Effect of the Threonine Analog @-Hydroxynorvaline on the Glycosylation and Secretion of cul-Acid Glycoprotein by Rat Hepatocytes*

The threonine analog B-hydroxynorvaline (Hnv) is an inhibitor of asparagine-linked glycosylation. In the presence of the analog hepatocytes synthesized immunoreactive al-acid glycoprotein with 0-6 oligosaccharide chains. Pulse-chase experiments were conducted to compare the rates of secretion of al-acid glycoprotein from untreated, tunicamycin-treated, and Hnvtreated cells. Partially glycosylated (1-5 oligosaccharide chains) and unglycosylated (tunicamycin-inhibited) molecules exited the cells more slowly than native cyl-acid glycoprotein. In addition, secretion of fully glycosylated (6 oligosaccharide chains) al-acid glycoprotein was retarded in Hnv-treated cells when compared to controls. The slowest rate of secretion was exhibited by the unglycosylated form from Hnvtreated cells. These results suggest that Hnv-induced changes either in the extent of glycosylation or in the peptide sequence of al-acid glycoprotein can interfere with its transport through the cell. The major intracellular forms of al-acid glycoprotein from control and Hnv-treated cells were endoglycosidase H-sensitive and contained GlcNAcz oligosaccharide structures. The oligosaccharide chains on the secreted molecules from control and Hnv-treated c lls were entirely of the endoglycosidase H-resistant, complex type.