Subcellular organization of bile acid amidation in human liver: a key issue in regulating the biosynthesis of bile salts

To extend our knowledge of how the synthesis of free bile acids and bile sa lts is regulated within the hepatocyte, bile acid-CoA:amino acid N-acyltran sferase and bile acid-CoA thioesterase activities were measured in subcellu lar fractions of human liver homogenates. Some bile acids, both conjugated and unconjugated, have been reported to be natural ligands for the farnesoi d X receptor (FXR), an orphan nuclear receptor. The conversion of [C-14]cho loyl-CoA and [C-14]chenodeoxycholoyl-CoA into the corresponding tauro- and glyco-bile acids or the free bile acids was measured after high-pressure li quid radiochromatography. There was an enrichment of the N-acyltransferase in the cytosolic and the peroxisomal fraction. Bile acid-CoA thioesterase a ctivities were enriched in the cytosolic, peroxisomal, and mitochondrial fr actions. The highest amidation activities of both choloyl-CoA and chenodeox ycholoyl-CoA were found in the peroxisomal fraction (15-58 nmol/mg protein/ min). The K-m was higher for glycine than taurine both in cytosol and the p eroxisomal fraction. These results show that the peroxisomal de novo synthe sis of bile acids is rate limiting for peroxisomal amidation, and the micro somal bile acid-CoA synthetase is rate limiting for the cytosolic amidation , The peroxisomal location may explain the predominance of glyco-bile acids in human bile. Both a cytosolic and a peroxisomal bile acid-CoA thioestera se may influence the intracellular levels of free and conjugated bile acids .