Enhanced proteolytic degradation of normal β-galactosidase in the lysosomal storage disease with combined β-galactosidase and neuraminidase deficiency

Abstract

The endocytosis of β-galactosidase purified from bovine and monkey testis was studied in cultured fibroblasts from patients with G M1-gangliosidosis, mucolipidosis I and combined β-galactosidase and neuraminidase deficiency (β-gal −/neur −). The uptake of 3H-labelled β-galactosidase was of the same order of magnitude in all cell strains. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate of cell free extracts after endocytosis indicated that in β-gal −/neur − cells the majority of ingested enzyme was degraded within 4 h, whereas the molecular weight remained unchanged in the other cell strains for at least 12 h. The intracellular accumulation of β-galactosidase activity in β-gal −/neur − cells levels off rapidly and the ultimate activity remains very low. In contrast, the β-galactosidase activity ingested by other cell strains increased almost linearly in time, and eventually exceeded the endogenous β-galactosidase activity in normal cells. Chase experiments indicated that the ingested β-galactosidase activity was short-lived in β-gal −/neur − cells and long-lived in the other cell strains. The enhanced degradation of ingested normal β-galactosidase in β-gal −/neur − fibroblasts could be prevented by addition of leupeptin, which inhibits the action of thiol-cathepsins. We conclude from these various experiments that the basic defect in combined β-galactosidase and neuraminidase deficiency results in an inadequate protection of both endogenous and exogenous β-galactosidase towards intralysosomal proteolytic degradation. The relationship to neuraminidase deficiency is discussed.