Lysosomal degradation of glycoproteins and glycosaminoglycans. Efflux and recycling of sulphate and N-acetylhexosamines.

Abstract

Lysosomal degradation of the carbohydrate portion of glycoproteins and glycosaminoglycans produces monosaccharides and sulphate, which must efflux from the lysosomes before re-entering biosynthetic pathways. We examined the degradation of glycoproteins and glycosaminoglycans by lysosomes isolated from cultured human diploid fibroblasts. Cells were grown for 24 h in medium containing [3H]glucosamine and [35S]sulphate. When lysosomes are isolated from these cells, they contain label primarily in macromolecules (glycoproteins and glycosaminoglycans). Glycoprotein degradation by isolated lysosomes was followed by measuring the release of tritiated sugars from macromolecules and efflux of these sugars from the organelles. Glycosaminoglycan degradation was monitored by the release of both tritiated sugars and [35S]sulphate. During macromolecule degradation, the total amounts of free [35S]sulphate, N-acetyl[3H]glucosamine and N-acetyl[3H]galactosamine found outside the lysosome parallels the amounts of these products released by degradation. The total degradation of glycoproteins and glycosaminoglycans by intact cultured cells was also examined. The lysosomal contribution to degradation was assessed by measuring inhibition by the lysosomotropic amine NH4Cl. After 48 h incubation, inhibition by NH4Cl exceeded 55% of glycoprotein and 72% of glycosaminoglycan degradation. Recycling of [3H]hexosamines and [35S]sulphate by intact cells was estimated by measuring the appearance of 'newly synthesized' radioactively labelled macromolecules in the medium. Sulphate does not appear to be appreciably recycled. N-Acetylglucosamine and N-acetylgalactosamine, on the other hand, are reutilized to a significant extent.