Conjugation reactions catalyzed by bifunctional proteins related to beta-oxidation in bile acid biosynthesis

The conjugation reactions of hydration and dehydrogenation catalyzed by the dehydratase and dehydrogenase activities of D-3-hydroxyacyl-CoA dehydratas e/D-3-hydroxyacyl-CoA dehydrogenase bifunctional protein (DBP) and enoyl-Co A hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional protein (LBP) in t he side chain degradation step of bile acid biosynthesis were investigated using chemically synthesized C-27-bile acid CoA esters as substrates. The h ydration catalyzed by DBP showed high diastereoselectivity for (24E)-3 alph a ,7 alpha ,12 alpha -trihydroxy- and (24E)-3 alpha ,7 alpha -dihydroxy-5 b eta -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 -ch olestan-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 de hydratase activity of LBP displayed a different diastereoselectivity produc ing the (24S,25S)-isomer, and dehydrogenase activity of LBP was stereospeci fic for the (24S,25R)-isomer to give the above 24-oxo-derivative. The hydra tion and dehydrogenation reactions catalyzed by DBP were effectively conjug ated to convert (24E)-5 beta -cholestenoyl CoA to 24-oxo-5 beta -cholestano yl CoA. However, the reactions catalyzed by LBP were not conjugated. These results indicate that DBP plays an important role in the biosynthesis of bi le acid. (C) 2001 Elsevier Science Inc. All rights reserved.