In the present study, using the C24 bile acid chenodeoxycholic acid as substrate, rat liver bile acid CoA ligase activity (rBAL) was purified 200- fold from detergent-solubilized microsomes using a combination of Q-Sepharose anion exchange, hydroxyapatite, and CM-Sepharose chromatography. Purified rBAL had a molecular weight of 65 kDa by SDS-PAGE analysis. Gel filtration of purified rBAL indicated that rBAL activity forms a complex with other proteins with an apparent aggregate molecular weight of 243 kDa. A monoclonal antibody raised against the 65-kDa protein and covalently coupled to 6B- Sepharose completely absorbed rBAL activity from a semipurified preparation of rat liver microsomes. Western blot analysis confirmed the elution of the 65-kDa protein from the affinity phase at low pH. Optimum rBAL activity was found at pH 8.5, and activity was dependent on the divalent cation Mg2+. In the presence of 50 μM CoA and 2.5 mM MgCl2, kinetic analysis revealed that the apparent K(m)s of ATP and chenodeoxycholic acid of the purified enzyme were 548 ± 247 and 18.0 ± 6.2 μM, respectively, and the apparent V(max) was 9.53 ± 2.0 nmol min-1 mg protein-1. The formation of chenodeoxycholyl-CoA by rBAL was strongly inhibited by hydrophobic bile acids (the C24 monohydroxy bile acid lithocholic acid and 3α,7α,12α- trihydroxy-5β-cholestanoic acid, the C27 homolog of cholic acid), but only weakly by cholic acid. Chenodeoxycholyl-CoA and 3α,7α,12α-trihydroxy-5β- cholestan-27-oyl-CoA were confirmed as reaction products of purified rBAL by HPLC-electrospray ionization mass spectrometry.