The Feed–Fast Cycle - 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 24. The Feed–Fast Cycle

I. OVERVIEW OF THE ABSORPTIVE STATE

The absorptive (well-fed) state is the 2- to 4-hour period after ingestion of a normal meal. During this interval, transient increases in plasma glucose, amino acids, and triacylglycerols (TAG) occur, the latter primarily as components of chylomicrons synthesized by the intestinal mucosal cells (see p. 228). Islet tissue of the pancreas responds to the elevated levels of glucose with an increased secretion of insulin and a decreased release of glucagon. The elevated insulin-to-glucagon ratio and the ready availability of circulating substrates make the absorptive state an anabolic period characterized by increased synthesis of TAG and glycogen to replenish fuel stores and enhanced synthesis of protein. During this absorptive period, virtually all tissues use glucose as a fuel, and the metabolic response of the body is dominated by alterations in the metabolism of liver, adipose tissue, skeletal muscle, and brain. In this chapter, an “organ map” is introduced that traces the movement of metabolites between tissues. The goal is to create an expanded and clinically useful vision of whole-body metabolism.

Figure 24.1 Control mechanisms of metabolism and some typical response times. [Note: Response times may vary according to the nature of the stimulus and from tissue to tissue.]

II. ENZYMIC CHANGES IN THE ABSORPTIVE STATE

The flow of intermediates through metabolic pathways is controlled by four mechanisms: 1) the availability of substrates; 2) allosteric regulation of enzymes; 3) covalent modification of enzymes; and 4) induction-repression of enzyme synthesis, primarily through regulation of transcription. Although this scheme may at first seem redundant, each mechanism operates on a different timescale (Figure 24.1) and allows the body to adapt to a wide variety of physiologic situations. In the well-fed state, these regulatory mechanisms ensure that available nutrients are captured as glycogen, TAG, and protein.

A. Allosteric effectors

Allosteric changes usually involve rate-determining reactions. For example, glycolysis in the liver is stimulated following a meal by an increase in fructose 2,6-bisphosphate, an allosteric activator of phosphofructokinase-1([PFK-1] see p. 99). In contrast, gluconeogenesis is inhibited by fructose 2,6-bisphosphate, an allosteric inhibitor of fructose 1,6-bisphosphatase (see p. 121).

B. Covalent modification

The activity of many enzymes is regulated by the addition (via kinases, such as cyclic adenosine monophosphate [cAMP]-activated protein kinase A [PKA] and adenosine monophosphate-activated protein kinase [AMPK]) or removal (via phosphatases) of phosphate groups from specific serine, threonine, or tyrosine residues of the protein. In the absorptive state, most of the covalently regulated enzymes are in the dephosphorylated form and are active (Figure 24.2). Three exceptions are glycogen phosphorylase kinase (see p. 132), glycogen phosphorylase (see p. 132), and hormone-sensitive lipase (HSL) of adipose tissue (see p. 190), which are inactive in their dephosphorylated form. [Note: In liver, the phosphatase domain of bifunctional phosphofructokinase-2 (PFK-2) is inactive when the protein is dephosphorylated (see p. 100).]

C. Induction and repression of enzyme synthesis

Increased (induction of) or decreased (repression of) enzyme synthesis leads to changes in the number of enzyme molecules, rather than influencing the activity of existing enzyme molecules. Enzymes subject to regulation of synthesis are often those that are needed under specific physiologic conditions. For example, in the fed state, elevated insulin levels result in an increase in the synthesis of key enzymes, such as acetyl coenzyme A (CoA) carboxylase ([ACC] see p. 184) and fatty acid synthase (see p.184), involved in anabolic metabolism. In the fasted state, glucagon induces expression of phosphoenolpyruvate carboxykinase (PEPCK) of gluconeogenesis (see p.120). Both hormones affect transcription factors.

Figure 24.2 Important reactions of intermediary metabolism regulated by enzyme phosphorylation. Blue text = intermediates of carbohydrate metabolism; brown text = intermediates of lipid metabolism. P = phosphate; CoA = coenzyme A.

III. LIVER: NUTRIENT DISTRIBUTION CENTER

The liver is uniquely situated to process and distribute dietary nutrients because the venous drainage of the gut and pancreas passes through the hepatic portal vein before entry into the general circulation. Thus, after a meal, the liver is bathed in blood containing absorbed nutrients and elevated levels of insulin secreted by the pancreas. During the absorptive period, the liver takes up carbohydrates, lipids, and most amino acids. These nutrients are then metabolized, stored, or routed to other tissues. In this way, the liver smooths out potentially broad fluctuations in the availability of nutrients for the peripheral tissues.

A. Carbohydrate metabolism

Liver is normally a glucose-producing rather than a glucose-using tissue. However, after a meal containing carbohydrate, the liver becomes a net consumer, retaining roughly 60 of every 100 g of glucose presented by the portal system. This increased use reflects increased glucose uptake by the hepatocytes. Their insulin-independent glucose transporter (GLUT-2) has a low affinity (high Km) for glucose and, therefore, takes up glucose only when blood glucose is high (see p. 97). Additional mechanisms by which hepatic glucose metabolism is increased include the following. [Note: The numbers in colored circles in the text refer to Figure 24.3.]

1. Increased phosphorylation of glucose: The elevated levels of glucose within the hepatocyte (as a result of elevated extracellular levels) allow glucokinase to phosphorylate glucose to glucose 6-phosphate (Figure 24.3, ). (Recall that glucokinase has a high Km for glucose, is not subject to direct product inhibition, and has a sigmoidal reaction curve; see p.98.).

2. Increased glycogenesis: The conversion of glucose 6-phosphate to glycogen is favored by the activation of glycogen synthase, both by dephosphorylation and by increased availability of glucose 6-phosphate, its allosteric effector (see Figure 24.3, ).

3. Increased activity of the pentose phosphate pathway: The increased availability of glucose 6-phosphate, combined with the active use of nicotinamide adenine dinucleotide phosphate (NADPH) in hepatic lipogenesis, stimulates the pentose phosphate pathway ([PPP] see p. 145). This pathway typically accounts for 5%–10% of the glucose metabolized by the liver (see Figure 24.3, ).

Figure 24.3 Major metabolic pathways in liver in the absorptive state. [Note: The acetyl CoA is also used for cholesterol synthesis.] The numbers in circles, which appear both in the figure and in the text, indicate important pathways for carbohydrate, fat, or protein metabolism. Blue text = intermediates of carbohydrate metabolism; brown text = intermediates of lipid metabolism; green text = intermediates of protein metabolism. P = phosphate; PPP = pentose phosphate pathway; TCA = tricarboxylic acid (cycle); CoA = coenzyme A; VLDL = very-low-density lipoprotein; GLUT = glucose transporter; NADPH = nicotinamide adenine dinucleotide phosphate.

4. Increased glycolysis: In liver, glycolysis is significant only during the absorptive period following a carbohydrate-rich meal. The conversion of glucose to pyruvate is stimulated by the elevated insulin-to-glucagon ratio that results in increased amounts of the regulated enzymes of glycolysis: glucokinase, PFK-1, and pyruvate kinase ([PK] see p. 102). Additionally, PFK-1 is allosterically activated by fructose 2,6-bisphosphate generated by the active (dephosphorylated) kinase domain of bifunctional PFK-2. PK is dephosphorylated and active. Pyruvate dehydrogenase (PDH), which converts pyruvate to acetyl CoA, is active (dephosphorylated) because pyruvate inhibits PDH kinase (see Figure 24.3, ). The acetyl CoA either is used as a substrate for fatty acid (FA) synthesis or is oxidized for energy in the tricarboxylic acid (TCA) cycle. (See Figure 24.4 for the central role of glucose 6-phosphate.)

5. Decreased production of glucose: Although glycolysis and glycogenesis (pathways that promote glucose storage) are stimulated in liver in the absorptive state, gluconeogenesis and glycogenolysis (pathways that generate glucose) are decreased. Pyruvate carboxylase (PC), which catalyzes the first step in gluconeogenesis, is largely inactive due to low levels of acetyl CoA, its allosteric activator (see p. 119). [Note: The acetyl CoA is being used for fatty acid synthesis.] The high insulin-to-glucagon ratio also favors inactivation of other gluconeogenic enzymes such as fructose 1,6-bisphosphatase (see Figure 8.17, p. 100). Glycogenolysis is inhibited by dephosphorylation of glycogen phosphorylase and phosphorylase kinase.

Figure 24.4 Central role of glucose 6-phosphate in metabolism. [Note: The presence of glucose 6-phosphatase in liver allows the production of free glucose from glycogenolysis and gluconeogenesis.] NADPH = nicotinamide adenine dinucleotide phosphate; P = phosphate.

B. Fat metabolism

1. Increased fatty acid synthesis: Liver is the primary tissue for de novo synthesis of FAs (see Figure 24.3, ). FA synthesis, a cytosolic process, is favored in the absorptive period by availability of the substrates acetyl CoA (from glucose and amino acid metabolism) and NADPH (from glucose metabolism) and by the activation of ACC, both by dephosphorylation and by the presence of its allosteric activator, citrate. [Note: Inactivity of AMPKfavors dephosphorylation.] ACC catalyzes the formation of malonyl CoA from acetyl CoA, the rate-limiting reaction for FA synthesis (see p. 183). [Note: Malonyl CoA inhibits carnitine palmitoyltransferase-I (CPT-I) of FA oxidation (see p.191). Citrate, thereby, directly activates FA synthesis and indirectly inhibits FA degradation.]

a. Source of cytosolic acetyl coenzyme A: Pyruvate from aerobic glycolysis enters mitochondria and is decarboxylated by PDH. The acetyl CoA product is combined with oxaloacetate (OAA) to form citrate via citrate synthase. Citrate leaves the mitochondria (as a result of the inhibition of isocitrate dehydrogenase by adenosine triphosphate [ATP]) and enters the cytosol. Citrate is cleaved by ATP-citrate lyase (induced by insulin), producing the acetyl CoA substrate of ACC and OAA. The OAA is reduced to malate, which is oxidatively decarboxylated to pyruvate by malic enzyme as NADPH is formed (see p. 187).

2. Increased triacylglycerol synthesis: TAG synthesis is favored because fatty acyl CoAs are available both from de novo synthesis from acetyl CoA and from hydrolysis of the TAG component of chylomicron remnants removed from the blood by hepatocytes (see p. 178). Glycerol 3-phosphate, the backbone for TAG synthesis, is provided by glycolysis (see p. 189). The liver packages TAG into very-low-density lipoprotein (VLDL) particles that are secreted into the blood for use by extrahepatic tissues, particularly adipose and muscle tissues (see Figure 24.3, ).

Figure 24.5 Colorized transmission electron micrograph of adipocytes.

C. Amino acid metabolism

1. Increased amino acid degradation: In the absorptive period, more amino acids are present than the liver can use in the synthesis of proteins and other nitrogen-containing molecules. The surplus amino acids are not stored but are either released into the blood for other tissues to use in protein synthesis or deaminated, with the resulting carbon skeletons being degraded by the liver to pyruvate, acetyl CoA, or TCA cycle intermediates. These metabolites can be oxidized for energy or used in FA synthesis (see Figure 24.3, ). The liver has limited capacity to degrade the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. They pass through the liver essentially unchanged and are preferentially metabolized in muscle (see p. 266).

2. Increased protein synthesis: The body does not store protein in the same way that it maintains glycogen or TAG reserves (see p. 327). However, a transient increase in the synthesis of hepatic proteins does occur in the absorptive state, resulting in replacement of any proteins that may have been degraded during the previous postabsorptive period (see Figure 24.3, ).

IV. ADIPOSE TISSUE: ENERGY STORAGE DEPOT

Adipose tissue is second only to the liver in its ability to distribute fuel molecules. In a 70-kg man, white adipose tissue (WAT) weighs approximately 14 kg, or about half as much as the total muscle mass. Nearly the entire volume of each adipocyte in WAT can be occupied by a droplet of TAG (Figure 24.5).

A. Carbohydrate metabolism

1. Increased glucose transport: Circulating insulin levels are elevated in the absorptive state, resulting in an influx of glucose into adipocytes via insulin-sensitive GLUT-4 recruited to the cell surface from intracellular vesicles (Figure 24.6, ). The glucose is phosphorylated by hexokinase.

2. Increased glycolysis: The increased intracellular availability of glucose results in an enhanced rate of glycolysis (see Figure 24.6, ). In adipose tissue, glycolysis serves a synthetic function by supplying glycerol 3-phosphate for TAG synthesis (see p. 189). Recall that adipose tissue lacks glycerol kinase.

3. Increased activity of the pentose phosphate pathway: Adipose tissue can metabolize glucose by means of the PPP, thereby producing NADPH, which is essential for fat synthesis (see p. 186 and Figure 24.6, ). However, in humans, de novo synthesis is not a major source of FA in adipose tissue, except when refeeding a previously fasted individual (see Figure 24.6, ).

Figure 24.6 Major metabolic pathways in adipose tissue in the absorptive state. [Note: The numbers in the circles, which appear both in the figure and in the corresponding text, indicate important pathways for adipose tissue metabolism.] GLUT = glucose transporter; P = phosphate; PPP = pentose phosphate pathway; CoA = coenzyme A; TCA = tricarboxylic acid; TAG = triacylglycerol; VLDL = very-low-density lipoprotein; LPL = lipoprotein lipase.

B. Fat metabolism

Most of the FAs added to the TAG stores of adipocytes after consumption of a lipid-containing meal are provided by the degradation of exogenous (dietary) TAG in chylomicrons sent out by the intestine and endogenous TAG in VLDL sent out by the liver (see Figure 24.6, ). The FAs are released from the lipoproteins by the action of lipoprotein lipase (LPL), an extracellular enzyme attached to the capillary walls in many tissues, particularly adipose and muscle. In adipose tissue, LPL is upregulated by insulin. Thus, in the fed state, elevated levels of glucose and insulin favor storage of TAG (see Figure 24.6, ), all the carbons of which are supplied by glucose. [Note: Elevated insulin favors the dephosphorylated (inactive) form of HSL (see p. 190), thereby inhibiting lipolysis in the fed state.]

V. RESTING SKELETAL MUSCLE

In the fed state, muscle takes up glucose via GLUT-4 (for energy and glycogen synthesis) and amino acids (for energy and protein synthesis). [Note: The energy metabolism of skeletal muscle is unique in being able to respond to substantial changes in the demand for ATP that accompanies muscle contraction. At rest, muscle accounts for approximately 30% of the O2 consumption of the body, whereas during vigorous exercise, it is responsible for up to 90% of the total O2 consumption. This graphically illustrates the fact that skeletal muscle, despite its potential for transient periods of anaerobic glycolysis, is an oxidative tissue. In contrast to liver, there is no covalent regulation of PFK-2 in skeletal muscle. In the cardiac isozyme, however, the kinase domain is activated by epinephrine-mediated phosphorylation.]

A. Carbohydrate metabolism

1. Increased glucose transport: The transient increase in plasma glucose and insulin after a carbohydrate-rich meal leads to an increase in glucose transport into muscle cells by GLUT-4 (see p. 97 and Figure 24.7, ), thereby reducing blood glucose. Glucose is phosphorylated to glucose 6-phosphate by hexokinase and metabolized to provide the energy needs of the cells.

2. Increased glycogen synthesis: The increased insulin-to-glucagon ratio and the availability of glucose 6-phosphate favor glycogen synthesis, particularly if glycogen stores have been depleted as a result of exercise (see p. 126, and Figure 24.7, ).

Figure 24.7 Major metabolic pathways in skeletal muscle in the absorptive state. [Note: The numbers in circles, which appear both in the figure and in the text, indicate important pathways for carbohydrate or protein metabolism.] CoA = coenzyme A; P = phosphate; GLUT = glucose transporter; BCAAs = branched-chain amino acids; TCA = tricarboxylic acid.

B. Fat metabolism

FAs are released from chylomicrons and VLDL by the action of LPL (see pp. 228 and 231). However, fatty acids are of secondary importance as a fuel for resting muscle during the fed state, in which glucose is the primary source of energy.

C. Amino acid metabolism

1. Increased protein synthesis: An increase in amino acid uptake and protein synthesis occurs in the absorptive period after ingestion of a meal containing protein (see Figure 24.7, and ). This synthesis replaces protein degraded since the previous meal.

2. Increased uptake of branched-chain amino acids: Muscle is the principal site for degradation of the BCAAs (leucine, isoleucine, and valine) because it contains the required transaminase (see p. 266). The BCAAs escape metabolism by the liver and are taken up by muscle, where they are used for protein synthesis (see Figure 24.7, ) and as sources of energy.

VI. BRAIN

Although contributing only 2% of the adult weight, the brain accounts for a consistent 20% of the basal O2 consumption of the body at rest. Because the brain is vital to the proper functioning of all organs of the body, special priority is given to its fuel needs. To provide energy, substrates must be able to cross the endothelial cells that line the blood vessels in the brain (the blood–brain barrier [BBB]). In the fed state, the brain exclusively uses glucose as a fuel (GLUT-1 of the BBB is insulin independent), completely oxidizing approximately 140 g/day to CO2 and H2O. The brain contains no significant stores of glycogen and is, therefore, completely dependent on the availability of blood glucose (Figure 24.8, ). [Note: If blood glucose levels fall below 40 mg/100 ml (normal fasted blood glucose is 70–99 mg/100 ml), cerebral function is impaired (see p. 315).] The brain also lacks significant stores of TAG, and the FAs circulating in the blood make little contribution to energy production because FAs bound to albumin do not efficiently cross the BBB. The intertissue exchanges characteristic of the absorptive period are summarized in Figure 24.9.

Figure 24.8 Major metabolic pathways in brain in the absorptive state. [Note: The numbers in circles, which appear both in the figure and in the text, indicate important pathways for carbohydrate metabolism.] CoA = coenzyme A; TCA = tricarboxylic acid; P = phosphate; GLUT = glucose transporter.

VII. OVERVIEW OF FASTING

Fasting begins if no food is ingested after the absorptive period. It may result from an inability to obtain food, the desire to lose weight rapidly, or clinical situations in which an individual cannot eat (for example, because of trauma, surgery, cancer, or burns). In the absence of food, plasma levels of glucose, amino acids, and TAG fall, triggering a decline in insulin secretion and an increase in glucagon and epinephrine release. The decreased insulin/counterregulatory hormone ratio and the decreased availability of circulating substrates make the period of nutrient deprivation a catabolic period characterized by degradation of TAG, glycogen, and protein. This sets into motion an exchange of substrates among liver, adipose tissue, skeletal muscle, and brain that is guided by two priorities: 1) the need to maintain adequate plasma levels of glucose to sustain energy metabolism in the brain, red blood cells, and other glucose-requiring tissues and 2) the need to mobilize fatty acids from adipose tissue and the synthesis and release of ketone bodies from the liver to supply energy to other tissues. [Note: Maintaining glucose requires that the substrates for gluconeogenesis (such as pyruvate, alanine, and glycerol) be available.]

A. Fuel stores

The metabolic fuels available in a normal 70-kg man at the beginning of a fast are shown in Figure 24.10. Note the enormous caloric stores available in the form of TAG compared with those contained in glycogen. [Note: Although protein is listed as an energy source, each protein also has a function (for example, as a structural component of the body, an enzyme, and so forth). Therefore, only about one third of the body’s protein can be used for energy production without fatally compromising vital functions.]

Figure 24.9 Intertissue relationships in the absorptive state and the hormonal signals that promote them. [Note: Small circles on the perimeter of muscle and the adipocyte indicate insulin-dependent glucose transporters.] P = phosphate; PPP = pentose phosphate pathway; CoA = coenzyme A; NADPH = nicotinamide adenine dinucleotide phosphate; TCA = tricarboxylic acid; VLDL = very-low-density lipoprotein.

B. Enzymic changes in fasting

In fasting (as in the fed state), the flow of intermediates through the pathways of energy metabolism is controlled by four mechanisms: 1) the availability of substrates, 2) allosteric regulation of enzymes, 3) covalent modification of enzymes, and 4) induction–repression of enzyme synthesis. The metabolic changes observed in fasting are generally opposite to those described for the absorptive state (see Figure 24.9). For example, although most of the enzymes regulated by covalent modification are dephosphorylated and active in the fed state, they are phosphorylated and inactive in the fasted state. Three exceptions are glycogen phosphorylase (see p. 132), glycogen phosphorylase kinase (see p. 132), and HSL of adipose tissue (see p. 190), which are active in their phosphorylated states. In fasting, substrates are not provided by the diet but are available from the breakdown of stores and/or tissues, such as glycogenolysis with release of glucose from liver, lipolysis with release of FAs and glycerol from TAG in adipose tissue, and proteolysis with release of amino acids from muscle. Recognition that the changes in fasting are the reciprocal of those in the fed state is helpful in understanding the ebb and flow of metabolism.

Figure 24.10 Metabolic fuels present in a 70-kg man at the beginning of a fast. The fat stores are sufficient to meet energy needs for about 80 days.

VIII. LIVER IN FASTING

The primary role of liver in energy metabolism during fasting is maintenance of blood glucose through the production of glucose from glycogenolysis and gluconeogenesis for glucose-dependent tissues and the synthesis and distribution of ketone bodies for use by other tissues. Therefore, “hepatic” metabolism and “extrahepatic” or “peripheral” metabolism are distinguished.

A. Carbohydrate metabolism

The liver first uses glycogen degradation and then gluconeogenesis to maintain blood glucose levels to sustain energy metabolism of the brain and other glucose-requiring tissues in the fasted (postabsorptive) state. [Note: Recall that the presence of glucose 6-phosphatase in the liver allows the production of free glucose both from glycogenolysis and from gluconeogenesis (see Figure 24.4).]

1. Increased glycogen degradation: Figure 24.11 shows the sources of blood glucose after ingestion of 100 g of glucose. During the brief absorptive period, ingested glucose is the major source of blood glucose. Several hours later, blood glucose levels have declined sufficiently to cause increased secretion of glucagon and decreased release of insulin. The increased glucagon-to-insulin ratio causes a rapid mobilization of liver glycogen stores (which contain about 80 g of glycogen in the fed state) due to PKA-mediated phosphorylation (and activation) of glycogen phosphorylase kinase that phosphorylates (and activates) glycogen phosphorylase (see p. 130). Figure 24.11shows that liver glycogen is nearly exhausted after 10–18 hours of fasting, and therefore, hepatic glycogenolysis is a transient response to early fasting. Figure 24.12, , shows glycogen degradation as part of the overall metabolic response of the liver during fasting. [Note: Phosphorylation of glycogen synthase simultaneously inhibits glycogenesis.]

Figure 24.11 Sources of blood glucose after ingestion of 100 g of glucose. [Note: See Section B.2. for an explanation as to why gluconeogenesis declines.]

2. Increased glucose synthesis: The synthesis of glucose and its release into the circulation are vital hepatic functions during short- and long-term fasting (see Figure 24.12, ). The carbon skeletons for gluconeogenesis are derived primarily from glucogenic amino acids and lactate from muscle and glycerol from adipose tissue. Gluconeogenesis, favored by activation of fructose 1,6-bisphosphatase (due to decreased availability of its inhibitor fructose 2,6-bisphosphate; see p. 121) and by induction of PEPCK by glucagon (see p. 122), begins 4–6 hours after the last meal and becomes fully active as stores of liver glycogen are depleted (see Figure 24.11). [Note: The decrease in fructose 2,6-bisphosphate simultaneously inhibits glycolysis at PFK-1 (see p. 99).]

Figure 24.12 Major metabolic pathways in liver during fasting. [Note: The numbers in circles, which appear both in the figure and in the corresponding citation in the text, indicate important metabolic pathways for carbohydrate or fat.] P = phosphate; CoA = coenzyme A; TCA = tricarboxylic acid; NADH = nicotinamide adenine dinucleotide.

B. Fat metabolism

1. Increased fatty acid oxidation: The oxidation of FAs obtained from TAG hydrolysis in adipose tissue is the major source of energy in hepatic tissue in the postabsorptive state (see Figure 24.12, ). The fall in malonyl CoA due to phosphorylation (inactivation) of ACC by AMPK removes the brake on CPT-1, allowing β-oxidation to occur (see p. 191). FA oxidation generates NADH, FADH2, and acetyl CoA. The NADH inhibits the TCA cycle. The acetyl CoA is an allosteric activator of PC and an allosteric inhibitor of PDH, thereby favoring use of pyruvate in gluconeogenesis (see Figure 8.24). [Note: The acetyl CoA cannot be used as a substrate for gluconeogenesis, in part because the PDH reaction is irreversible.] Oxidation of NADH and FADH2 coupled with oxidative phosphorylation supplies the energy required by the PC and PEPCK reactions of gluconeogenesis.

Figure 24.13 Concentrations of fatty acids and 3-hydroxybutyrate in the blood during fasting. [Note: 3-Hydroxybutyrate is made from the reduction of acetoacetate.]

2. Increased ketone body synthesis: The liver is unique in being able to synthesize and release ketone bodies, primarily 3-hydroxybutyrate but also acetoacetate, for use as fuel by peripheral tissues (see p. 195) but not by the liver itself because liver lacks thiophorase. Ketogenesis, which starts during the first days of fasting (Figure 24.13), is favored when the concentration of acetyl CoA from FA oxidation exceeds the oxidative capacity of the TCA cycle. [Note: Ketogenesis releases CoA, ensuring its availability for continued FA oxidation.] The availability of circulating water-soluble ketone bodies is important in fasting because they can be used for fuel by most tissues, including brain, once their level in the blood is sufficiently high. This reduces the need for gluconeogenesis from amino acid carbon skeletons, thus preserving essential protein (see Figure 24.11). Ketogenesis as part of the overall hepatic response to fasting is shown in Figure 24.12, . [Note: Ketone bodies are organic acids and, when present at high concentrations, can cause ketoacidosis.]

Figure 24.14 Major metabolic pathways in adipose tissue during fasting. [Note: The numbers in the circles, which appear both in the figure and in the corresponding citation in the text, indicate important pathways for fat metabolism.] CoA = coenzyme A; TCA = tricarboxylic acid.

IX. ADIPOSE TISSUE IN FASTING

A. Carbohydrate metabolism

Glucose transport by insulin-sensitive GLUT-4 into the adipocyte and its subsequent metabolism are depressed due to low levels of circulating insulin. This results in decreased TAG synthesis.

B. Fat metabolism

1. Increased degradation of fat: The PKA-mediated phosphorylation and activation of HSL (see p. 190) and subsequent hydrolysis of stored fat are enhanced by the elevated catecholamines norepinephrine and epinephrine. These hormones, which are released from the sympathetic nerve endings in adipose tissue and/or from the adrenal medulla, are physiologically important activators of HSL (Figure 24.14, ).

2. Increased release of fatty acids: FAs obtained from hydrolysis of stored TAG are primarily released into the blood (see Figure 24.14, ). Bound to albumin, they are transported to a variety of tissues for use as fuel. The glycerol produced from TAG degradation is used as a gluconeogenic precursor by the liver, which contains glycerol kinase. [Note: FA can also be oxidized to acetyl CoA, which can enter the TCA cycle, thereby producing energy for the adipocyte. They also can be re-esterified to glycerol 3-phosphate (from glyceroneogenesis, see p. 190), generating TAG and reducing plasma FA concentration.]

3. Decreased uptake of fatty acids: In fasting, LPL activity of adipose tissue is low. Consequently, circulating TAG of lipoproteins is not available to adipose tissue.

Figure 24.15 Major metabolic pathways in skeletal muscle during fasting. [Note: The numbers in the circles, which appear both in the figure and in the corresponding citation in the text, indicate important pathways for fat or protein metabolism.] CoA = coenzyme A; TCA = tricarboxylic acid.

X. RESTING SKELETAL MUSCLE IN FASTING

Resting muscle switches from glucose to FAs as its major fuel source in fasting. [Note: By contrast, exercising muscle initially uses its glycogen stores as a source of energy. During intense exercise, glucose 6-phosphate derived from glycogen is converted to lactate by anaerobic glycolysis (see p. 103). The lactate is used by liver for gluconeogenesis (Cori cycle; see p.118). As these glycogen reserves are depleted, free FAs provided by the mobilization of TAG from adipose tissue become the dominant energy source. The contraction-based rise in AMP activates AMPK that phosphorylates and inactivates the muscle isozyme of ACC, decreasing malonyl CoA and allowing FA oxidation (see p. 183).

A. Carbohydrate metabolism

Glucose transport into skeletal muscle cells via insulin-sensitive GLUT-4 (see p. 97) and subsequent glucose metabolism are depressed because of low levels of circulating insulin. Therefore, the glucose from hepatic gluconeogenesis is unavailable to muscle (and adipose tissue).

B. Lipid metabolism

During the first 2 weeks of fasting, muscle uses FA from adipose tissue and ketone bodies from the liver as fuels (Figure 24.15, and ). After about 3 weeks of fasting, muscle decreases its use of ketone bodies (thus sparing them for brain) and oxidizes FA almost exclusively. [Note: The acetyl CoA from FA oxidation indirectly inhibits PDH (by activation of PDH kinase) and spares pyruvate, which is transaminated to alanine and used by liver for gluconeogenesis (glucose–alanine cycle; see p. 253).]

Figure 24.16 Major metabolic pathways in the brain during fasting. [Note: The numbers in the circles, which appear both in the figure and in the corresponding citation in the text, indicate important pathways for metabolism of fat or carbohydrates.] CoA = coenzyme A; TCA = tricarboxylic acid; P = phosphate.

C. Protein metabolism

During the first few days of fasting, there is a rapid breakdown of muscle protein, providing amino acids that are used by the liver for gluconeogenesis (see Figure 24.15, ). Because muscle does not have glucagon receptors, muscle proteolysis is initiated by a fall in insulin and sustained by a rise in glucocorticoids. [Note: Alanine and glutamine are quantitatively the most important gluconeogenic amino acids released from muscle. They are produced by the catabolism of BCAAs (see p. 267).] The glutamine is used as a fuel by enterocytes, for example, which send out alanine that is used in hepatic gluconeogenesis. In the second week of fasting, the rate of muscle proteolysis decreases, paralleling a decline in the need for glucose as a fuel for the brain, which has begun using ketone bodies as a source of energy.

XI. BRAIN IN FASTING

During the early days of fasting, the brain continues to use only glucose as a fuel (Figure 24.16, ). Blood glucose is maintained by hepatic gluconeogenesis from glucogenic precursors, such as amino acids from proteolysis and glycerol from lipolysis. In prolonged fasting (beyond 2–3 weeks), plasma ketone bodies (see Figure 24.12) reach significantly elevated levels and replace glucose as the primary fuel for the brain (see Figure 24.16, , and Figure 24.17). This reduces the need for protein catabolism for gluconeogenesis: ketone bodies spare glucose and, thus, muscle protein. [Note: As the duration of a fast extends from overnight to days to weeks, blood glucose levels initially drop and then are maintained at the lower level (65–70 mg/dl).] The metabolic changes that occur during fasting ensure that all tissues have an adequate supply of fuel molecules. The response of the major tissues involved in energy metabolism during fasting is summarized in Figure 24.19 (see below).

Figure 24.17 Fuel sources used by the brain to meet energy needs in the well fed and starved states.

XII. KIDNEY IN LONG-TERM FASTING

As fasting continues into early starvation and beyond, the kidney plays important roles. The kidney expresses the enzymes of gluconeogenesis, including glucose 6-phosphatase, and in late fasting about 50% of gluconeogenesis occurs here. [Note: A portion of this glucose is used by the kidney itself.] The kidney also provides compensation for the acidosis that accompanies the increased production of ketone bodies (organic acids). The glutamine released from the muscle’s metabolism of BCAAs is taken up by the kidney and acted upon by renal glutaminase and glutamate dehydrogenase (see p. 256), producing α-ketoglutarate that can be used as a substrate for gluconeogenesis, plus ammonia (NH3). The NH3 picks up protons from ketone body dissociation and is excreted in the urine as ammonium (NH4+), thereby decreasing the acid load in the body (Figure 24.18). In long-term fasting, then, there is a switch from nitrogen disposal in the form of urea to disposal in the form of ammonia. [Note: As ketone body concentration rises, enterocytes, typically consumers of glutamine, become consumers of ketone bodies. This allows more glutamine to be available to the kidney.]

Figure 24.18 Use of glutamine from BCAA catabolism in muscle to generate ammonia (NH3) used for the excretion of protons (H+) as ammonium (NH4+) in kidney.

XIII. CHAPTER SUMMARY

The flow of intermediates through metabolic pathways is controlled by four mechanisms: 1) the availability of substrates, 2) allosteric activation and inhibition of enzymes, 3) covalent modification of enzymes, and 4) induction-repression of enzyme synthesis. In the absorptive state, the 2–4-hour period after ingestion of a meal, these regulatory mechanisms ensure that available nutrients are captured as glycogen, triacylglycerol (TAG), and protein (Figure 24.20). During this interval, transient increases in plasma glucose, amino acids, and TAG occur, the last primarily as components of chylomicrons synthesized by the intestinal mucosal cells. The pancreasresponds to the elevated levels of glucose with an increased secretion of insulin and a decreased secretion of glucagon by the pancreas. The elevated insulin-to-glucagon ratio and the ready availability of circulating substrates make the absorptive state an anabolic period during which virtually all tissues use glucose as a fuel. In addition, the liver replenishes its glycogen stores, replaces any needed hepatic proteins, and increases TAGsynthesis. The latter are packaged in very-low-density lipoproteins, which are exported to the peripheral tissues. Adipose tissue increases TAG synthesis and storage, whereas muscle increases protein synthesis to replace protein degraded since the previous meal. In the fed state, the brain uses glucose exclusively as a fuel. In the absence of food, plasma levels of glucose, amino acids, and TAG fall, triggering a decline in insulin secretion and an increase in glucagon and epinephrine release. The decreased insulin/counterregulatory hormone ratio and the decreased availability of circulating substrates make the fasting state a catabolic period. This sets into motion an exchange of substrates among liver, adipose tissue, skeletal muscle, and brain that is guided by two priorities: 1) the need to maintain adequate plasma levels of glucose to sustain energy metabolism of the brain and other glucose-requiring tissues and 2) the need to mobilize fatty acids (FAs) from adipose tissue and release ketone bodies from liver to supply energy to other tissues. To accomplish these goals, the liver degrades glycogen and initiates gluconeogenesis, using increased fatty acid oxidation as a source of the energy and reducing equivalents needed for gluconeogenesis and to supply the acetyl coenzyme A building blocks for ketogenesis. The adipose tissue degrades stored TAG, thus providing FAs and glycerol to the liver. The muscle can also use FAs as fuel as well as ketone bodies supplied by the liver. Muscle protein is degraded to supply amino acids for the liver to use in gluconeogenesis, but deceases as ketone bodies increase. The brain can use both glucose and ketone bodies as fuels. From late fasting into starvation, the kidneys play important roles by synthesizing glucoseand excreting the protons from ketone body dissociation as ammonium (NH4+).

Figure 24.19 Intertissue relationships during starvation and the hormonal signals that promote them. P = phosphate; TCA = tricarboxylic acid; CoA = coenzyme A.

Figure 24.20 Key concept map for feed-fast cycle. VLDL = very-low-density lipoprotein.

Study Questions

Choose the ONE best answer.

24.1 Which one of the following is elevated in plasma during the absorptive (fed) period as compared with the postabsorptive (fasted) state?

A. Acetoacetate

B. Chylomicrons

C. Free fatty acids

D. Glucagon

Correct answer = B. Triacylglycerol-rich chylomicrons are synthesized in (and released from) the intestine following ingestion of a meal. Acetoacetate, free fatty acids, and glucagon are elevated in the fasted state, not the absorptive state.

24.2 Which one of the following statements concerning liver in the fed state is correct?

A. Fructose 2,6-bisphosphate is elevated.

B. Insulin stimulates the uptake of glucose.

C. Most enzymes that are regulated by covalent modification are in the phosphorylated state.

D. The oxidation of acetyl coenzyme A is increased.

E. The synthesis of glucokinase is repressed.

Correct answer = A. The increased insulin and decreased glucagon levels characteristic of the fed state promote the synthesis of fructose 2,6-bisphosphate, which allosterically activates phosphofructokinase-1 of glycolysis. Most covalently modified enzymes are in the dephosphorylated state and are active. Acetyl coenzyme A is not oxidized in the fed state because it is being used in fatty acid synthesis. Uptake of glucose (by glucose transporter-2) into the liver is insulin independent. Synthesis of glucokinase is induced by insulin in the fed state.

24.3 Which one of the following enzymes is phosphorylated and active in an individual who has been fasting for 12 hours?

A. Arginase

B. Carnitine palmitoyltransferase-1

C. Fatty acid synthase

D. Glycogen synthase

E. Hormone-sensitive lipase

F. Phosphofructokinase-1

G. Pyruvate dehydrogenase

Correct answer = E. Hormone-sensitive lipase of adipocytes is phosphorylated and activated by protein kinase A in response to epinephrine. Choices A, B, C, and F are not regulated covalently. Choices D and G are regulated covalently but are inactive if phosphorylated.

24.4 For a 70-kg man, in which one of the periods listed below do ketone bodies supply the major portion of the caloric needs of brain?

A. Absorptive period

B. Overnight fast

C. Three-day fast

D. Four-week fast

E. Five-month fast

Correct answer = D. Ketone bodies, made from the acetyl coenzyme A product of fatty acid oxidation, increase in the blood in fasting but must reach a critical level to cross the blood–brain barrier. Typically this occurs in the second to third week of a fast. Fat stores in a 70-kg man would not be able to supply his energy needs for 5 months.

24.5 The diagram below shows inputs to and outputs from pyruvate, a central molecule in energy metabolism.

 Which letter on the diagram represents a reaction that requires biotin and is activated by acetyl coenzyme A?

Correct answer = C. Pyruvate carboxylase, a mitochondrial enzyme of gluconeogenesis, requires biotin (and adenosine triphosphate) and is allosterically activated by acetyl coenzyme A from fatty acid oxidation. None of the other choices meets these criteria.