Our laboratory has demonstrated recently that conjugates of 2-fluoro-β-alanine (FBAL) and bile acids are the major biliary metabolites of 5-fluorouracil (FUra) in cancer patients. Bile acids are normally conjugated with glycine or taurine, and therefore the identification of the FBAL-bile acid conjugates suggested that FBAL may also be a substrate for the bile acid conjugating enzyme, bile acid CoA : amino acid : N-acyltransferase. Enzyme activity detected using glycine and taurine as substrates was purified 8-fold from human liver cytosol using a DEAE-cellulose column. This preparation when tested for its activity towards β-alanine and FBAL using cholyl CoA as the bile acid substrate only catalyzed the formation of FBAL-cholate. β-Alanine was not a substrate. Confirmation of FBAL-cholate as the enzymatic product was demonstrated by (1) coelution of the product of this reaction on HPLC with authentic FBAL-cholate, (2) specific hydrolysis of this product by cholylglycine hydrolase, and (3) molecular weight of the product (497) being identical to that of the authentic FBAL-cholate. Kinetic experiments demonstrated that the enzyme had an affinity for FBAL (Km 1.45 mM) comparable to taurine (Km 1.32 mM), but greater than glycine (Km, 6.45 mM). Formation of FBAL-cholate was inhibited competitively by taurine (Ki 1.27 mM) and glycine (Ki 4.47 mM), suggesting that a single enzyme is responsible for conjugation of glycine, taurine and FBAL with bile acids. These data indicate that the formation of the FBAL-bile acid conjugates in patients receiving FUra results from high affinity of the bile acid conjugating enzyme for FBAL. © 1990.