Thiolase involved in bile acid formation.

The formation of cholic acid and chenodeoxycholic acid through cleavage of the side chains of CoA esters of 3alpha,7alpha,12alpha-trihydroxy-5beta-choles tan-26-oic acid and 3alpha,7alpha-dihydroxy-5beta-cholestan-26-oic acid is believed to occur in peroxisomes. Recently, we found a new peroxisomal enzyme, D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase bifunctional protein, and suggested that this bifunctional protein is responsible for the conversion of 3alpha,7alpha,12alpha-trihydroxy-5beta-cholest-2 4-en-26-oyl-CoA and 3alpha,7alpha-dihydroxy-5beta-cholest-24-en-26-oyl-CoA to their 24-oxo-forms. In the present study, the products of this bifunctional protein reaction were analyzed by gas chromatography-mass spectrometry, and the formation of 24-oxo-27-nor-cholestanes was confirmed. Previously, we found a new thiolase in Caenorhabditis elegans, P-44, and suggested that P-44 and sterol carrier protein x, a peroxisomal protein, constitute a second group of 3-oxoacyl-CoA thiolases. The production of cholic acid and chenodeoxycholic acid from the precursors on incubation with the bifunctional protein and sterol carrier protein x or P-44 was confirmed by gas chromatography.