Case Files Biochemistry, 3rd Edition (2015)

CASE 34

A 49-year-old woman presents to your clinic for follow-up after initiating a new medication (lovastatin) for her elevated cholesterol. She is currently without complaints and is feeling well. On repeat serum cholesterol screening, there is noted to be a decrease in the cholesterol level. The patient asks if she needs to continue the medication and what the potential side effects and benefits might be. Her physician explains that this medication inhibits the rate-limiting step and key enzyme in cholesterol biosynthesis.

What is the mechanism of action of this medication?

ANSWER TO CASE 34:

Statin Medications

Summary: A 49-year-old woman with history of elevated cholesterol on lovastatin that appears to be improving serum cholesterol levels.

• Mechanism action of lovastatin: β-Hydroxy-β-methylglutaryl coenzyme A (HMG CoA) reductase inhibitor.

CLINICAL CORRELATION

Hyperlipidemia is one of the most treatable risk factors of coronary heart disease. Initially, when the fasting low-density lipoprotein (LDL) cholesterol is measured and found to be elevated, lifestyle modification is recommended such as dietary adjustments, exercise, and weight loss. The lipids are measured again after a 3- to 6-month interval. If the LDL cholesterol level is again found to be above threshold, then pharmacologic therapy is entertained. One of the most common medications is a statin agent, acting to inhibit HMG-CoA reductase. The potential side effects include elevated liver function tests, increased muscle creatine phosphokinase (CPK) secondary to myopathy and rarely rhabdomyolysis. Other agents that may be considered include bile acid sequestrants, nicotinic acid, fibric acid, and fish oils.

APPROACH TO:

Cholesterol Synthesis

OBJECTIVES

1. Describe the role of HMG-CoA reductase in cholesterol synthesis.

2. Explain the mechanism of action of statin medications.

3. Describe how cholesterol gives rise to the steroid hormones.

4. Be aware that niacin decreases lipolysis in adipose tissue and very-low-density lipoprotein (VLDL) synthesis in liver.

DEFINITIONS

CYTOCHROME P450 ENZYME SYSTEM: The cytochromes P450 are mixed-function oxidases that require both NADPH and O₂. They are involved in a number of reactions in the conversion of lanosterol to cholesterol, as well as important steps in the synthesis of steroid hormones. Cytochromes P450 are very important in the detoxification of xenobiotics and in the metabolism of drugs.

HMG-CoA REDUCTASE: β-Hydroxy-β-methylglutaryl-CoA reductase; the enzyme that catalyzes the rate limiting and committed step in the synthesis of cholesterol. It converts β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) to mevalonate.

ISOPRENOID: Any of a number of hydrophobic compounds derived from the polymerization of isopentenyl pyrophosphate and its isomer, dimethylallyl pyrophosphate. The isoprene unit is a 5-carbon branched hydrocarbon (2-methyl-1, 3-butadiene).

STATIN: Any of a number of drugs that competitively inhibit the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase.

DISCUSSION

Cholesterol is mainly synthesized in the liver by a 3-stage process. All 27 carbon atoms in the cholesterol molecule are derived from acetyl-CoA. The first stage is the synthesis of the activated 5-carbon isoprene unit, isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate then condense to form squalene in a sequence of reactions that also synthesize isoprenoid intermediates that are important in protein isoprenylation modifications. The characteristic 4-ring structure of cholesterol is then formed by cyclizing of the linear squalene molecule. Several demethylations, the reduction of a double bond, and the migration of another double bond result in the formation of cholesterol. Figure 34-1 provides an overview of cholesterol biosynthesis.

Figure 34-1. Overview of cholesterol synthesis.

The key enzyme in the synthesis of cholesterol is β-hydroxy-β-methylglutaryl-CoA reductase (HMG-CoA reductase), which catalyzes the synthesis of mevalonate from HMG-CoA in an irreversible, rate-limiting reaction. Mevalonate is the immediate 6-carbon precursor to isopentenyl pyrophosphate. HMG-CoA reductase is localized on the membrane of the endoplasmic reticulum and spans the membrane. The active site for this enzyme is found on the cytosolic side of the membrane. HMG-CoA reductase is inhibited by cholesterol in a feedback mechanism and the levels of mRNA for the enzyme are also regulated by the levels of cholesterol. Low concentrations of cholesterol increase the level of messenger RNA (mRNA) for HMG-CoA reductase, whereas high concentrations of cholesterol decrease the mRNA level. Because the enzyme HMG-CoA reductase is the rate-limiting step of cholesterol biosynthesis, this enzyme is the target for many cholesterol-lowering drugs.

The 5 major classes of steroid hormones are derived from cholesterol by the pathway illustrated in Figure 34-2. Hydroxylation is important in these conversions. The hydroxylation reactions require NADPH and O₂ and are carried out by the cytochrome P450 enzyme system. The enzyme 21-hydroxylase is required for the synthesis of mineralocorticoids and glucocorticoids.

Figure 34-2. General biosynthetic pathway of the steroid hormones.

Another important hormone derived from cholesterol is vitamin D. This steroid-like hormone is involved in regulating calcium and phosphorus metabolism. The complete synthesis of vitamin D requires ultraviolet light to convert 7-dehydrocholesterol to previtamin D₃. The reaction scheme is shown in Figure 34-3. The active hormone 1,25-dihydroxycholecalciferol (calcitriol) requires reactions that occur in the liver and kidneys and acts in a manner similar to steroid hormones to activate transcription, thereby regulating gene expression.

Figure 34-3. Biosynthesis of active vitamin D₃.

Lovastatin is a member of a class of drugs (atorvastatin and simvastatin are others in this class) called statins that are used to treat hypercholesterolemia. The statins act as competitive inhibitors of the enzyme HMG-CoA reductase. These molecules mimic the structure of the normal substrate of the enzyme (HMG-CoA) and act as transition state analogues. Although the statins are bound to the enzyme, HMG-CoA cannot be converted to mevalonic acid, thus inhibiting the whole cholesterol biosynthetic process. Recent studies indicate that there may be important secondary effects of statin therapy because some of the medical benefits of statins are too rapid to be a result of decreasing atherosclerotic lesions. Statin therapy has been associated with reduced risks of dementia, Alzheimer disease, ischemic cerebral stroke, and other diseases that are not correlated with high cholesterol levels. Although this is still an active area of research, it appears that the pleiotropic effects of statins may be a result of a reduction in the synthesis of isoprenoid intermediates that are formed in the pathway of cholesterol biosynthesis.

Niacin is a vitamin that is used in high doses to treat hypercholesterolemia. Niacin acts to decrease VLDL and LDL plasma levels. Its mechanism of action is not clearly understood but probably involves inhibition of VLDL secretion, which in turn decreases the production of LDL. Niacin inhibits the release of free fatty acids from adipose tissue, which leads to a decrease of free fatty acids entering the liver and decreased VLDL synthesis in the liver. This decreases the availability of VLDL for conversion to LDL (containing cholesterol esters). Niacin also increases high-density lipoprotein (HDL; the “good cholesterol”) by an unknown mechanism.

COMPREHENSION QUESTIONS

34.1 Which of the following compounds directly inhibits the expression of the HMG-CoA reductase gene?

A. Squalene

B. HMG-CoA

C. Lanosterol

D. Isopentenyl pyrophosphate

E. Cholesterol

34.2 You decide to treat a patient who has very high levels of serum cholesterol with the statin drug atorvastatin. You know that this drug acts in the metabolic pathway leading to the synthesis of cholesterol. The substrate for the enzyme inhibited by the statin drugs is which of the following?

A. Acetoacetyl-CoA

B. HMG-CoA

C. Farnesol pyrophosphate

D. Isopentenyl pyrophosphate

E. Mevalonate

34.3 Which of the following vitamins can be used in high doses to treat hypercholesterolemia?

A. Niacin

B. Riboflavin

C. Pyridoxine

D. Folic acid

E. Thiamine

ANSWERS

34.1 E. The major regulatory enzyme of cholesterol metabolism, HMG-CoA reductase, is regulated by 3 distinct mechanisms. The first is phosphorylation by a cAMP dependent protein kinase. Phosphorylation of HMG-CoA reductase inactivates the enzyme. The other 2 mechanisms involve the levels of cholesterol. The degradation of the enzyme is controlled by cholesterol levels. The half-life of HMG-CoA reductase is regulated by cholesterol levels with high concentrations of cholesterol leading to a shorter half-life. The final regulatory mechanism involves control of the expression of the HMG-CoA reductase gene. High levels of cholesterol lead to a decrease in the mRNA levels coding for HMG-CoA reductase.

34.2 B. The first step in the biosynthesis of cholesterol is the condensation of 2 molecules of acetyl-CoA to form acetoacetyl-CoA. The addition of a third acetyl CoA molecule gives rise to HMG-CoA. HMG-CoA reductase is converted to mevalonate. HMG-CoA reductase is the target of the statin drugs and the substrate used by this enzyme is HMG-CoA.

34.3 A. Niacin is the vitamin that can be used, in high doses, to treat hypercholesterolemia. Niacin acts to decrease VLDL and LDL plasma levels. Its mechanism of action is not clearly understood but probably involves inhibition of VLDL secretion, which, in turn, decreases the production of LDL. Niacin inhibits the release of free fatty acids from adipose tissue, which leads to a decrease of free fatty acids entering the liver and decreased VLDL synthesis in the liver. This decreases the availability of VLDL for conversion to LDL (containing cholesterol esters). Niacin also increases HDL (the “good cholesterol”) by an unknown mechanism.

BIOCHEMISTRY PEARLS

HMG-CoA reductase catalyzes the synthesis of mevalonate from HMG-CoA in an irreversible rate-limiting reaction.

The enzyme 21-hydroxylase is required for the synthesis of mineralocorticoids and glucocorticoids.

The statins act as competitive inhibitors of the enzyme HMG-CoA reductase.

REFERENCES

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th ed. New York: Freeman; 2002:722-731.

Devlin TM, ed. Textbook of Biochemistry with Clinical Correlations. 7th ed. New York: Wiley-Liss; 2010.

Granner DK. The diversity of the endocrine system. In: Murray RK, Bender DA, Botham KM, et al, eds. Harper’s Illustrated Biochemistry. 29th ed. New York: Lange Medical Books/McGraw-Hill; 2012.

Liao JK. Isoprenoids as mediators of the biological effects of statins. J Clin Invest. 2002;110(3):285-288.

Mayes PA, Botham KM. Cholesterol synthesis, transport, & excretion. In: Murray RK, Bender DA, Botham KM, et al, eds. Harper’s Illustrated Biochemistry. 29th ed. New York: Lange Medical Books/McGraw-Hill, 2012.

Goldberg AC. The Merck Manual of Diagnosis and Therapy. 19th ed. Whitehouse Station, NJ: Merck Sharp & Dohme Corp.; 2013. http://www.merckmanuals.com/professional/endocrine_and_metabolic_disorders/lipid_disorders/overview_of_lipid_metabolism.html.