Purification and characterization of a rat liver bile acid coenzyme A ligase from rat liver microsomes

In the present study, using the C-24 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 mu M 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 mu M, 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 C-24 monohydroxy bile acid lithocholic acid and 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid, the C-27 homolog of cholic acid), but only weakly by cholic acid, Chenodeoxycholyl-CoA and 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-27-oyl-CoA were confirmed as reaction products of purified rBAL by HPLC-electrospray ionization mass spectrometry. (C) Academic Press.