Cholic acid biosynthesis

CYP8A1 is the complementary enzyme to CYP5 in that it synthesizes prostacyclin in the arachidonic acid cascade. CYP8B1 catalyzes the steroid 12-alpha hydroxylation in the cholic acid biosynthesis. 

Fig. 10 (b). The sequence leading to the oxidation and cleavage of the side chain of 5P-cholestane-3a,7a,12a-triol pathway for side-chain cleavage in cholic acid biosynthesis. 

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-... 

Hoshita et al. have shown that liver microsomes from the green iguana, in which the major biliary bile salt is tauroallocholate, convert 7a,12a-dihydroxycholest-4-en-3-one (XVII) into 5a-cholestane-3a,7a,12a-triol (XVIII) rather than into 5)8-choles-tane-3 ,7a,12a-triol (VIII) which is involved in cholic acid biosynthesis [164]. On the basis of this result and that obtained from studies with carp liver [151], it can be assumed that 5a-bile acids and alcohols are formed from cholesterol by a modification of the biosynthetic pathway to the corresponding 5y8 isomers in which the only difference is the stereospedfic saturation of the A double bond of the intermediate XVII. 

Fig. 6. The stereochemical course of the reduction of the d bond during cholic acid biosynthesis.

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. 

Ox bile, which contains cholic acid as its principal constituent, provided one of the earliest mammalian sources of steroid raw materials for the commercial manufacture of the androgens. In nature, cholesterol itself is the mammalian precursor of the androgens, the biosynthesis passing through progesterone (XII). 

Our study on the distribution of electron transferring proteins in animal sources is still in progress. From present knowledge, adrenodoxin can be found in adrenal cortexes from pig, beef, and rat. Further, a similar protein was isolated from pig testis (see II-A-2), and it was also found in the ovary. However, brain, heart, liver, kidney, and pancreas appear to lack adrenodoxin-like protein. If this is correct, the proteins of the ferredoxin family are located solely in the glands which directly act in the biosynthesis of steroid hormones. It is of interest that adrenodoxin-like protein does not participate in the steroid hydroxylation involved in cholesterol and cholic acid biosyntheses. All of these reactions without the participation of adrenodoxin are similar to enzymes responsible for microsomal non-specific hydroxylation, which consist of the following sequence of electron transfer ... 

Fig. 9. The elaboration of the cholesteol nucleus in bile acid synthesis. (Cholic acid and chenodeoxycholic acid biosynthesis pathway).

Oftebro, H., Bjorkhem, I., Skrede, S., Screiner, A. and Pedersen, J.I. (1980). Cerebrotendinous xanthomatosis a defect in mitochondrial 26-hydroxy lation required for normal biosynthesis of cholic acid. J. Clin. Invest.65 1418-1430. 

Recent investigations concerning the biosynthesis of Salamandra alkaloids 17) involving in vitro and in vivo experiments showed that these alkaloids are formed like other steroids from acetate via cholesterol. The alkaloids substituted at C-17 (samandaridine, samandenone, and samandinine) might be considered as intermediates on the way from cholesterol (XXVIII) to the alkaloids without side chains. A cholic acid intermediate (XXIX) could be postulated as one route to the isopropyl... 

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. 

The exact contributions of these alternate pathways to total hepatic bile acid synthesis in normal subjects is not certain, although 26-hydroxylation is usually regarded as the major pathway. In addition, it should be pointed out that current views of hepatic cholic acid and chenodeoxycholic acid synthesis are based primarily oh studies carried out in the rat. More recent studies, which have involved the administration of labeled bile acid intermediates to patients, have suggested that bile acid biosynthesis is more complex than previously thought and that multiple pathways exist to convert cholesterol to bile acids (Vll). 

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... 

S2). More recent studies have shown that patients with cirrhosis are able to efficiently convert 7a-hydroxy-cholesterol into cholic acid (G8, P8), suggesting that 12a-hydroxylase activity is near normal. Other evidence from in vivo studies in man with labeled preciusors suggests that 12a-hydroxylase activity is not important in the regulation of the ratio between cholic acid and chenodeoxycholic add in human bile (B21). The possibility that different pools of cholesterol are utilized for the biosynthesis of cholic acid and chenodeoxycholic acid is now being investigated. 

In 1972, large amounts of THCA were firund in the bile of two unrelated young children with intrahepatic bile duct abnormalities (E5), suggesting a block in the biosynthesis of cholic acid. Tliree years later, this bile acid intermediate was also found in the bile, serum, and urine of a brother and sister with a similar paucity of intrahepatic bile ducts and cholestatic liver disease, which proved fotal (H3). DHCA or varanic acid could not be detected in these patients, so that the metabolic defect in this condition appeared to be specific for the enzyme involved in the conversion of THCA to varanic acid. Hanson et a/, speculated that THCA might have caused the... 

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

Introduction of a 26-hydroxyl group almost completely prevents introduction of a 12 -hydroxyl group in rat liver. In contrast, there is an efficient conversion of 5)8-cholestane-3a,7a,26-triol and 7a,26-dihydroxy-4-cholesten-3-one into both cholic and chenodeoxychohc acid in human hver [180-182], From these findings it is possible to formulate a number of different pathways in the biosynthesis of cholic acid in human hver, with introduction of the 26- and the 12a-hydroxyl groups at different stages. 

Salen et al. reported that liver microsomes from 2 patients with CTX had a decreased capacity to 24/8-hydroxylate 5)8-cholestane-3a,7a,12a,25-tetrol [185]. It was suggested that the basic metabolic defect is a relative deficiency of the 24j8-hy-droxylase. To explain the severe metabolic consequences of such a defect, it must be assumed that the 25-hydroxylase pathway is the major pathway in the biosynthesis of cholic acid. This hypothesis does not explain the marked reduction in the biosynthesis of chenodeoxycholic acid. In view of the very low activity of the microsomal 25-hydroxylase towards 5)3-cholestane-3a,7a-diol in human hver [41] it is evident that a 25-hydroxylase pathway cannot be of importance in the normal... 

Results of various in vivo experiments with labelled bile acid precursors in patients with CTX have been published [185,190,195]. All these experiments show that there is a defect in the oxidation of the steroid side chain in the biosynthesis of cholic acid but are not fully conclusive with respect to the site of defect. Bjorkhem et al. administered a mixture of [ H]7a,26-dihydroxy-4-cholesten-3-one and [ " C]7a-hy-droxy-4-cholesten-3-one to a patient with CTX [195]. The ratio between and C in the cholic acid and the chenodeoxycholic acid isolated was 40 and 60 times higher, respectively, than normal. Similar results were obtained after simultaneous administration of H-labelled 5)3-cholestane-3a,7a,26-triol and 4- C-labelled 5j8-cholestane-3a,7a-diol. The results of these experiments are in consonance with the contention that the basic defect in CTX is the lack of the 26-hydroxylase, but do not per se completely exclude other defects in the oxidation of the side chain. 

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