A MEMBRANE-BOUND FORM OF PROTEIN DISULFIDE-ISOMERASE (PDI) AND THE HEPATIC-UPTAKE OF ORGANIC-ANIONS

Protein disulfide isomerase (PDI) was considered to be involved in the hepatic uptake of certain organic anions because the protein is photoaffinity labeled by photolabile derivatives of the bile acid taurocholate. Several lines of evidences including photoaffinity labeling experiments indicated a close relationship between the uptake of bile acids and the organic anion bumetanide. The possible involvement of PDI in hepatic transport processes of these organic anions was tested with polyclonal antibodies raised against a PDI-beta-galactosidase fusion protein. Western blot analysis and immunofluorescence of intact hepatocytes showed that protein disulfide isomerase is located in sinusoidal rat liver plasma membranes. This protein is immunologically identical with microsomal PDI prepared from bovine liver. The plasma membrane form of PDI is, however, not labeled by photoactivated bumetanide as revealed by two-dimensional gel electrophoresis. These results indicate that, although a membrane-bound form of the PDI is present in the sinusoidal plasma membrane of rat hepatocytes, this protein is not involved in the hepatocellular uptake of the organic anion bumetanide.