THE SUBCELLULAR-LOCALIZATION OF A BILE-ACID GLUCOSYLTRANSFERASE IN RAT-LIVER

Formation of bile acid glucosides occurs in rat liver homogenate with a specific enzyme activity of 0.014 +/- 0.001 nmol per min per mg prot ein. Subcellular fractionation of rat liver by differential centrifuga tion revealed an enrichment of bile acid glucosyltransferase activity both in the mitochondrial-lysosomal fraction and in microsomes with a recovery of 38.8 +/- 4.6% and 37.7 +/- 1.7%, respectively, of enzyme a ctivity in the homogenate. Subfractionation of the mitochondrial-lysos omal fraction after treatment of rats with Triton WR 1339 showed an al most exclusive association of bile acid glucosyltransferase activity w ith purified lysosomes (''tritosomes''). After subfractionation of mic rosomes by analytical gradients, bile acid glucosyltransferase was bim odally distributed with peaks at modal densities of 1.09 g/cm(3) and 1 .16 g/cm(3), respectively. If microsomes were pretreated with pyrophos phate, a membrane perturbant known to strip ribosomes, only the peak o f bile acid glucosyltransferase at higher density (1.16 g/cm(3)) and U DP-glucuronosyltransferase (marker of endoplasmic reticulum) shifted t o a similar lower equilibrium density. Both enzymes were unaffected in their distribution by pretreatment of microsomes with digitonin. In c ontrast, markers of plasma membranes (5'-nucleotidase) and the Golgi-c omplex (galactosyltransferase) shifted to higher equilibrium densities after digitonin treatment, but were unaltered in their distribution a fter pyrophosphate. Bile acid glucosyltransferase activity in the lowe r density range with a peak at 1.09 g/cm(3) did not show any associati on with the density distributions of known marker enzymes. In purified microsomal fractions obtained by preparative gradients, bile ace gluc osyltransferase activity was enriched in enzyme activity by 1.4-fold i n rough and by 2.3-fold in smooth endoplasmic reticulum, respectively. In highly purified membranes of the Golgi-complex assessed by the 83- fold enrichment of its marker galactosyltransferase, the bile glucosyl transferase showed a 4.8-fold increase of its specific activity as com pared to homogenate. Since galactosyltransferase is mainly localized i n the trans-Golgi-compartment and was affected in its density distribu tion by digitonin treatment, in contrast to bile acid glucosyltransfer ase, our results obtained by analytical and preparative gradients sugg est that microsomal bile acid glucosyltransferase is partly associated with membranes of the Golgi-complex, distinct from galactosyltransfer ase containing trans-Golgi, and partly with membranes of the endoplasm ic reticulum. The distribution of rat liver bile acid glucosyltransfer ase activities among membranes of lysosomes, endoplasmic reticulum and Golgi-complex is species-specific in comparison to human liver, where glucosyltransferase activity is confined to the smooth endoplasmic re ticulum. (C) Journal of Hepatology.