Formation of Cholic Acid

Kase, F., Bjorkhem, I., and Pedersen, J. I., Formation of cholic acid from 3a,7a,12a-trihydroxy-5P-cholestan-26-oic acid by rat liver peroxisomes. J. lipid Res. 24,1560-1567 (1983). 

Conjugation of the carboxylic group in cholic acid involves participation of a cholic acid CoA ligase and a cholyl-CoA glycine (taurine) acyltransferase [153-158]. Since the primary product of the thiolytic cleavage of the CoA ester of 3a,7a,12a-tri-hydroxy-24-oxo-5/8-cholestanoic acid is the CoA ester of cholic acid, it is probable that only the transferase may be involved in the primary formation of cholic acid in the liver. A description of the properties of the CoA synthetase and the transferase is given in Chapter 11. 

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. 

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-... 

Two recent investigations on the formation of cholic acid in rats with a biliary fistula indicate the presence of additional pathways for the formation of cholic acid involving other sequences of changes in the steroid nucleus than those discussed above. Nair et al. (63) have found that in rats with a biliary fistula labeled 5,7-cholestadien-3j -ol (7-dehydrocholesterol) is converted into cholic acid apparently without the intermediary formation of cholesterol, since hepatic as well as biliary cholesterol was unlabeled. No information is available concerning intermediates in this pathway for cholic acid formation, and 5j5-cholestane-3a,7a,12a-triol may or may not be an intermediate. It can, however, be concluded that 12a-hydroxylation must be an early step, since 5,7-cholestadien-3/5-ol did not give rise to chenodeoxy-cholic acid. The contribution to cholic acid formation of a pathway from 5,7-cholestadien-3j -ol is unknown. It should be pointed out that Nair et al. 

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... 

The synthesis of cholic acid from cholesterol in rodents proceeds via 7a-OH-cholesterol, leading to a trihydroxy derivative which finally loses a side chain, resulting in the formation of cholic acid (1). The synthesis appears to proceed similarly in the human liver (12). Trihydroxycoprostanic acid, which is a precursor of cholic acid and which is formed from cholesterol... 

It seems to be a general observation that the proportion of chenodeoxy-cholic acid is increased in liver cirrhosis. Thus the ratio cholic acid/cheno-deoxycholic acid has been found to be decreased in the bile (23), serum (52,134,193,195-198), and urine (88,199) of cirrhotic subjects. Since the ratios of cholic acid, chenodeoxycholic acid, and deoxycholic acid appear to be approximately the same in bile and serum (200,201), and perhaps also in urine, it seems quite obvious that the bile acid pattern in any of these three sources is similar to that produced by the liver. Simultaneous determinations of bile acids from bile, serum, and urine have not been made, however. The relative increase of chenodeoxycholic acid has been interpreted to indicate a hindrance of 12a-hydroxylation in liver injury when the formation of cholic acid is decreased in favor of chenodeoxycholic acid (202). This, on the other hand, changes the pattern of secondary bile acids so that relatively more lithocholic acid is formed in the colon (191,200,202), the amount of deoxycholic acid being reduced (23,52,134,193,195-198), particularly because quantitatively only a small portion of the bile acids escapes daily from the ileum to the colon (23). 

Early in vitro studies showed that mitochondria from livers of hyperthyroid rats did not oxidize cholesterol-26- C to C02 at a faster rate than similar preparations from normal animals (12). A more recent study (13) led to the conclusion that the effects of thyroid hormones on bile acid metabolism must take place at a biosynthetic step preceding side-chain oxidation, perhaps involving hydroxylation of the steroid nucleus. However, it must be realized that the normal substrate for side-chain oxidation leading to the formation of cholic acid from cholesterol is not cholesterol itself but presumably 3a,7afl2a-trihydroxy-5/5-cholestane (14,15), and the substrate for the side-chain oxidation leading to chenodeoxycholate is, presumably, 3a,7a-dihydroxy-5/5-cholestane (16). Thus results of in vitro experiments in which cholesterol is employed as the substrate must be interpreted with caution, since mitochondria do not have the enzyme system required for formation of the triol and diol from cholesterol. 

Formation of the 3j8-acetoxyeti-5-enic esters has been used to obtain optically pure samples of (+)- and (—)-tran5-verbenoP and to resolve an alcohol intermediate in the synthesis of the witchweed seed germination stimulant (+)-strigol. A general synthesis of thiol esters from carboxylic acids, exemplified by the formation of the n-propylthio-, isopropylthio-, and t-butylthio-esters of cholic acid, comprises reaction with diethyl chlorophosphate-triethylamine, followed by the thallium(i) salt of the appropriate thiol. 

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. 

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... 

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. 

Chimaerol, S S-bufol, 5j8-cyprinol, and scymnol are the 24-, 25-, and 27-hy-droxylated, and 24,27-dihydroxylated derivatives of 27-deoxy-5j8-cyprinol (IX), respectively. It is possible that these naturally occurring bile alcohols could be intermediates in alternative pathways for the formation of choUc acid (XIV) from 27-deoxy-5)8-cyprinol (IX). To test this possibility, these cholestanepolyols were labeled with tritium and given to guinea pigs or rats with a biliary fistula [133-136]. Of the tested bile alcohols, 5)3-chimaerol and 5 -cyprinol were converted efficiently to cholic acid [135,136]. However, these results do not provide conclusive evidence for alternative pathways of cholic acid formation since the conversion of these bile alcohols to cholic acid may merely reflect a lack of specificity of the enzyme systems involved in the conversion of 27-deoxy-5/8-cyprinol (IX) to cholic acid (XIV) via trihydroxy-5)3-cholestanoic acid (XII). 

The continuing work on the modification of cholic acid in an approach to quassinoids has resulted in the formation of 5-lactones related to (103) and in several publications. — ... 

Funasaki, N., Hada, S., Neya, S. (1990). Micelle formation of a sulfobetaine derivative of cholic acid, Chem. Lett., p. 1075. 

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. 

The conjugation of bile acids with added hydroxylamine by rat liver microsomes in the presence of ATP, CoA, and fluoride to yield bile acid hydroxamates has been used to demonstrate the formation of cholyl-CoA as an intermediate in the enzymatic system catalyzing the synthesis of the natural conjugates (12,13). In pig liver, there appear to be two distinctly different enzyme systems capable of the synthesis of cholylhydroxamic acid (15). A partially solubilized preparation from pig liver catalyzed the rapid formation of cholylhydroxamic acid in the presence of cholic acid and hydroxylamine but in the absence of ATP and CoA. This is in contrast to the synthesis of hydroxamates of bile acids by the microsomes, which required both ATP and CoA. Since fluoride inhibits the system that functions in the absence of ATP and CoA, the reaction appears to be similar to that of a lipase (15)... 

The formation of the other major primary bile acid, chenodeoxycholic acid, from cholesterol is similar to that of cholic acid, except that no 12-hydroxylation takes place. 

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