title: Evidence for lysosomal enzyme recognition by human fibroblasts via a phosphorylated carbohydrate moiety.
abstract: Adsorptive endocytosis of five different lysosomal enzymes from various human and non-human sources was susceptible to inhibition by mannose and l-fucose, methyl alpha-d-mannoside, alpha-anomeric p-nitrophenyl glycosides of mannose and l-fucose, mannose 6-phosphate and fructose 1-phosphate. A few exceptions from this general scheme were observed for particular enzymes, particularly for beta-glucuronidase from human urine. The inhibition of alpha-N-acetylglucosaminidase endocytosis by mannose, p-nitrophenyl alpha-d-mannoside and mannose 6-phosphate was shown to be competitive. The loss of endocytosis after alkaline phosphatase treatment of lysosomal enzymes supports the hypothesis that the phosphorylated sugars compete with a phosphorylated carbohydrate on the enzymes for binding to the cell-surface receptors [Kaplan, Achord & Sly (1977) Proc. Natl. Acad. Sci. U.S.A.74, 2026-2030]. Endocytosis of ;low-uptake' forms of alpha-N-acetylglucosaminidase and alpha-mannosidase was likewise susceptible to inhibition by sugar phosphates and by alkaline phosphatase treatment, suggesting that ;low-uptake' forms are either contaminated with ;high-uptake' forms or are internalized via the same route as ;high-uptake' forms. The existence of an alternative route for adsorptive endocytosis of lysosomal enzymes is indicated by the unaffected adsorptive endocytosis of rat liver beta-glucuronidase in the presence of phosphorylated sugars and after treatment with alkaline phosphatase.
mesh_terms: D000118:Acetylglucosaminidase / Q000037:antagonists & inhibitors; D000469:Alkaline Phosphatase / Q000494:pharmacology; D002241:Carbohydrates / Q000494:pharmacology; D002478:Cells, Cultured; D004705:Endocytosis / Q000187:drug effects*; D005347:Fibroblasts / Q000378:metabolism*; D005966:Glucuronidase / Q000378:metabolism; D006023:Glycoproteins / Q000494:pharmacology; D006801:Humans; D008247:Lysosomes / Q000201:enzymology*; D008361:Mannosidases / Q000378:metabolism; D013403:Sugar Phosphates / Q000494:pharmacology*
chemical_list: D002241:Carbohydrates; D006023:Glycoproteins; D013403:Sugar Phosphates; D000469:Alkaline Phosphatase; D008361:Mannosidases; D005966:Glucuronidase; D000118:Acetylglucosaminidase