Cholesterol a-hydroxylase

Tissue where bile acids are synthesized, and the regulated step Bile acids are synthesized in the liver. The rate-limiting step is catalyzed by cholesterol-7-a-hydroxylase, which is activated by cholesterol and inhibited by bile acids. 

The conversion of cholesterol to bile salts begins when hydroxyl groups are introduced into the phenanthrene ring of cholesterol by the action of cholesterol 7-a-hydroxylase, followed by modification of the side chain. Cholic acid and chenodeoxycholic acid are produced, as shown in Fig. 13-24. 

The rate-limiting step of bile acid synthesis is cholesterol 7-a-hydroxylase. The changes in enzyme activity are the result of altered levels of cholesterol 7-a-hydroxylase mRNA. 

Quazi S, Takahata M, Horio F, et al 1984. Hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7-a-hydroxylase activities in rats fed PCB. Nutr Rep Int 30(3) 617627. 

Noshiro, M. and K. Okuda (1990). Molecular cloning and sequence analysis of cDNA encoding human cholesterol 7 a-hydroxylase. FEES Lett. 268, 137-140. 

The principal LCPUFA of oo-6 series is arachidonic acid (AA 20 4) acting as a precursor for eicosanoids synthesized from LA. LCPUFAs of 00-6 series have been considered as activators of PPAR-y. Their metabolic effects include increased synthesis of cholesterol, increased activity of LDL receptors, increased activity of cholesterol 7 a-hydroxylase (Cyp 7A1), and decreased conversion of VLDL to LDL. As ligands of PPAR-y, co-6 PUFAs may improve insulin sensitivity, change fat distribution, and affect adipocyte differentiation (Chiang et ah, 2001 Corton and Anderson, 2000). 

Chiang, J. Y. L., Kimmel, R., and Stroup, D. (2001). Regulation of cholesterol 7 a-hydroxylase gene (CYP7A1) transcription by the liver orphan receptor (LXRa). Gene 262,257-265. 

Diet-induced reduction in plasma HDL shows a physiological and a genetic correlation with repression of cholesterol-7-a-hydroxylase, the liver specific enzyme that regulates the conversion of cholesterol into bile acids. Constitutively expressing this enzyme in mice prevented them from developing atherosclerosis and developing decreased HDL-C levels (498). 

They are used to soften and purify water, to purify fruit juices, in the separation of metals from each other (for example, separating plutonium and uranium in nuclear reactors), in the manufacture and purification of sugars and in the manufacture of pharmaceutical products. The ion exchange polymers colestyramine, colestipol and colesevelam are also known as bile acid sequestrants and are used to lower serum cholesterol concentrations. They are not absorbed from the intestine, where they bind bile acids, reducing their reabsorption after biliary excretion. The pool of bile acids becomes depleted, resulting in upregulation of cholesterol 7-a-hydroxylase, which increases conversion of cholesterol to bile acids. 

Figure 1.1 The classic pathway for the conversion of cholesterol into the primary bile acids CA and CDCA, involving the 7 a-hydroxylase enzyme (also known as CYP7A1). Simplified from Dr John Chiang/ The 7 OH group is highlighted with the shaded circle. This group is cleaved to produce the secondary BAs DCA and LCA.

Figure 1.1 illustrates a condensed version of the classical pathway of bile-acid synthesis, a series of 12 enzymatic reactions that convert cholesterol, which is insoluble, into BAs, which are water soluble. The cholesterol is first converted to 7 alpha-hydroxy cholesterol, followed by the series of enzymatic transformations, eventually producing cholic and chenodeoxycholic acids (not all steps shown). The rate-limiting enzyme in this pathway is cholesterol 7 alpha-hydroxylase (CYP 7A1), which originates from microsomal cytochrome P-450 enzymes, expressed only in the liver hepatocytes. 

Bile salts are exclusively synthesized in the liver (see A). The slowest step in their biosynthesis is hydroxylation at position 7 by a 7-a-hydroxylase. Cholic acid and other bile acids inhibit this reaction (end-product inhibition). In this way, the bile acids present in the liver regulate the rate of cholesterol utilization. 

The lowered concentration of bile acids returning to the liver by the enterohepatic circulation results in derepression of 7-a-hydroxylase, the rate-limiting enzyme for conversion of cholesterol to bile acids. This results in increased use of cholesterol to replace the excreted bile acids and lowering of hepatic cholesterol (mechanism VI in Fig. 23.2). Thus, similar to the statins, the ultimate actions of the bile acid-sequestering resins are up-regulation of transcription of the LDL receptor gene, increased hepatic receptor activity, and lowering of plasma LDL cholesterol (mechanism VII in Fig. 23.2). 

An a-hydroxyl group is added to carbon 7 of cholesterol. A 7 a-hydroxylase, which is inhibited by bile salts, catalyzes this rate-limiting step. 

Patel, D. D Knight, B. L Soutar, A. K Gibbons, G. F and Wade, D. P. (2000) The effect of peroxisome-proliferator-activated receptor-alpha on the activity of the cholesterol 7 alpha-hydroxylase gene. Biochem. J. 351 (Pt. 3), 747-753. 

Bile salts are synthesized in the liver from cholesterol by reactions that hydroxy-late the steroid nucleus and cleave the side chain. In the first reaction, an a-hydroxyl group is added to carbon 7 (on the a side of the B ring). The activity of the 7 a-hydroxylase that catalyzes this rate-limiting step is decreased by bile salts (Fig. 34.9). 

The increased CETP activity also downregulated mRNA levels of hepatic LDL receptors, 3-hydroxy-3-methylglu-taiyl-coenzyme A reductase and 7-a-hydroxylase (4). These mice also had increased hepatic TC, free cholesterol (EC), and cholesteryl ester (CE) concentrations (4). The development of fatty liver by the accumulation of triglycerides (TG) has also been shown in simian-CETP transgenic mice (9). 

The mechanisms by which ascorbate supplementation prevents the exacerbation of hypercholesterolemia and related CVD include an increased catabolism of cholesterol. In particular, ascorbate is known to stimulate 7-a-hydroxylase, a key enzyme in the conversion of cholesterol to bile acids and to increase the expression of LDL receptors on the cell surface. Moreover, ascorbate is known to inhibit endogenous cholesterol synthesis as well as oxidative modification of LDL (for review see 1). 

The regulatory implication of such a proposal is shown in Fig. 9. In the event of cholesterol excess, such as dietary cholesterol entering the cell, the regulatory adjustment would be as follows HMG-CoA reductase activity would decline, as a consequence of phosphorylation, whereas the activities of AC AT and 7 a-hydroxylase enzymes would be stimulated. In the instance of cholesterol deprivation, for example a cholesterol-free diet or a cultured liver cell grown in lipid-deficient medium, the regulatory adjustment would be HMG-CoA reductase activity would increase as a consequence of dephosphorylation, but AC AT and 7a-hydroxylase activities would decline under these conditions. 

The initial steps in BA synthesis are characterised by the introduction of a hy-droxylic group in the la position, or in position 27, followed by another in the la position into the cholesterol nucleus. Both synthetic pathways (the neutral and the acidic pathways) possess a distinct microsomal 7-oxysterol hydroxylase, which is regulated by different genes. The most recently described disorder of BA synthesis is cholesterol 7a-hydroxylase deficiency, in which their decreased production through the classical pathway is partially balanced by activation of the alternative pathway. Cholesterol levels increase in the liver, with a consequent low-density lipoprotein hypercholesterolemia, and cholesterol gallstones may result, although there is no liver disease. In contrast, a defect in the conversion of 27-hydroxy-cholesterol to la,27-dihydroxy-cholesterol due to deficiency of the oxysterol 7a-hydroxylase specific for the alternate pathway, causes severe neonatal liver disease [8]. 

The mechanism of action of o-thyroxine appears to be stimulation of oxidative cataboli.sm of cholesterol in the liver through stimulation of 7-a-cholcstcrol hydroxylase, the rate-limiting enzyme in the conversion of cholesterol to bile acids. The bile acids arc conjugated with glycine or taurine and excreted by the biliary route into the feces. Although thyroxine docs not inhibit cholesterol bio.synthesis. it increases the number of LDL receptors, enhancing removal of LDL from plasma. 

Shefer, S., Gheng, F. W., Hauser, S., Batta, A. K., and Salen, G., Regulation ofbile acid synthesis. Measurement of cholesterol 7 -hydroxylase activity in rat liver microsomal preparations in the absence of endogenous cholesterol. J. Lipid Res. 22, 532-536... 

Answer D. Cholestyramine and colestipol are resins that sequester bile acids in the gut, preventing their reabsorption. This leads to release of their feedback inhibition of 7-alpha hydroxylase and the diversion of cholesterol toward new synthesis of bile acids. Increase in high-affinity LDL receptors on hepatocyte membranes decreases plasma LDL. These drugs have a small but significant effect to increase plasma HDL rather than decrease it, but their ability to increase TGs precludes their clinical use in the management of hypertriglyceridemias. 

The regulation of the overall biosynthesis of bile acids has been studied intensively during the last decade, and only a small fraction of all the pubhcations can be reviewed here. Cholesterol 7a-hydroxylase is the rate-limiting enzyme in the biosynthesis of both chohc acid and chenodeoxycholic acid. The publications in which a correlation has been demonstrated between bile acid biosynthesis and 7a-hydroxyl-ation of cholesterol have been reviewed by Myant and Mitropoulos [59]. In the present review, emphasis will be put on the feedback regulation of the cholesterol 7a-hydroxylase by the bile-acid flux through the hver, the relation between HMG-CoA reductase and cholesterol 7 -hydroxylase and possible mechanisms for the regulation. 

The bile acids returning to the liver in the portal blood inhibit both cholesterol 7a-hydroxylase and HMG-CoA reductase. Size, circulation rate and composition of the bile acid pool are of importance. The inhibitory effect of bile acids on cholesterol 7a-hydroxylase is mediated by an effect on synthesis or breakdown of proteins, most likely the specific species of cytochrome P-450 involved in the hydroxylation. Evidence that the rate of synthesis of the specific species of cytochrome P-450 is of major importance can be obtained only when it is possible to measure accurately the amount of specific cytochrome P-450 by means other than enzyme activity. Although less likely from the data available, it cannot be excluded at the present state of knowledge that the inhibitory effect of bile acids on cholesterol 7a-hydroxylase is mediated by the effect of bile acids on HMG-CoA reductase. Since the substrate pool for cholesterol 7 -hydroxylase does not seem to be affected [59,222], a hitherto unknown mechanism must then be responsible for the coupling between the two rate-limiting enzymes. 

Oxysterols have diverse roles in cholesterol efflux, a critical topic in foam cell biology. On the one hand, cells incubated with 7-ketocholesterol and 25-hydroxycholesterol have decreased cholesterol efflux. Possible mechanisms include inhibition of membrane desorption of cholesterol or phospholipids or, as mentioned above, inhibition of lysosomal sphingomyelinase leading to lysosomal sequestration of cholesterol (M. Aviram, 1995). On the other hand, the conversion of cholesterol by macrophage sterol 27-hydroxylase to 27-hydroxycholesterol and 3[l-hydroxy-5-cholestenoic acid, which are efficiently effluxed from cells, has been proposed to promote sterol efflux from foam cells (1. Bjorkhem,... 

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