A highly specific 125I-radioimmunoassay for cholic acid conjugates

Several modifications of the immunization procedure permitted development of a highly specific radioimmunoassay (RIA) for cholic acid conjugates. Antiserum was produced in guinea pigs using cholic acid-thyroglobulin complex as immunogen. 125-I-Cholyglycylhistamine was prepared as radioactive ligand according to a modification of the method of Spenney et al. (Spenney, J.G., Johnson, B.J., Hirschowitz, B.I., Mihas, A.A. and Gibson, R. (1977) Gastroenterology 72, 305--311). The association constant of the antisera to taurocholic acid was 1.8 x 10(7) l/mol, the working range of the assay between 9.5--890 pmol. Cross-reactivities of the antiserum to bile acids other than cholic acid species were less than 3%, which is lower than for any published bile acid RIA. Concentrations of cholic acid conjugates in sera obtained from 17 healthy fasting volunteers ranged from 0.4--1.9/mumul/l.