Suitability of (l 1,l 2-3H,)chenodeoxycholic acid and (ll ,12-3H2)Iithocholic acid for isotope dilution studies of bile acid metabolism

Studies were carried out to assess the stability of the label in ( 1 1,l 2-3Hz)chenodeoxycholic acid and ( 1 1,l 2-3Hz)lithocholic acid during enterohepatic cycling and thus define the suitability of these tracers for isotope dilu- tion studies of bile acids in man. To do this, (l 1,12-3H2)- chenodeoxycholic acid and ( 24-'4C)chenodeoxycholic acid were administered simultaneously to six healthy adults. The 3H/'4C ratio of biliary chenodeoxycholic acid and the specific activity of biliary chenodeoxycholic acid were de- termined to permit calculation of pool size and turnover rate by the conventional isotope dilution procedure of Lindstedt (1957. Acta Physzol. Scund. 40: 1). Excretion of label in urine was measured as well as the 3H/'4C ratio of fecal bile acids. Similar studies were carried out with ( 1 1, 12-3Hz)lithocholic acid and (24-14C)lithocholic acid. With (l 1,12-3Hz)chenodeoxycholic acid, 10-15% of the label was lost during the first 24 hours, after which the 3H/'4C ratio of biliary chenodeoxycholic acid remained constant. The loss of 3H caused a consistent overestimate of pool size by 10-15%, but since the fractional turnover rate of the two isotopes was identical, the calculated rate of synthesis was 10-15% greater when deter- mined using 3H. The loss of 3H was attributed to the presence of 3H in positions other than 11 and 12 oc- curring during the labeling procedure when the un- saturated precursor was exposed to carrier-free tritium gas. The 3H/'4C ratio of feces was consistently below that of bile, suggesting bacterial removal of additional 3H during distal intestinal passage. In contrast, ( 11, 12-3H2)lithocholic acid lost 3H continuously, so that by 2 days after ad- ministration, the 3H/14C ratio had fallen to nearly half of that of the administered mixture. It is concluded that isotope dilution studies may be carried out with one preparation of ( 11 ,12-3H2)chenodeoxycholic acid with an error of 10-15%; possibly a more stable preparation can be prepared with a entler tritiation procedure. One preparation of ( 11, 12-3H~lithocholic acid was unsatisfactory for isotope dilution studies in man.