Subcellular distribution of hepatic bile acid-conjugating enzymes

1. The subcellular location of enzymes conjugating bile acids with glycine or taurine was investigated by centrifugation of rat liver homogenates. 2. [14C]Cholic acid-conjugating activity was predominantly associated with the soluble-microsomal region of the gradient after centrifugation in a Ti-15 zonal rotor but the bulk of the conjugating activity sedimented with mitochondrial-lysosomal fractions in differential pelleting experiments. 3. Cholate: CoA ligase (EC 6.2.1.7) and cholyltransferase (EC 2.3.1) were not enriched in purified Golgi or plasma-membrane fractions. Cholate: CoA ligase was distributed evenly between rough- and smooth-surfaced microsomal subfractions but cholyltransferase showed a dual soluble-rough microsomal activity distribution. 4. Sedimentation of cholyltransferase in mitochondria-enriched fractions prepared by differential centrifugation appears to be an artefact of sedimentation of rough microsomal membranes in mitochondrial fractions. 5. The subcellular distribution of bile acid-conjugating enzymes is discussed with reference to hepatic processing of bile acids.