Chenodeoxycholic acid formation

The conversion of cholesterol into chenodeoxycholic acid was not studied in the same detail as the conversion of cholesterol into cholic acid. It was found that many different C2 -steroids were converted into chenodeoxycholic acid in rats with a biliary fistula. It was not possible to deduce a pathway for chenodeoxycholic acid formation that could include all these C27-steroids as intermediates. 

With respect to the oxidation of the side chain in chenodeoxycholic acid formation, it may be inferred from the early studies with mitochondrial preparations that it involves an co-oxidation followed by a / -oxidation (cf. Section IIB). More direct evidence has been presented by Dean and White-house (87,91), who showed that mitochondrial preparations from rat liver catalyze the oxidation of 5-cholestene-3/ ,26-diol into 3/ -hydroxy-5-choles-tenoic acid and the formation of propionic acid from 3/5-hydroxy-5-choles-tenoic acid. Mitropoulos and Myant (97) have shown that mitochondrial preparations from rat liver catalyze the conversion of cholesterol into 5-cholestene-3/ ,26-diol, 3/ -hydroxy-5-cholestenoic acid, 3/5-hydroxy-5-chole-noic acid, lithocholic acid, and chenodeoxycholic acid (Fig. 5). Additional evidence for a pathway to chenodeoxycholic acid involving the successive, intermediary formation of above-mentioned compounds is provided by the finding that 3/ -hydroxy-5-cholenoic acid is converted into lithocholic acid and chenodeoxycholic acid by mitochondrial preparations (98). 

The importance, quantitatively, of the different pathways for chenodeoxycholic acid formation has not been fully established. Several lines of evi-... 

Patients also develop cholesterol gallstones from a defect in bile acid synthesis. The defect is in the mitochondrial C27-steroid 27-hydroxylase. In these patients, the reduced formation of normal bile acids, particularly chenodeoxycholic acid, leads to the up-regulation of the rate limiting enzyme Tct-hydroxylase of the bile acid synthetic pathway (discussed later). This leads to accumulation of 7a-hydroxylated bile acid intermediates that are not normally utilized. 

Formation, enterohepatic circulation, and disposition of the bile acids. CDCA = Chenodeoxycholic acid. 

When chenodeoxycholic acid therapy was first introduced, there was some anxiety that this bile acid, or its bacterial metabolite lithcholic acid, might cause liver damage in man. This possible complication has not eventuated. Lithocholic acid is toxic to the liver in many animal species but in man, it is converted to sulfolithocholate and excreted (A5). Nevertheless, up to one-third of patients undergoing chenodeoxycholic acid treatment do show transient rises in serum levels of aspartate aminotransferase activity. The mechanism of this hypertransaminasemia is obscure, although it could possibly be related to lithocholate formation (D8). In any case, hepatotoxicity very rarely occurs at a clinically significant level (SI4). 

F7. Fromm, H., Sarva, R. P., and Bazzoli, F., Formation of ursodeoxycholic acid from chenodeoxycholic acid in the human colon studies of the role of 7-ketolithocholic acid as an intermediate. /. Lipid Res. 24, 841-853 (1983). 

From the above investigations, summarized in Fig. 2, it was concluded that 7a-hydroxylation of cholesterol may be the first step in the conversion of cholesterol into bile acids, and that 5/S-cholestane-3a,7a,12a-triol probably is an intermediate in cholic acid formation. Since 5)S-cholestane-3a,7a-diol was rapidly converted into chenodeoxycholic acid and only to a small part into cholic acid [19], it was concluded that 5i8-cholestane-3a,7a-diol is a corresponding intermediate in the formation of chenodeoxychohc acid. Samuelsson showed that the conversion of cholesterol into bile acids most probably involves a ketonic intermediate, since [3a- H]cholesterol lost its tritium when converted into chohc acid [1,20]. Since... 

In a study by Ali and Elliott it was shown that 5a-cholestane-3 ,7a-diol was an even better substrate for the 12a-hydroxylase in rabbit liver microsomes than 7a-hydroxy-4-cholesten-3-one (156%) [104]. This reaction is probably of importance in the formation of allocholic add. The high specificity of the 12 -hydroxylase towards the coplanar 5a-sterol nucleus is also evident from the finding that allochenodeoxycholic acid can be converted into allocholic acid in rats, both in vivo and in vitro [105,106, Chapter 11]. Based on the known structural requirements of the 12a-hydroxylase, Shaw and Elliott prepared competitive inhibitors with different substitutions in the C,2 position [107]. The best inhibitor of those tested was found to be 5a-cholest-ll-ene-3a,7 ,26-triol. Theoretically, such inhibitors may be used to increase the endogenous formation of chenodeoxycholic acid in connection with dissolution of gallstones. 

From in vitro experiments it may be concluded that there are several possible pathways for formation of chenodeoxycholic acid in rat liver. A number of different 26-hydroxylated steroids are converted into chenodeoxycholic acid in bile fistula... 

Yamasaki et al. have studied a pathway for formation of chenodeoxycholic acid in the rat involving intermediate formation of 7a-hydroxycholesterol, 3 -hydroxy-5-cholenoic acid and 7a-hydroxy-3-oxo-4-cholenoic acid [176-179]. In this pathway, changes in the side chain occur after the rate-limiting step. From the data available, it is not possible to evaluate the quantitative importance of this pathway. 

Oftebro et al. reported that the mitochondrial fraction of a liver homogenate from a biopsy of a CTX patient was completely devoid of 26-hydroxylase activity [193]. The possibility that there had been a general inactivation of the mitochondrial fraction seems excluded since there was a significant 25-hydroxylase activity towards vitamin D,. There was a substantial accumulation of 5 -cholestane-3a,7a,12a-triol, the immediate substrate for the 26-hydroxylase in cholic acid biosynthesis. It was suggested that the accumulation of 5)3-cholestane-3a,7a,12a-triol would lead to increased exposure to the action of the microsomal 23-, 24- and 25-hydroxylases. The alternative 25-hydroxylase pathway would then be of importance for the formation of cholic acid in patients with CTX (Fig. 13). If the 25-hydroxylase pathway has an insufficient capacity, this would explain the accumulation of the different 25-hydroxylated intermediates in patients with CTX. A lack of the mitochondrial 26-hydroxylase would also lead to accumulation of intermediates in chenodeoxycholic acid biosynthesis such as 5)8-cholestane-3a,7a-diol and 7a-hy-droxy-4-cholesten-3-one. Such accumulation would lead to increased exposure to the microsomal 12a-hydroxylase which would yield a relatively higher biosynthesis of cholic acid. This would explain the marked reduction in the biosynthesis of chenodeoxycholic acid in patients with CTX. 

Microsomal 12a-hydroxylation is the only unique step in the formation of chohc acid and is likely to be of regulatory importance for the ratio between newly synthesized cholic and chenodeoxycholic acid. Introduction of a 26-hydroxyl group seems to prevent subsequent 12a-hydroxylation in rat liver and the 26-hydroxylase could thus also have a regulatory role. It is possible that there are different precursor pools for the synthesis of cholic acid and chenodeoxycholic acid in rats. If so, the relative size of the two pools could be of importance for the relative rate of formation of the two bile acids. 

Mitropoulos et al. have measured the rate of excretion and the specific activities of cholic acid and chenodeoxycholic acid in bile fistula rats fed [ H]cholesterol and infused with [ " C]mevalonate or [ C]7a-hydroxycholesterol [255]. It was concluded that newly synthesized hepatic cholesterol was the preferred substrate for the formation of cholic acid. It could not be excluded, however, that part of the chenodeoxycholic acid had been formed from a pool of cholesterol different from that utilized in cholic acid biosynthesis. The mitochondrial pathway, starting with a 26-hydroxylation, could have accounted for a significant fraction of the chenodeo-... 

The major pathway for the formation of chenodeoxycholic add is thought to be the same as that for cholic acid, with the exception that no 12a-hydroxylation occurs. Thus, 3a,7a-dihydroxy-5i8-cholestan-26-oic acid is a probable intermediate, and Hanson has shown that this acid can be made from cholesterol and is effidently converted to chenodeoxycholic acid but only to a very limited extent to cholic acid [131]. 

Yamasaki and his co-workers have proposed an alternative pathway to chenodeoxycholic acid via the side chain degradation of cholest-5-ene-3/S,7a-diol to form 3jS,7a-dihydroxychol-5-en-24-oic acid [119]. The formation of chenodeoxycholic acid from 3jS,7a-dihydroxychol-5-en-24-oic acid was demonstrated in rats [146] and hamsters [147]. 

Biosynthesis The primary B. are synthesized in the liver from cholesterol by a complicated, multi-step reaction sequence. The 7o-hydroxy group is introduced first while the 12-hydroxy group is added later to a further intermediate with subsequent formation of both chenodeoxycholic acid and cholic acid. Deoxycholic acid is not synthesized in the liver but rather in the intestines by 7a-dehydroxylation of cholic acid by intestinal bacteria. 

Use A low conversion rate of cholesterol to B. in the liver leads to an oversaturation of bile with cholesterol and can result in the formation of cholesterol gallstones. Oral administration of a large amount of chenodeoxycholic acid can dissolve small gallstones. However, the 7a-dehydroxylation of the administered exogenous chenodeoxycholic acid by intestinal bacteria can result in the formation of high concentrations of toxic lithocholic acid (risk of development of liver and bile cirrhosis). 

The structural changes involved in the conversion of cholesterol into chenodeoxycholic acid are the same as those in the formation of cholic acid with the exception that no 12a-hydroxyl group is introduced. It has been shown that the mechanisms of conversion of the zl -3i5-hydroxy configuration of cholesterol into the 3a,7a-dihydroxy-5/5 configuration of chenodeoxycholic acid are the same as those in the formation of cholic acid. Similarly, the mechanisms of oxidation of the side chain are the same for chenodeoxycholic acid and cholic acid. Whereas it is now possible to formulate a few probable sequences for these events in cholic acid formation, available information... 

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