The conjugation reactions of hydration and dehydrogenation catalyzed by the dehydratase and dehydrogenase activities of D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase bifunctional protein (DBP) and enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional protein (LBP) in the side chain degradation step of bile acid biosynthesis were investigated using chemically synthesized C-27-bile acid CoA esters as substrates. The hydration catalyzed by DBP showed high diastereoselectivity for (24E)-3 alpha ,7 alpha ,12 alpha -trihydroxy- and (24E)-3 alpha ,7 alpha -dihydroxy-5 beta -cholest-24-en-26-oyl CoA to give (24R,25R)-3 alpha ,7 alpha ,12 alpha ,24-tetrahydroxy- and (24R, 25R)-3 alpha ,7 alpha ,24-trihydroxy-5 beta -cholestan-26-oyl CoAs, respectively, and the dehydrogenation catalyzed by DBP also showed high stereospecificity for the above (24R,25R)-isomers to give 3 alpha ,7 alpha ,12 alpha -trihydroxy- and 3 alpha ,7 alpha -dihydroxy-24-oxo-5 beta -cholestan-26-oyl CoAs, respectively. On the other hand, the dehydratase activity of LBP displayed a different diastereoselectivity producing the (24S,25S)-isomer, and dehydrogenase activity of LBP was stereospecific for the (24S,25R)-isomer to give the above 24-oxo-derivative. The hydration and dehydrogenation reactions catalyzed by DBP were effectively conjugated to convert (24E)-5 beta -cholestenoyl CoA to 24-oxo-5 beta -cholestanoyl CoA. However, the reactions catalyzed by LBP were not conjugated. These results indicate that DBP plays an important role in the biosynthesis of bile acid. (C) 2001 Elsevier Science Inc. All rights reserved.