Cholesterol biosynthesis HMG-CoA reductase

Several studies have demonstrated that intensive lowering of serum cholesterol or LDL cholesterol may retard progression of coronary atherosclerosis [32]. At present the inhibitors of key enzyme of cholesterol biosynthesis (HMG-CoA-reductase inhibitors) used in the clinical conditions as one of the most effective lipid-lowering drugs [32]. Note should be taken that HMG-CoA-reductase inhibitors may depress not only cholesterol but also ubiquinon Qio biosynthesis so far as biosynthesis of both this substances involved a common precursor [33] (Figure 16). 

Cholesterol can be obtained from the diet or it can be synthesized de novo. An adult on a low-cholesterol diet typically synthesizes about 800 mg of cholesterol per day. The liver is the major site of cholesterol synthesis in mammals, although the intestine also forms significant amounts. The rale of cholesterol formation by these organs is highly responsive to the cellular level of cholesterol. This feedback regulation is mediated primarily by changes in the amount and activity of 3-hydroxy 3 methylglutaryl CoA reductase. As described earlier (p. 739), this enzyme catalyzes the formation of meval-onate, the committed step in cholesterol biosynthesis. HMG CoA reductase is controlled in multiple ways ... 

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

Lipid homeostasis, Cholesterol biosynthesis HMG-CoA reductase, SCAP, SREBP, ACAT-1, ACAT-2, ACACA, ACACB, LXR-a, DGATl, LDLR, LRP2, ABCAl Hyperlipidemia, Dys-lipidemia, Obesity, Alzheimer s, Coronary heart disease, Atherosclerosis... 

HMG-CoA reductase is the major regulatory enzyme in cholesterol biosynthesis. HMG-CoA reductase is controlled hormonally by insulin and glucagon and transcription and translation of the enzyme can be suppressed by the presence of cholesterol in cells. Mevalonate is converted in the cytosol to the five carbon building blocks of isoprene synthesis-isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DPP)-in the reactions shown in Figure 19.19. Subsequently, IPP and DPP form famesyl pyrophosphate in the cytosol (Figure 19.20)... 

Ferrous ion-induced Hpid peroxidation of rat liver mitochondria was accelerated by phosphate (Yamamoto et al. 1974). Preincubation of rat liver microsomes with iron (Fe)/ascorbate (50 pM/ 200 pM), known to induce peroxidation, resulted in a significant inhibition of (i) the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (46 %, P <0.01, (ii) the crucial enzyme control-Hng the conversion of cholesterol in bile acids, cholesterol 7a-hydroxylase (48%, P <0.001), and (iii) the central enzyme for cholesterol esterification, acyl-CoAxholesterol acyltransferase (ACAT, 80%, P <0.0001) (Brunet etal. 2000). The disturbances of these key enzymes coincided with a high rate of malondialdehyde production (350%, P <0.007) and the loss of polyunsaturated fatty adds (36.19 1.06% vs. 44.24 0.41% in controls, P <0.0008). While a-tocopherol simultaneously neutrahsed lipid peroxidation, preserved microsomal fatty acid status, and restored ACAT activity, it was not effective in preventing Fe/ascorbate-induced inactivation of both HMG-CoA reductase (44%, P <0.01) and cholesterol 7a-hydroxylase (71%, P< 0.0001). 

To summarize, vitamin C is an important mediator of cholesterol metabolism and lipoprotein profile. The rate-limiting enzymes of both cholesterol biosynthesis (HMG-CoA reductase) and cholesterol catabolism to bile acids (cholesterol 7a-hy-droxylase) are affected by vitamin C status. Interestingly, the effect of vitamin C on these enzymes is parabolic with the extremes of vitamin C deficiency and excess... 

The first two steps in cholesterol biosynthesis from acetyl-CoA are identical to those of ketone body formation (Figure 19.10). The difference is that ketone bodies are formed in the mitochondria, whereas cholesterol synthesis initially takes place in the ER. A thiolase catalyzes the condensation of two acetyl-CoA molecules to acetoacetyl-CoA, and the combination of a third acetyl-CoA with acetoacetyl-CoA to form /8-hydroxymethylglutaryl-CoA (HMG-CoA) is catalyzed by HMG-CoA synthase. Although HMG-CoA is split into acetoacetate and acetyl-CoA in the mitochondria, in cholesterol biosynthesis, HMG-CoA is reduced by a microsomal enzyme, HMG-CoA reductase, to mevalonate (see Figure 19.17). The reducing agent is NADPH. 

A feedback mechanism operates in which intracellular free cholesterol inhibits HMG-CoA reductase. When the diet is rich in cholesterol, intracellular cholesterol increases in the liver and the biosynthesis of cholesterol is suppressed. Conversely, a low-cholesterol diet, but one with adequate triglyceride, stimulates cholesterol biosynthesis. 

As previously mentioned, the rate-limiting enzyme in the biosynthesis of cholesterol is HMG-CoA reductase. It catalyzes the reduction of 3-hydroxy-3-methyl-glutaryl-coen-zyme A to mevalonic acid (Fig. 11-4). The enzyme is also the point in the sequence where the end product cholesterol acts as its own feedback inhibitor. 7-Oxocholesterol, a nonbioactive steroid, retards the cell growth that cholesterol promotes and strongly inhibits HMG-Co-A reductase activity. The compound thus acts as a false feedback inhibitor. This inhibition is reversible by the addition of either cholesterol or mevalonic acid. 

Several target enzymes in the cholesterol biosynthesis pathway have been targeted in attempts to lower blood cholesterol. The HMG-CoA reductase inhibitor (106) is 10 times more potent than lovastatin <93JMC3658>, and both SQ-33600 (107) and XU 62-320 (108) also inhibit this enzyme . The indole compound SaH 57-118 (109) inhibits acyl CoA cholesterol acyltransferase and lowers cholesterol absorption in rabbits by 65% . 

The search for inhibitors of this pathway began with the first key regulatory enzyme, HMG CoA reductase. Several clinically useful inhibitors of HMG CoA reductase are now known. One of the most successful, Mevacor, produced by Merck, is one of the pharmaceutical industry s best selling products. However, the problem with inhibiting a branched biosynthetic pathway at an early point is that the biosynthesis of other crucial biomolecules may also be inhibited. Indeed, there is some evidence that levels of ubiquinone and the dolichols are affected by some HMG CoA reductase inhibitors. Consequently, efforts have recently been directed towards finding inhibitors of squalene synthase, the enzyme controlling the first step on the route to cholesterol after the FPP branch point. 

HMG-CoA-Reductase Inhibitors. Figure 1 Mechanism of action of statins - cholesterol synthesis pathway. The conversion of acetyl CoA to cholesterol in the liver. The step of cholesterol biosynthesis inhibited by HMG-CoA reductase inhibitors (statins) is shown. 

Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, interrupting the conversion of HMG-CoA to mevalonate, the rate-limiting step in de novo cholesterol biosynthesis. Reduced synthesis of LDL and enhanced catabohsm of LDL mediated through LDL-Rs appear to be the principal mechanisms for lipid-lowering effects. 

The therapeutic class that uniquely exemplifies lactone prodrugs are the statins, i.e., the cholesterol-lowering agents that act by inhibiting 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.34). This microsomal enzyme catalyzes conversion of HMG-CoA to mevalonate, an important rate-limiting step in cholesterol biosynthesis. Cholesterol synthesis occurs mainly... 

An enzyme (see Section 2.6) called HMG-CoA reductase is involved in the biosynthesis of cholesterol. Drugs such as atorvastatin (Lipitor) and simvastatin (Zocor) are competitive inhibitors of HMG-CoA reductase. They inhibit cholesterol synthesis by increasing the number of LDL receptors to take up the LDL. 

Competitive blocker of a-adrenergic receptors in heart and blood vessels Inhibits the enzyme HMG-CoA reductase and reduces the biosynthesis of cholesterol Acts as an angiotensin II receptor antagonist Inhibits the synthesis of prostaglandins via the selective inhibition of the enzyme cyclooxygenase-2... 

Inhibit Enzymes Many drugs are competitive inhibitors of key enzymes in pathways. The statin drugs (lovastatin, simvastatin), used to control blood cholesterol levels, competitively inhibit 3-hvdroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase in cholesterol biosynthesis. Methotrexate, an antineoplastic drug, competitively inhibits dihydrofolate reductase, depriving the cell of active folate needed for purine and deoxythymidine synthesis, thus interfering with DNA replication during S phase. 

Statins are compounds that inhibit HMG-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis, and they are the world s best-selling drugs and are used for lowering cholesterol. Statins are well studied and are believed to be quite safe. Because they reduce the levels of cholesterol, the precursor of the bile acids, statins may be the ideal drugs to use for BA-loweiing in these GI tract diseases. 

The subsequent conversion of HMG-CoA into MVA involves a two-step reduction of the thioester group to a primary alcohol (see Section 7.11), and provides an essentially irreversible and rate-limiting transformation. Drug-mediated inhibition of this enzyme, HMG-CoA reductase (HMGR), can be used to regulate the biosynthesis of the steroid cholesterol. High levels of blood cholesterol are known to contribute to the incidence of coronary heart disease and heart attacks. 

Formation of mevalonate. The conversion of acetyl CoA to acetoacetyl CoA and then to 3-hydroxy-3-methylglutaryl CoA (3-HMG CoA) corresponds to the biosynthetic pathway for ketone bodies (details on p. 312). In this case, however, the synthesis occurs not in the mitochondria as in ketone body synthesis, but in the smooth endoplasmic reticulum. In the next step, the 3-HMG group is cleaved from the CoA and at the same time reduced to mevalonate with the help of NADPH+H 3-HMG CoA reductase is the key enzyme in cholesterol biosynthesis. It is regulated by repression of transcription (effectors oxysterols such as cholesterol) and by interconversion... 

The answer is D. This patient s tests indicate that he has severe hypercholesterolemia and high blood pressure in conjunction with atherosclerosis. The deaths of several of his family members due to heart disease before age 60 suggest a genetic component, ie, familial hypercholesterolemia. This disease results from mutations that reduce production or interfere with functions of the LDL receptor, which is responsible for uptake of LDL-cholesterol by liver cells. The LDL receptor binds and internalizes LDL-choles-terol, delivers it to early endosomes and then recycles back to the plasma membrane to pick up more ligand. Reduced synthesis of apoproteins needed for LDL assembly would tend to decrease LDL levels in the bloodstream, as would impairment of HMG CoA reductase levels, the rate-limiting step of cholesterol biosynthesis. Reduced uptake of bile salts will also decrease cholesterol levels in the blood. 

Fluvastatin, launched in 1994, was the first S5mthetic HMG-CoA reductase inhibitor to be approved for the treatment of hypercholesterolemia. This statin was developed by Novartis (Sandoz), and it is marketed as a racemate under the trade name Lescol (Asberg and Holdaas, 2004). In laboratory studies, fluvastatin was shown to potently inhibit HMG-CoA reductase (IC50 = 8 nM) and effectively block cholesterol biosynthesis in hepatoc5de cells (IC50 = 52 nM) (Parker et al., 1990). Clinically, fluvastatin is prescribed less frequently than simvastatin, atorvastatin, or rosuvastatin, but it nevertheless remains an important medication for the treatment of hypercholesterolemia moreover, as the first completely S5mthetic statin to be approved, it offers a compelling synthetic story (Repic et al., 2001). 

Rosuvastatin (Crestor ) is the most recent HMG-CoA reductase inhibitor to be widely approved for the treatment of hypercholesterolemic patients. It has established an impressive record of clinical efficacy and is often prescribed to patients that do not respond well to earlier HMG-CoA reductase inhibitors (Schuster, 2003). This fully synthetic statin, developed by Astra-Zeneca and Shionogi Research Laboratories, has been shown to potently inhibit cholesterol biosynthesis in rat hepatocytyes (IC50 =1.12 nM). By comparison, pravastatin (5) was > 100-fold less active in the same assay (IC50 =198 nM) (Hirai et al., 1993 Watanabe et al., 1997). 

Fig. 8. Most important steps in the biosynthesis of cholesterol. The reduction of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) to yield mevalonic acid is an important rate-limiting step and also the site of attack of the HMG-CoA-reductase inhibitors (statins).

The statin family of six closely related hypocholesterolemic drugs are all potent competitive inhibitors of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase), the rate-limiting enzyme in cholesterol biosynthesis. The liver is their target organ, and decreased hepatic cholesterol synthesis ultimately leads to increased removal of LDL particles from the circulation. As a consequence, all other hypocholesterolemic drugs have been relegated to secondary status. 

Trifluoromethyl ketones and alcohol derivatives of squalene have been prepared in order to inhibit squalene epoxycyclase. This important enzyme regulates the biosynthesis of cholesterol. It bears a cysteine in its active site. Although these compounds have been shown to be good inhibitors, the involved mechanism is different from what was expected. Indeed, they do not inhibit squalene epoxycyclase, but they are substrates of this enzyme and are transformed into fluorohydroxysterols. The repression of the expression of HMG-CoA reductase is responsible for the observed inhibition of cholesterol biosynthesis. This repression comes from the back-regulation that is exerted by fluorohydroxysterols. Indeed, these compounds induce an important diminution of the cell activity of HMG-CoA reductase (Figure 7.66). °... 

HMG-CoA Reductase Inhibitors Statins inhibit HMG-CoA reductase, the enzyme synthesizing mevalonic acid (a key step in cholesterol biosynthesis). These drugs are indicated to treat hypercholesterolemia and to reduce LDL cholesterol. 

Mechanism of Action An HMG-CoA reductase inhibitor that interferes with cholesterol biosynthesis by preventing the conversion of HMG-CoA reductase to meva-lonate, a precursor to cholesteroh Therapeutic Effect Lowers serum LDL and VLDL cholesterol and plasma triglyceride levels increases serum HDL concentration. Pharmacokinetics Poorly absorbed from the G1 tract. Protein binding 50%. Metabolized in the liver (minimal active metabolites). Primarily excreted in feces via the biliary system. Not removed by hemodialysis. Half-life 2.7 hr. 

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