Cholic acid formation

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... 

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). 

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. 

At present, the sequence of changes in the steroid nucleus in cholic acid formation has been examined only in one species other than the rat, viz., man (62). The metabolism of cholesterol and several other C27-steroids in the presence of different subcellular fractions of homogenates of human liver was found to be the same as found previously for the rat, indicating the presence of the same pathways in man as in the rat. 

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... 

Bile Acids, Cholecystolithiasis, and Cholestasis 596 Cholic Acid Formation Secondary Bile Acid... 

Based on his examination of the bile salts of various species, Haslewood (1959) had concluded that an evolutionary pattern exists which is reflected in the pathway by which the cholesterol side chain is cleaved to yield the flve carbon acidic side chain of the bile acids. Thus, the older animals (shark) possess a side chain that has a 27 hydroxyl group, the reptiles mainly synthesize a 27 carboxylic acid and mammals a 25 carboxylic acid. Haslewood suggested that 3a, 7a, 12a-tri-hydroxycoprostanic acid might be an intermediate in cholic acid formation in 1952. 

Bile salt export pump (BSEP gene symbol ABCB11) mediates the biliary excretion of nonconjugated bile salts, such as taurocholic acid, glycocholic acid and cholic acid, and therefore is responsible for the formation of the bile acid-dependent bile flow [97, 98]. Its hereditary defect results in the acquisition of PFIC2, a potentially lethal disease which requires liver transplantation [17, 81, 82, 99]. As discussed in Section 12.5.2, the inhibition of BSEP following drug administration may result in cholestasis. 

However, subsequent studies did not find clear evidence to support the view that bile acids could independently stimulate tumour formation utilising rat models. Rather, the findings indicated that bile acids could enhance the effect of other carcinogens in these models. An example of such a study is by McSherry et al. in which male Fischer rats were fed diets supplemented with cholic acid (0.2%) and administered the colonic carcinogen, A-Methyl-A-nitrosurea (MNU), intra-rectally. Fifty-five per cent of MNU treated rats on standard diet developed tumours, a figure that increased to eighty per cent in MNU-treated rats given dietary cholic acid. Rats fed cholic acid supplemented diet alone did not develop tumours. 

Jenkins et al. demonstrated that the secondary bile acid, deoxycholic acid could induce micronuclei formation in the oesophageal adenocarcinoma cell line, OE33. The induction of micronuclei demonstrated a dose-dependent effect and occurred under both neutral and acidic pH conditions. An example of a micronucleus induced by treatment of the OE33 oesophageal adenocarcinoma cell line with deoxy cholic acid is shown in Figure 4.3. 

In the bile cholesterol is kept soluble by fats, phospholipids like lecithin and by bile acids. The important bile acids in human bile are cholic acid, chen-odeoxycholic acid or chenodiol and ursodeoxycholic acid or ursodiol. Bile acids increase bile production. Dehydrocholic acid, a semisynthetic cholate is especially active in this respect. It stimulates the production of bile of low specific gravity and is therefore called a hydrocholeretic drug. Chenodiol and ursodiol but not cholic acid decrease the cholesterol content of bile by reducing cholesterol production and cholesterol secretion. Ursodiol also decreases cholesterol reabsorption. By these actions chenodiol and ursodiol are able to decrease the formation of cholesterolic gallstones and they can promote their dissolution. 

Bouchard G, Yousef IM, Tuchweber B (1993) Influence of oral treatment with ursodeoxycholic and tauroursodeoxy-cholic acids on estrogen induced cholestasis in rats Effects on bile formation and liver plasma membranes. Liver 13 193-202... 

Bile-acid formation in rats involves hydroxylation to give 7a- and 6j8-hydroxy-derivatives. In many cases, no isotope effect was observed on hydroxylation of the appropriate labelled sterol. These examples involve cytochrome P-450 in the oxidation. However, oxidation of [7a- H,24- C]deoxycholic acid or tauro-deoxycholic acid to the corresponding cholic acid showed an isotope effect of 3.8 on examination of recovered starting material. 

The insertion of hydroxyl groups into the 23- or 24-position of 5P-cholestane-3a,7a,12a,25-tetrol was found to be stereospecific. Although all these compounds were potential precursors of bile acid, studies in vivo and in vitro experiments using [3P- H] and (24- C) 5P-cholestane-3a,7a,12a,25-tetrol (46) (Figs.6, 7), (24- C) 5p-cholestane-3a,7a,12a,24R,25-pentol and (24- C) 5P-cholestane-3a,7a,12a,24S,25-pentol demonstrated the existence of a new 25-hydroxylation pathway for the transformation of cholesterol to cholic acid in these patients (2,10). The reaction sequence involved the stereospecific formation of a 24S-hydroxy pentol, 5P-cholestane-3a,7a,12a,24S,25-pentol, 3a7a,12a,25-tetrahydroxy-5P-cholestan-24-one and did not involve SP-cholestanoic acids as intermediates (Fig. 8). The two bile pentols, SP-cholestane-3a,7a,12a,24R, 25-pentol and 5P-cholestane-3a,7a,12a,23R,25-... 

There are of course more organic compounds which can be used as host molecules in the formation of inclusion complexes. For most of them, e.g. urea and thiourea, crown ethers, cholic acid, hydroquinone, tri-o-thymotides and Di-anin s compound, the literature describing SS NMR applications in structural... 

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. 

The liver, and also bacteria in the small and large intestine, can cause other structural modifications to bile acids as they undergo their entero-hepatic cycle. The formation of sulfate esters, already mentioned with respect to lithocholate in Section 4.2.1, is carried out primarily in the liver in man by a sulfotransferase (Lll). Other bile acids can also be sulfoconjugated to a small extent, mainly at the 3a-hydroxyl position. Bacteria, which have been isolated anaerobically from human feces, are known to possess bile acid sulfatase activity, which removes the 3a-sul te group of chenodeoxycholic and cholic acids (H24). The action of this bacterial enzyme probably explains why only trace amounts of sul ted bile acids, which are poorly absorbed in the intestine, are detected in the feces (12). Another type of bile acid conjugate, which has been identified in the urine of healthy subjects and patients with hepatobiliary disease, is the glucuronide (A7, S41). Both the liver and extrahepatic tissues, such as the kidney and small intestinal mucosa, are capable of glucuronidation of bile acids in man (M14). 

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