Sterol 27-Hydroxylase

The existence of an alternate pathway for the synthesis of bile acids was suspected because it was possible for oxysterols to be converted into bile acids (N. Wachtel, 1968). It is now recognized that a variety of oxysterols produced by an assortment of cell types can be converted into bile acids. The production of these oxysterols is catalyzed by several sterol hydroxylases sterol 27-hydroxylase (CYP27A1) (J.J. Cali, 1991), cholesterol 25-hydroxylase (CH25H) (E.G. Lund, 1998), and cholesterol 24-hydroxylase (CYP46A1) (E.G. Lund, 1999). Cholesterol 25-hydroxylase is not a cytochrome P-450 monooxygenase, unlike the two other enzymes. Almost all of the 24-hydroxycholesterol that ends up in the liver originates from the brain, and it has been suggested that the production of... 

CYP27A1 catalyzes the side chain oxidation (27-hydroxylation) in bile acid biosynthesis. Because bile acid synthesis is the only elimination pathway for cholesterol, mutations in the CYP27A1 gene lead to abnormal deposition of cholesterol and cholestanol in various tissues. This sterol storage disorder is known as cerebrotendinous xanthomatosis. CYP27B1 is the 1-alpha hydroxylase of vitamin D3 that converts it to the active vitamin form. The function of CYP27C1 is not yet known. 

Figure 26-7. Biosynthesis and degradation of bile acids. A second pathway in mitochondria involves hy-droxylation of cholesterol by sterol 27-hydroxylase. Asterisk Catalyzed by microbial enzymes.

Hepatic CYP39A1 mRNA not induced by cholesterol or bile acids, unlike other sterol 7alpha-hydroxylases protein found in hepatic microsomal fraction (Li-Hawkins et al, 2000). 

Andersson U, Eggertsen G, Bjorkhem I. 1998. Rabbit liver contains one major sterol 12alpha-hydroxylase with broad substrate specificity. Biochim Biophys Acta 1389 150-154. 

Cali JJ, Hsieh CL, Francke U, Russell DW. 1991. Mutations in the bile acid biosynthetic enzyme sterol 27-hydroxylase underlie cerebrotendinous xanthomatosis. J Biol Chem 266 7779-7783. 

Eggertsen G, Olin M, Andersson U, Ishida H, Kubota S, et al. 1996. Molecular cloning and expression of rabbit sterol 12alpha-hydroxylase. J Biol Chem 271 32269-32275. 

Gafs els M, Olin M, Chowdhary BP, Raudsepp T, Andersson U. et al. 1999. Structure and chromosomal assignment of the sterol 12alpha-hydroxylase gene (CYPSBl) in human and mouse eukaryotic cytochrome P-450 gene devoid of introns. Genomics 56 184-196. 

Ishida H, Kuruta Y, Gotoh O, Yamashita C, Yoshida Y, et al. 1999. Structure, evolution, and liver-specific expression of sterol 12alpha-hydroxylase P450 (CYP8B). J Biochem (Tokyo) 126 19-25. 

Sugama S, Kimura A, Chen W, Kubota S, Sepma Y, et al. 2001. Frontal lobe dementia with abnormal cholesterol metabolism and heterozygous mutation in sterol 27-hydroxylase gene CYP27). J Inherit Metab Dis 24 379-392. 

Wakamatsu N, Hayashi M, Kawai H, Kondo H, Gotoda Y, et al. 1999. Mutations producing premature termination of translation and an amino acid substitution in the sterol 27-hydroxylase gene cause cerebrotendinous xanthomatosis associated with parkinsonism. J Neurol Neurosurg Psychiatry 67 195-198. 

The primary supply of vitamin in humans is not obtained from the diet but rather is derived from the ultraviolet photoconversion of 7-dehydrocholesterol to vitamin Ds in skin. Thus, vitamin Dj synthesis varies with the seasons. D3 is a prohormone and requires further metabolic conversion to exert biological activity in its target organs (Fig. 66.2). The liver and the kidney are the major sites of metabolic activation of this endogenous sterol hormone. The initial transformation of D3 occurs in the liver and is catalyzed by the enzyme 25-OH-D3-hydroxylase... 

In addition to the endogenous metabolites, some exogenous sterols possess biological activity similar to that of D3. Ergocalciferol (vitamin D2) is derived from the plant sterol ergosterol and may act as a substrate for both the 25-hydroxylase and the 1-hydroxylase enzyme systems of the liver and kidney to form 25-(OH)D2 and 1,25-(0H)2 D2, respectively. Ergocalciferol (vitamin D2) is the form used in commercial vitamins and supplemented dairy products. Dihydrotachysterol, another sterol that is used as a therapeutic agent, also functions as a substrate for the hydroxylase enzymes in the liver and kidney. 

The conversion of cholesterol to bile acids is quantitatively the most important mechanism for degradation of cholesterol. In a normal human adult approximately 0.5 g of cholesterol is converted to bile acids each day. The regulation of this process operates at the initial biosynthetic step catalyzed by an enzyme in the endoplasmic reticulum, la-hydroxylase (fig. 20.18). The 7a-hydroxylase is one of a group of enzymes called mixed-function oxidases, which are involved in the hydroxylation of the sterol molecule at numerous specific sites. A mixed-function oxidase is an enzyme complex that catalyzes hydroxylation of a substrate with a concomitant production of H20 from a single molecule of 02- The 7a-hydroxylase is one of several enzymes referred to as cytochrome P450. 

Babiker A, Andersson O, Lund E, et al. Elimination of cholesterol in macrophages and endothelial cells by the sterol 27-hydroxylase mechanism. Comparison with high-density lipoprotein-mediated reverse cholesterol transport. J Biol Chem 1997 272 26253-26261. 

Bjorkhem, I., Andersson, O., Diczfalusy, U., Sevastik, B., Xiu, R.-J., Duan, C., Lund, E.G. 1994. Atherosclerosis and sterol 27-hydroxylase evidence for a role of this enzyme in elimination of cholesterol from human macrophages. Proc. Natl. Acad Sci. USA 91, 8592-8596. 

While these studies were in progress, Reshef etal. reported the analysis of die sterol 27-hydroxylase gene mutant allele in a Jewish family of Algerian origin (64). They further stated that the CTX disorder which is characterized by extensive nervous system involvement, juvenile cataracts, tendon xanthomatosis and premature atheroselerosis was caused by the sterol 27-... 

Leitersdorf, E., Reshef, A., Meiner, V., Levitzki, R., Schwartz, S. P., Dann, E. J., Berkman, N., Cali, J. J., Klapholz, L. and Berginer, V. M. (1993). Frameshift and splice-j-junction mutations in the sterol 27-hydroxylase gene cause cerebrotendinous xanthomathosis in Jews of Moroccan origin., /. Clin. Invest. 91 2488-2496. 

Cali, J. J., Russell, D. W. (1991). Characterization of the human sterol 27-hydroxylase a mitochondrial P-4S0 that catalyzes mutiple oxidation reactions in bile acid biosyntheis. J. Biol. Chem. 266 lllA-111%. 

Patients with familial hypercholesterolaemia exhibit lower levels of plasma cholesterol after an operation for portacaval anastomosis, and it has now been shown in rats that such an operation causes an increase in HMG-CoA reductase and cholesterol 7a -hydroxylase activities. Many transplantable human and rodent hepatomas do not control the rate of sterol biosynthesis and HMG-CoA reductase levels in response to dietary cholesterol as normal liver cells do. However, certain hepatoma cells have now been found that, although lacking feedback regulation of choles-terologenesis in vivo, retain their regulatory ability in vitro It thus appears that malignant transformation is not necessarily linked to the loss of regulation by the cell of HMG-CoA reductase activity or sterol synthesis. 

For obvious reasons, inhibitors of cholesterol biosynthesis have aroused intense interest. Studies on a variety of species (mostly rats) have been made using 1-alkylimidazoles," colchicine," and chenodeoxycholic acid, the last work being particularly interesting as the metabolite affected HMG-CoA reductase but not cholesterol 7a-hydroxylase, the steps believed to be rate-limiting for the biosynthesis of sterols and of bile acids respectively. Arsenite, /8-mercaptoethanol, dithiothreitol, and ethanethiol all inhibited the biosynthesis of cholesterol from MVA in rat liver homogenates. The accumulation of 4,4-dimethyl-5a-cholest-8-en-3/3-ol together with the corresponding A -diene supported the view that... 

Inhibition of cytochrome P450 mediated sterol hydroxylase is one of the most common mechanisms of antifungal compounds. However, within vertebrates inhibition as well as induction of CYP has been observed. Some compounds are converted to nonspecific CYP inhibitors (for review see ref. 39). The imidazole derivative clotrimazole was shown to inhibit benzo(a)pyrene elimination in shad (Dorosoma cepedianum) presumably through a noncompetitive inhibition of CYP1A which was observed in vitro within shad and trout liver microsomes52. In contrast,... 

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