Secretion of high‐mannose‐type α 1 ‐proteinase inhibitor and α 1 ‐acid glycoprotein by primary cultures of rat hepatocytes in the presence of the mannosidase I inhibitor 1‐deoxymannojirimycin

Two different forms of α 1 ‐proteinase inhibitor and α 1 ‐acid glycoprotein were found in primary cultures of rat hepatocytes. After a 2.5‐h labeling period with [ 35 S]methionine the high‐mannose‐type precursor of α 1 ‐proteinase inhibitor ( M r 49000) and α 1 ‐acid glycoprotein ( M r 39000) and the mature‐complex‐type α 1 ‐proteinase inhibitor ( M r 54000) and α 1 ‐acid glycoprotein ( M r 43000–60000) could be immunoprecipitated from the cells, but only the complex‐type forms of the two glycoproteins were secreted into the hepatocyte media. When hepatocytes were incubated with the mannosidase I inhibitor 1‐deoxymannojirimycin at a concentration of 4 mM, the 49000‐ M r form of α 1 ‐proteinase inhibitor and the 39000‐ M r form of α 1 ‐acid glycoprotein could be detected in the cells as well as in their media. Neither the secretion of α 1 ‐proteinase inhibitor nor that of α 1 ‐acid glycoprotein was impaired by 1‐deoxymannojirimycin. While α 1 ‐proteinase inhibitor and α 1 ‐acid glycoprotein, secreted by control cells, were resistant to endoglucosaminidase H, α 1 ‐proteinase inhibitor and α 1 ‐acid glycoprotein, secreted by hepatocytes treated with 4mM 1‐deoxymannojirimycin, could be deglycosylated by endoglucosaminidase H. When the [ 3 H]mannose‐labeled oligosaccharides of α 1 ‐proteinase inhibitor, secreted by 1‐deoxymannojirimycin‐treated hepatocytes, were cleaved off by endoglucosaminidase H and analyzed by Bio‐Gel P‐4 chromatography, they eluted at the position of Man 9 GlcNAc, indicating that mannosidase I had been efficiently inhibited. 1‐Deoxymannojirimycin did not inhibit the synthesis or the cotranslational N ‐glycosylation of α 1 ‐proteinase inhibitor or α 1 ‐acid glycoprotein.