Biosynthesis of bile acids

Cholesterol is an essential substance manufactured by most cells in the body. Cholesterol is used to maintain cell wall integrity and for the biosynthesis of bile acids and steroid hormones.4... 

Oxysterols are defined as oxygenated derivatives of cholest-5-en-3(3-ol (cholesterol) (Figure 18.1) or precursors of CHOL that may be formed directly by autoxidation or by the action of a specific monooxygenase, or that may be secondary to enzymatic or nonenzymatic lipid peroxidation (Guardiola et al., 1996 Schroepfer, 2000 Bjorkhem and Diczfalusy, 2002). These OS may be formed in the human body by endogenous free-radical attack on CHOL or by enzymatic processes, mainly in the biosynthesis of bile acids and steroid hormones. In addition, OS may be formed exogenously by autoxidation of CHOL in foods. The nomenclature and abbreviations of OS are presented in Table 18.1. It should be emphasized at this point that the OS that occur in... 

Cholesterol 7a-hydroxylase, a cytochrome P450-dependent enzyme, catalyzes the first and rate-limiting step in the biosynthesis of bile acids from cholesterol. Potential mechanisms for regulation of the enzyme have been extensively studied. 

Regulation of the biosynthesis of bile acids is predominantly aehieved through feedback by the respeetive daily loss quota. Further regulations are effeeted by HMG-CoA reduetase and 7a-hydroxylase, whieh are themselves ehiefiy adjusted by ursodeoxyeholie aeid. (s. fig 3.3) With advaneing age, bile aeid synthesis in the liver decreases and excretion of eholesterol in the bile increases. The terms bile acids and bile salts are interchangeable. 

Salen, G., Shefer, S., Chen, F.W., Dayal, B., Batta, A. K. and Tint, G. S. (1985). Biosynthesis of bile acids in cerebrotendinous xanthomatosis. Relationship of bile acid pool sizes and synthesis rates to hydroxylation at C-12, C-25, and C-26. J. Clin Invest. 76 744-751. 

Improved methodology for the rapid assay of hepatic HMG-CoA reductase has been described and new and simplified assays are available for cholesterol 7a-hydroxylase, 4-methylsterol oxidase, 3/3-hydroxy-steroid dehydrogen-ase, the biosynthesis of bile acids, and microsomal cholesterol levels.The rate of biosynthesis of gibberellins has been monitored by a bioassay based on /3-amyrin production of Amaranthus seeds. 

Peroxisomes contain dihydroxyacetone phosphate acyl-transferase and alkyldihydroxyacetone phosphate synthase, which are involved in synthesis of the plasmalogens (Chapter 19). Peroxisomes may also participate in the biosynthesis of bile acids. The conversion of trihydrox-ycholestanoic acid to cholic acid (Chapter 19) has been localized to peroxisomes. 

Two pathways have been proposed for degradation of the cholestane side chain in the biosynthesis of bile acids. These differ in the site proposed for the first hydroxylation step in side-chain oxidation and are discussed below for the formation of cholic add. 

Cholic acid differs from chenodeoxycholic acid in having an extra hydroxyl group at C-12. The enzyme responsible for producing this difference, 7a-hydroxy-4-cholesten-3-one 12a-hydroxylase, thus acts at a key branch point in the biosynthesis of bile acids and might be expected to be regulated in order to control the relative amounts of cholic acid and chenodeoxycholic acid produced. Like other hydroxylation steps in bile acid biosynthesis, 12a-hydroxylation requires a specific form of cytochrome P-450, which is present in the cytochrome P-45OLM4 fraction of rabbit liver microsomes (H6). The activity of I2a-hydroxylase has been postulated to be decreased in patients with liver cirrhosis to explain the low proportion of cholic add relative to chenodeoxycholic add in the bile of these patients (V9). Conversely, the activity of this enzyme may be high in patients with cerebrotendinous xanthomatosis, as the bile of these individuals contains mostly cholic acid... 

Swell, L., Biosynthesis of bile acids in man. Multiple pathways to cholic acid and che-nodeoxycholic acid. /. Biol. Chem. 255, 2925-2933 (1980). 

Figure 8.8. Biosynthesis of bile acids and the enterohepatic circulation. Bile acids are synthesized from cholesterol in the liver under feedback regulation of the nuclear orphan receptors famesoid X receptor (FXR) and lignane X receptor (LXR). They are stored in the gallbladder and released through the bile duct into the duodenum, where they aid in the digestion of dietary fats. Intestinal uptake of bile acids takes place alongthe entire length of the small intestine, but active reabsorption is confined to the distal ileum to minimize loss of bile salts in the feces. The portal circulation carries bile acids from the intestine to the liver, where they are actively absorbed by hepatoc5Tesand secreted into bile.

II) Formulation of the sequence of reactions in the biosynthesis of bile acids... 

If the cholesterol 7a-hydroxylating system is of regulatory importance, a short half-life of the enzyme can be expected. Already in 1968 Einarsson and Johansson obtained evidence that this is the case [87]. It was calculated that the half-life for the breakdown of the 7a-hydroxylase was only 2-3 h. Other enzyme activities involved in the biosynthesis of bile acids from cholesterol, such as the 12a-hydroxylase, had a considerably longer half-life time. It seems likely that a cytochrome P-450 species with short half-life time is the component of the cholesterol 7a-hydroxylase. This... 

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 possibility that the biosynthesis of bile acids is regulated by a negative feedback mechanism was supported by early experiments by Thompson and Vars [206] and Eriksson [207], who showed that the rate of bile acid synthesis in rats increased about 10-fold when a bile fistula is made. Bergstrom and Danielsson demonstrated that duodenal infusion of taurochenodeoxycholic acid in bile fistula rats restored the increased synthesis to a normal rate [208]. Danielsson et al. [44] showed that the cholesterol 7a-hydroxylase activity increased in parallel with the bile acid synthesis after cannulation of the bile duct in rats. In a subsequent work by Mosbach et al., it was reported that the incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol into bile acids was inhibited by duodenal infusion of taurocholate to bile fistula rats [209]. The incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol was stimulated in perfused livers of cholestyramine-treated rabbits [210]. It was concluded that there are essentially no rate-limiting steps beyond 7a-hydroxycholesterol in the biosynthesis of bile acids from acetate. Since both cholesterol and bile acid biosynthesis was subjected to negative feedback inhibition by bile acids, it cannot be excluded that inhibition of cholesterol biosynthesis precedes inhibition of the bile acid biosynthesis, and that the latter inhibition is secondary to the former. 

Biochemical studies with isolated rat hepatocytes have largely been concerned with transport mechanisms [15], secretion of bile acids [17-19], or biosynthesis of bile acids [20]. The capacity of cultured hepatocytes to convert tauro- or glyco-chenodeoxycholate to a- and )8-muricholates [19,21] and to produce bile salts (glycine or taurine conjugates) during the dark phase of the diurnal cycle [21] has been established. Demonstrations of other metabolic transformations by hepatocytes are included in the following sections. 

Bremer and Gloor [40] concluded that enzymes for both reactions were present in hepatic microsomes, but recent studies with microsomes of rat [40-42] and human liver [43] have confirmed the presence of only one enzyme, CoA ligase. The assay system, essentially that for long-chain acyl-CoA ligase [42,43], includes 50 mM NaF, a phosphate buffer (pH 7.5), the enzyme preparation, and constituents of Eqn. 1. Product formation was linear up to 12 min with added protein (between 0.1 and 1.2 mg) from a crude microsomal fraction. Sterol carrier protein [44], cysteine or nicotinamide [38,40] were without effect. This rate-limiting enzyme in the two-step sequence catalyzing conjugation of bile acids exhibits a diurnal variation such that the time of maximum enzyme activity coincides with predicted maximum activity of cholesterol 7a-hydroxylase and the time of maximal biosynthesis of bile acids [45]. The enzyme has not been purified. 

Another demonstrable effect of thyroid hormones is their stimulation of the biosynthesis of bile acids from hepatic cholesterol. Hypercholesterolemia is known to accompany... 

Figure 19.23 Biosynthesis of bile acids and salts from cholesterol. 

Hansson, R. and K. Wikvall (1982). Hydroxylations in biosynthesis of bile acids Cytochrome P-450 LM4 and 12a-hydroxylation of 53-cholestane-3oi,7a-diol. Ear. J. Biochem. 125, 423-429. 

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