21 a-Hydroxylase

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.

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). 

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. 

Barnes, H.J., Arlotto, M.P. and Waterman, M.R. (1991) Expression and enzymatic activity of recombinant cytochrome P450 1 7-a-hydroxylase in Escherichia coli. Proc. Natl. Acad. Sci. USA, 88, 5597-601. 

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. 

Specific defects in bile acid synthesis Cerebrotendinous xanthomatosis Intrahepatic cholestasis familial neonatal hepatitis) 3-p-hydroxysteroid dehydrogenase/isomerase deficiency A -3-oxosteroid 5-P-reductase deficiency C24 steroid 7-a-hydroxylase deficiency Peroxisomal disorders... 

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. 

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 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. 

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). 

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. 

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. 

In studies conducted in our laboratory, we have observed that in microsomes obtained from rats given mevalonolactone, the 7o -hydroxylase activity increases relative to that in the control animals given only saline, consistent with our proposal. This effect of mevalonolactone on 7 a-hydroxylase activity is manifest 20 minutes following mevalonolactone. But there is no further increase in enzyme activity if it is measured 60 minutes after mevcilonolactone (Fig. 10). 

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