EFFECT OF GLYCOSIDASE INHIBITORS ON THE BIOSYNTHESIS OF ALPHA-2-PLASMIN INHIBITOR AND ANTITHROMBIN-III IN HEP G2 CELLS

We studied the effect of the glucosidase I inhibitor, N-methyl-1-deoxy nojirimycin (MdN) and the mannosidase inhibitor, 1-deoxymannojirimycin (dMM) on the biosynthesis and secretion of alpha 2-plasmin inhibitor (alpha 2-PI) and antithrombin III (ATIII) in cultures of human hepatom a (Hep-G2) cells. Incubation with 1 mM MdN decreased secreted alpha 2- PI activity and antigen levels by about 40%, whereas those of ATIII we re not affected. Neither inhibitor affected the messenger RNA levels a s determined by Northern blotting. Pulse-chase studies using [S-35]-me thionine showed that MdN decreased alpha 2-PI and ATIII secretion rate s. By the 18 h chase, MdN had decreased secreted alpha 2-PI to 50-60%, with little effect on ATIII. Intracellular forms of alpha 2-PI or ATI II synthesized by cells treated with 1 mM MdN were sensitive to endogl ycosidase H (Endo H), whereas almost all the secreted forms were resis tant, suggesting the presence of complex-type oligosaccharides. In the presence of 1 mM dMM, cells synthesized Endo H-sensitive alpha 2-PI a nd ATIII with similar secretion rates. These results suggest that rete ntion of glucose on N-linked oligosaccharides not only retards the exi t of alpha 2-PI and ATIII, but also changes the catabolic rate of alph a 2-PI in the endoplasmic reticulum.