Alio bile acids

Separation of 26 alio bile acids as their methyl esters the combined use of NP 73... 

Bile acids are C-24 to C-28 carboxylic acids with a steroid nucleus containing hydroxylic substituents and part or all of the side chain of 5 -cholestane (Fig. 1). The A/B ring junction is cis (Sp hydrogen) in the bile acids of most higher vertebrates, although A/B trans (alio) bile acids (5a hydrogen) occur in lower vertebrates (HIO). Bile acids do not occur in invertebrates. Most unconjugated bile acids have 24 carbon atoms with the basic structure of 5p-... 

Conversion of 5/i-bile acids to alio bile acids... 

Bjorkhem and Einarsson showed that rat liver microsomes in the presence of NADPH are able to convert 7a-hydroxy-4-cholesten-3-one and 7a,12a-dihydroxy-4-cholesten-3-one into the corresponding 3-oxo-5a-steroids (Fig. 11) [168], Liver microsomes from female rats were 3-4 times more active than those from male rats. A similar sex difference was not found in human hver microsomes. The 3-oxo-5a-steroids were efficiently converted into alio bile acids in bile fistula rats. In the presence of both NADPH and NADH, the 3-oxo-5a-steroids were converted into the corresponding 3/3-hydroxy-5a-steroids. As mentioned above, the 3-oxo-5a-steroids are efficiently converted into the corresponding 3a-hydroxy-5a-steroids in the cytosol. 

The quantitative importance of the pathway involving 7a-hydroxy-4-cholesten-3-one and 7a,12a-dihydroxy-4-cholesten-3-one as intermediates in formation of alio bile acids is not known. At least in the rat, the capacity of the microsomal A" -3-oxosteroid 5a-reductase is high, and thus it is surprising that such small amounts of alio bile acids are formed under normal conditions in mammals. It seems probable that this enzyme is inhibited under in vivo conditions. Hoshita et al. found an efficient conversion of 7a,12a-dihydroxy-4-cholesten-3-one into 7a,12a-dihy-droxy-5a-cholestan-3-one in the microsomal fraction of liver from Iguana iguana [170], Alio bile acids are predominant in this species of iguana and it was suggested that the microsomal A -3-oxosteroid 5a-reductase is of major importance for their formation. 

Fig. 11. Sequence of reactions in the conversion of cholesterol into alio bile acids with 7a-hydroxy-4-cholesten-3-one and 7a,12a-dihydroxy-4-cholesten-3-one as intermediates.

Sulfated alio bile acids occur more abundantly or exclusively in bile of female rats compared to male rats [63]. Allochenodeoxycholate monosulfate was the major sulfated alio acid from the female none was detected in bile of the male. Other monosulfates identified include allocholate, and the 3 epimers of allochenodeoxycholate and allocholate. From female rat bile collected 48-60 h after cannulation allochenodeoxycholate constituted about 63% of the mono- and disulfate fractions none was found in bile from male rats. Sulfates of allocholate and the 3j8 epimers of allochenodeoxycholate and allocholate were present in smaller amounts. Initial collections of bile (0-12 h) contained allodeoxycholate and its 3j8 epimer in the monosulfate fraction. 

The alio bile acids are the 5a-cholanoic acids, i.e., derivatives of cholanoic acid in which the hydrogen at C5 is alpha-oriented rather than beta-oriented as in the large number of naturally occurring bile acids. The sequence of numbering in the cholane nucleus is the same as in the normal series. The... 

TABLE IV. Infrared Bands of Alio Bile Acids ... 

Fig. 11. Preparation of isomeric 3,6-dihydroxy alio bile acids.

Three biosynthetic pathways for the elaboration of alio bile acids have been identified. Each appears to be an extension of the metabolism of a C27 sterol derived from acetate via mevalonate. The following sections delineate... 

The origin of alio bile acids from cholestanol is clearly established (121, 36,28,122,40,123). After separation of two unidentified biliary acids similar to, but not identical with, cholic and chenodeoxycholic acids, Harold et al. 

If the pathway of biosynthesis of alio bile acids from cholestanol is similar to that proposed for the 5p bile acids from cholesterol, 7a-hydroxyla-tion should be the next step. The microsomal fraction of rat liver properly fortified with NADPH and oxygen was observed to hydroxylate cholestanol and cholesterol to the respective 7a-hydroxy derivatives at equivalent rates (127). With cholestanol the 7 -hydroxylase was inhibited by the product, or the 7j3-ol or 7-keto sterol. The rate of hydroxylation could be enhanced by prior treatment of the rats with cholestyramine (a bile acid sequestrant) and/or phenobarbital. A C5 double bond was not considered a requirement for enzymatic hydroxylation at C7. 

Although this class of substances has observed a fiftieth anniversary in man s knowledge of bile acids, the subject has not sparked biochemical interest until recent times, when, aided by modern techniques, the biochemist has been able to study smaller quantities of materials at the subcellular level. It seems likely that this is the level where new knowledge of the biological importance of alio bile acids will be generated. Where there are current explanations for the roles of the 5/3 bile acids in nature, these answers have not yet appeared for the allo-acids, partially because of the newness of the subject and the paucity of materials. Until the role of the allo-acids in nature is fully detailed, investigations in this area will continue. 

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