Enzyme-linked immunoassay of ursodeoxycholic acid in serum

A sensitive and specific enzyme-linked immunoassay for the measurement of ursodeoxycholic acid in human serum was developed. Ursodeoxycholic acid conjugated to alkaline phosphatase (from calf intestine) was used as a tracer. An antiserum to ursodeoxycholic acid serum was raised in rabbits using ursodeoxycholic acid--bovine serum albumin conjugate as an antigen. The binding required 1 hr at 42 degrees C; separation of the bound tracer was achieved by addition of a second antibody, and alkaline phosphatase activity of this bound complex was measured colorimetrically. The ratio of bound to total enzyme activity decreased linearly with a logarithmic increase in ursodeoxycholic acid concentration from 20 to 900 pmol. The specificity and sensitivity of this enzyme-linked immunoassay were similar to those of a radioimmunoassay reported previously. The serum ursodeoxycholic acid levels measured by this method correlated well with those determined by gas--liquid chromatography and radioimmunoassay. Based on these findings, this enzyme-linked immunoassay of bile acid might be useful as a tool for the routine clinical analysis of serum bile acids.