Suitability of [11,12-3H2]chenodeoxycholic acid and [11,12-3H2]lithocholic acid for isotope dilution studies of bile acid metabolism in man

Abstract

Studies were carried out to assess the stability of the label in [11,12-3H2]chenodeoxycholic acid and [11,12-3H2]lithocholic acid during enterohepatic cycling and thus define the suitability of these tracers for isotope dilution studies of bile acids in man. To do this, [11,12-3H2]-chenodeoxycholic acid and [24-14C]chenodeoxycholic acid were administered simultaneously to six healthy adults. The 3H/14C ratio of biliary chenodeoxycholic acid and the specific activity of biliary chenodeoxycholic acid were determined to permit calculation of pool size and turnover rate by the conventional isotope dilution procedure of Lindstedt (1957. Acta Physiol. Scand. 40: 1). Excretion of label in urine was measured as well as the 3H/14C ratio of fecal bile acids. Similar studies were carried out with [11,12-3H2]lithocholic acid and [24-14C]lithocholic acid.With [11,12-3H2]chenodeoxycholic acid, 10–15% of the label was lost during the first 24 hours, after which the 3H/14C 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 determined using 3H. The loss of 3H was attributed to the presence of 3H in positions other than 11 and 12 occurring during the labeling procedure when the unsaturated precursor was exposed to carrier-free tritium gas. The 3H/14C 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 administration, 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 gentler tritiation procedure. One preparation of [11,12-3H2]lithocholic acid was unsatisfactory for isotope dilution studies in man.

Keywords

• tritium labeling
• bile acids
• isotope dilution