| Abstract: | The effects of alpha-D-mannopyranosylmethyl-p-nitrophenyltriazene (alpha-ManMNT) on the degradation and biosynthesis of oligosaccharide chains on alpha 1-acid glycoprotein (AGP) were studied. Addition of the triazene to a perfused liver prevented the complete degradation of endocytosed N-acetyl14C]glucosamine-labeled asialo-AGP and caused the accumulation of Man2GlcNAc1 fragments in the lysosome-enriched fraction of the liver homogenate. This compound also reduced the reincorporation of lysosomally derived 14C]GlcNAc into newly secreted glycoproteins. Cultured hepatocytes treated with the inhibitor synthesized and secreted fully glycosylated AGP. However, the N-linked oligosaccharide chains on AGP secreted by the alpha-ManMNT-treated hepatocytes remained sensitive to digestion with endoglycosidase H, were resistant to neuraminidase, and consisted of Man9-7GlcNAc2 structures as analyzed by high resolution Bio-Gel P-4 chromatography. As measured by their resistance to cleavage by endoglycosidase H, the normal processing of all six carbohydrate chains on AGP to the complex form did not completely resume until nearly 24 h after triazene treatment. Since alpha-ManMNT is likely to irreversibly inactivate alpha-D-mannosidases, the return of normal AGP secretory forms after 24 h probably resulted from synthesis of new processing enzymes. |
