Bile Acids and Steroids 96. ON THE MECHANISM OF THE BIOLOGICAL FORMATION OF DEOXYCHOLIC ACID FROM CHOLIC ACID

Deoxycholic acid occurs in varying amounts in the bile of several species (1). In all cases recently studied by isotopic experiments (2-5) it has been demonstrated that deoxycholic acid is not formed primarily in the liver but by the action of intestinal microorganisms on cholic acid during the enterohepatic circulation. The microorganisms responsible for this reaction have not yet been isolated. A similar reaction, viz. microbiological elimination of the hydroxyl group at C-7 in hyocholic acid (3ar, 6a,7c~-trihydroxycholanic acid) with formation of hyodeoxycholic acid (3a(, 6adihydroxycholanic acid) has been observed in the pig (6). Thus there seems to be a general pathway for the metabolism of bile acids irrespective of the species, i.e. the formation of a trihydroxycholanic acid in the liver by degradation of cholesterol, elimination of the hydroxyl at C-7 of this acid, and in some cases hydroxylation by liver enzymes of the deoxycholic acid formed (7a-hydroxylations in the rat (7) and mouse (8) and 16hydroxylation in constrictor and python snakes (5)). Only one exception, the guinea pig, has been noted (8). Earlier experiments on the mechanism of deoxycholic acid formation by means of cholic acid-7/3-H3 ,24-Cl4 demonstrated the almost complete retention of tritium in deoxycholic acid formed from the doubly labeled cholic acid (9, 10). This fact excluded one of the previously suggested pathways, viz. the intermediate formation of 7-ketodeoxycholic acid (3ar, 12a-dihydroxy-7-ketocholanic acid) (11) and made it probable that the elimination consists of dehydration to an unsaturated intermediate followed by reduction or some type of direct elimination. In addition it was noted that the tritium retained in deoxycholic acid was lost on 7a-hydroxylation in rat liver (9, 10). In view of recent investigations (12-15) on the stereochemistry of steroid hydroxylations which demonstrated that the hydroxyl is introduced by displacement of the hydrogen atom in the position which is hydroxylated and with retention of the other hydrogen in the original position this result strongly indicated than an inversion of 7/?-H3 to 7a-H3 occurred in the reactions leading to deoxycholic acid. By the synthesis of cholic acid-6a, 6/3,8P-H3,24-Cl4 as well as the use of cholic acid-6a-Ha, formed from cholesterol-6-H3, it has been possible to evaluate more thoroughly the mechanism of the dehydroxylation, i.e. to differentiate between the mechanisms proposed above and to give more information concerning the stereochemistry of the elimination reaction. EXPERIMENTAL