PREFERENTIAL LOCALIZATION OF RAT-LIVER D-MYO-INOSITOL 1,4,5-TRISPHOSPHATE/1,3,4,5-TETRAKISPHOSPHATE 5-PHOSPHATASE IN BILE-CANALICULAR PLASMA-MEMBRANE AND LATE ENDOSOMAL VESICLES

Previous studies have shown that most of the inositol 1,4,5-trisphosphate/inositol 1,3,4,5-tetrakisphosphate 5-phosphatase activity of rat hepatocytes is associated with the plasma membrane [Shears, Parry, Tang, Irvine, Michell and Kirk (1987) Biochem. J. 246, 139-147]. We now show that the specific activity of this enzyme is highest in the bile-canalicular domain of the plasma membrane, at the opposite pole of the hepatocyte from the presumed site of receptor-mediated formation of inositol 1,4,5-trisphosphate. In intact hepatocytes and in sealed membrane vesicles originating from the bile-canalicular domain of the plasma membrane, the 5-phosphatase activity was mostly latent and therefore located at the cytoplasmic surface. A substantial amount of 5-phosphatase was also found in rat liver endosomal fractions, particularly a ''late'' endosomal subfraction, indicating that this enzyme may be transported between the sinusoidal plasma membrane and other cellular membranes.