title: Effect of the co-existence of galactosyl and phosphomannosyl residues on beta-hexosaminidase on the processing and transport of the enzyme in mucolipidosis I fibroblasts.
abstract: Cultured fibroblasts from patients with the lysosomal storage disease, mucolipidosis II, produce complex glycosylated lysosomal enzymes which are preferentially excreted presumably due to the absence of specific phosphomannosyl recognition residues needed for intracellular retention. Complex glycosylated hydrolases are also produced by fibroblasts from patients with mucolipidosis I but an abnormal excretion is not apparent in this disorder. Intra- and extracellular distribution, lectin binding, and specific endocytosis were criteria used to compared the properties of intra- and extracellular beta-hexosaminidase derived from mucolipidosis I and normal fibroblast cultures. Mucolipidosis I fibroblasts did not hyperexcrete beta-hexosaminidase when maintained in serum-free medium. Using the specificity of ricin binding to terminal galactosyl residues, the most galactosylated forms of the enzyme derived from mucolipidosis I cell extracts and culture fluids were found in the mucolipidosis I cell extracts (50% of total enzyme). Mucolipidosis I-excreted beta-hexosaminidase which was eluted from ricin-120-Sepharose, was a high-uptake form in endocytosis experiments while unbound enzyme was a low-uptake form. These data suggest that beta-hexosaminidase molecules contained phosphomannosyl residues necessary for receptor-mediated endocytosis as well as galactosyl residues on the same molecule. The co-existence of complex chains with high-mannose chains did not interfere with the phosphomannose-mediated endocytosis of beta-hexosaminidase nor with the retention of endogenous enzyme. We can speculate that since complex oligosaccharide chains in the mucolipidosis I cellular enzyme persist due to a sialidase deficiency, more extensive sialylation of cellular enzyme in normal fibroblasts probably occurs at some point during post-translational processing. However, the presence of sialidase in normal cells initiates complex chain trimming in the lysosomes resulting in a less glycosylated end product.
mesh_terms: D001692:Biological Transport; D002478:Cells, Cultured; D055598:Chemical Phenomena; D002621:Chemistry; D004705:Endocytosis; D005347:Fibroblasts / Q000201:enzymology; D005690:Galactose / Q000378:metabolism*; D006596:Hexosaminidases / Q000378:metabolism*; D006600:Hexosephosphates / Q000378:metabolism*; D006801:Humans; D008360:Mannosephosphates / Q000378:metabolism*; D009081:Mucolipidoses / Q000201:enzymology*; D009844:Oligosaccharides / Q000378:metabolism*; D012276:Ricin; D001619:beta-N-Acetylhexosaminidases
chemical_list: D006600:Hexosephosphates; D008360:Mannosephosphates; D009844:Oligosaccharides; D012276:Ricin; D006596:Hexosaminidases; D001619:beta-N-Acetylhexosaminidases; D005690:Galactose