SECRETION, SURFACE LOCALIZATION, TURNOVER, AND STEADY-STATE EXPRESSION OF PROTEIN DISULFIDE-ISOMERASE IN RAT HEPATOCYTES

Protein disulfide isomerase in isolated rat hepatocytes was present at a concentration of 7 mu g/mg cell protein, representing a similar to 2-fold enrichment compared to isolated hepatic non-parenchymal cells. Though localized mainly in microsomal fractions of hepatocyte, direct immunofluorescence and cell surface radioiodination followed by immuno precipitation revealed the presence of M(r) 57,000 disulfide isomerase at the cell surface. Electrostatic interaction of the protein with th e cell surface was suggested by susceptibility to carbonate washing. M etabolic radiolabeling and immunoprecipitation studies also indicated that some of the newly synthesized M(r) 57,000 disulfide isomerase was secreted. Treatment of cells with colchicine markedly reduced the rec overy of disulfide isomerase from the media, indicating microtubular d irected secretion of the protein. Partial staphlococcal V8 proteolytic digestion of the secreted protein revealed a peptide pattern similar to that of the cellular protein. Immunoprecipitation with antibody spe cific to the -KDEL peptide retention sequence confirmed the presence o f this sequence in the secreted protein. Studies of the turnover of di sulfide isomerase revealed a half-life of approximately 96 h. Treatmen t of cells with tunicamycin or heat shock resulted in an increased rec overy of newly synthesized disulfide isomerase from cell lysates but d iminished recovery from the media. The secretion and cell surface dist ribution of disulfide isomerase in hepatocytes may be important for th e pathogenesis of immune mediated liver injury.