration. Blood studies reveal a macrocytic anemia, reduced levels of hemoglobin, elevated levels of homocysteine, and normal levels of methylmalonic acid. Which of the following is most likely deficient in this woman?

A. Folic acid

B. Folic acid and vitamin B12

C. Iron

D. Vitamin C

Correct answer = A. Macrocytic anemia is seen with deficiencies of folic acid, vitamin B12, or both. Vitamin B12 is utilized in only two reactions in the body: the remethylation of homocysteine to methionine, which also requires folic acid (as tetrahydrofolate [THF]), and the conversion of methymalonyl coenzyme A to succinyl coenzyme A, which does not require THF. The elevated homocysteine and normal methylmalonic acid levels in the patient’s blood reflect a deficiency of folic acid as the cause of the macrocytic anemia. Iron deficiency causes microcytic anemia, as can vitamin C deficiency.

28.7 A 10-month-old African-American girl, whose family recently located from Maine to Virginia, is being evaluated for the bowed appearance of her legs. The parents report that the baby is still being breastfed and takes no supplements. Radiologic studies confirm the suspicion of vitamin D–deficient rickets. Which one of the following statements concerning vitamin D is correct?

A. A deficiency results in an increased secretion of calbindin.

B. Chronic kidney disease results in overproduction of 1,25-dihydroxycholecalciferol (calcitriol).

C. 25-Hydroxycholecalciferol (calcidiol) is the active form of the vitamin.

D. It is required in the diet of individuals with limited exposure to sunlight.

E. Its actions are mediated through binding to G protein–coupled receptors.

F. It opposes the effect of parathyroid hormone.

Correct answer = D. Vitamin D is required in the diet of individuals with limited exposure to sunlight, such as those living at northern latitudes like Maine and those with dark skin. Note that breast milk is low in vitamin D, and the lack of supplementation increases the risk of a deficiency. Vitamin D deficiency results in decreased synthesis of calbindin. Chronic kidney disease decreases production of calcitriol (1,25-dihydroxycholecalciferol), the active form of the vitamin. Vitamin D binds to nuclear receptors and alters gene transcription. Its effects are synergistic with parathyroid hormone.

28.8 Why might a deficiency of vitamin B6 result in a fasting hypoglycemia? Deficiency of what other vitamin could also result in hypoglycemia?

Vitamin B6 is required for glycogen degradation by glycogen phosphorylase. A deficiency would result in fasting hypoglycemia. Additionally, a deficiency of biotin (required by pyruvate carboxylase of gluconeogenesis) would also result in fasting hypoglycemia.