Formation of Secondary Bile Acids in Man

The predominant bile salts in feces are not the same as those in bile. When i C-labeled cholesterol is given intravenously to an adult human, the two major metaboUtes excreted in approximately equal quantities in the feces are deoxycholate and lithocholate, with much smaller amounts of other secondary bile salts. Deoxycholate and lithocholate, which account for slightly more than half of the excreted activity (28), are derived from 7a-dehydroxylation of cholate and chenodeoxycholate, respectively, by colon bacteria. The process has been shown to proceed by direct removal of the 7a-OH group with the adjacent 6/5-hydrogen, yielding a Zl -cholenoic acid this unsaturated acid is then hydrogenated to give deoxycholate (29). 

With isotopically labeled primary bile salts, cholate and chenodeoxycholate, it is possible to show that in addition to deoxycholate a wide variety of secondary bile salts are derived from cholate, the chief ones being 12a-hydroxy-3-keto-5/ -cholanoic acid, 3), 12a-dihydroxy-5) -cholanoic acid, 3a-hydroxy-12 keto-5/5-cholanoic acid, and 3/ -hydroxy-12-keto-5j -cholanoic acid (30). A smaller number are derived from chenodeoxycholate, mainly lithocholate, 3/ -hydroxy-5i -cholanoic acid, and 3-keto-5i -cholanoic acid (31). The secondary bile acids which have been identified in man are listed in Table III. 

TABLE in. Bile Acids Identified in Human Feces  

7a-Dihydroxy l2-keto-5i9-cholanoic acid 3a, 12a-Dihydroxy-7-keto-5j5-cholanoic acid 3a,7)5,12a-Trihydroxy-5iS-cholanoic acid 3)5,7)5,12a-Trihydroxy-5)5-cholanoic acid 3)5,7a,12a-Trihydroxy-5)5-cholanoic acid 3a,7a, 12a-Trihydroxy-5)5-cholanoic acid 3a, 12a-Dihydroxy-5)5-cholanoic acid 3,12-Diketo-5)5-cholanoic acid 3a-Hydroxy-l 2-keto-5)5-cholanoic acid 3)5-Hydroxy-l 2-keto-5)5-cholanoic acid 12a-Hydroxy-3-keto-5)5-cholanoic acid 3a, 12)5-Dihydroxy-5)5-cholanoic acid 3)5,12a-Dihydroxy-5)5-cholanoic acid 3)5,12)5-Dihydroxy-5)5-cholanoic acid 3a,7a-Dihydroxy-5)5-cholanoic acid 7a-Hydroxy-3-keto-5)5-cholanoic acid 3)5,7a-Dihydroxy-5)5-cholanoic acid 3a-Hydroxy-7-keto-5)5-cholanoic acid 3a,7)5-Dihydroxy-5)5-cholanoic acid 7)5-Hydroxy-5)5-cholanoic acid 3a-Hydroxy-5/5-cholanoic acid 3)5-Hydroxy-5)5-cholanoic acid 3-Keto-5)5-cholanoic acid 5)5-Cholanoic acid 

Secondary bile acids produced in the colon are absorbed by the colon into the enterohepatic circulation to a limited extent. Deoxycholate is usually the chief secondary bile salt found in human bile, with smaller amounts of lithocholate (1). The predominance of deoxycholate in human bile suggests that it is more readily absorbed by the colon than lithocholate. 

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