Metabolism of steroid and amino acid moieties of conjugated bile acids in man

Abstract

Cholyl-2,4-³H-glycine-1-¹⁴C was administered orally to eight healthy subjects with indwelling nasoduodenal tubes. The distribution of radioactivity among bile acids and the specific activity of cholylglycine were determined in bile collected at intervals for 7 days. ³H and ¹⁴C were measured in stool. ¹⁴C in breath was calculated from interval ¹⁴CO₂ specific activity determinations. The daily fractional turnover of the glycine moiety (mean ±SE, 106±17%) was three times greater than that of the cholyl moiety (38±7%). On the basis of certain assumptions, it was calculated that about 18% of the cholylglycine pool was deconjugated per enterohepatic cycle. The extent of deconjugation appeared to be unrelated to the efficiency of absorption of the cholyl moiety, which averaged 90-95% per enterohepatic cycle. ¹⁴C was recovered predominantly in breath (52±5% of administered dose), and 24 hr ¹⁴CO₂ excretion correlated highly (r = 0.95) with daily fractional turnover of the glycine moiety. ³H excretion occurred predominantly in feces, and the rate correlated highly (r = 0.92) with the daily fractional turnover of the cholyl moiety. Deoxycholylglycine became labeled with ³H rapidly, indicating the occurrence of bacterial 7-dehydroxylation of the cholyl moiety and absorption of deoxycholic acid. This biotransformation occurred in all eight subjects but varied in degree and was unrelated to the degree of deconjugation. Since ingested glycine-1-¹⁴C was not incorporated into bile acid glycine, appearance of ¹⁴C in deoxycholylglycine (observed in three of eight subjects) indicated that 7-dehydroxylation of cholylglycine can occur without deconjugation. Dehydroxylation was also observed in vitro when fecal homogenates were incubated with cholylglycine.