Molecular defect in combined beta-galactosidase and neuraminidase deficiency in man.
In normal human fibroblasts, an enzymically active 85,000-dalton precursor form of beta-galactosidase is processed, via a number of intermediates, into a mature 64,000-dalton form. In addition there is an enzymically inactive 32,000-dalton component and its 54,000-dalton precursor. In fibroblasts from patients with a combined deficiency of beta-galactosidase and neuraminidase these last two components are absent and hardly any mature beta-galactosidase can be demonstrated. Nevertheless, in the mutant fibroblasts, precursor beta-galactosidase is synthesized and processed normally. The excessive intralysosomal degradation that is responsible for the deficiency of mature beta-galactosidase can be partially corrected by addition of the protease inhibitor leupeptin, which results in the accumulation of 85,000-dalton precursor beta-galactosidase and of a partially processed 66,000-dalton form. When mutant cells were grown in the presence of a "corrective factor" purified from the medium of NH4Cl-stimulated cell cultures, both beta-galactosidase and neuraminidase activities were restored to low control levels. The immunoprecipitation pattern was completely normal after addition of the corrective factor, and mature 64,000-dalton beta-galactosidase accumulated in the mutant fibroblasts. We propose that the combined beta-galactosidase/neuraminidase deficiency is caused by a defective 32,000-dalton glycoprotein which is normally required to protect beta-galactosidase and neuraminidase against excessive intralysosomal degradation and to give these enzymes their full hydrolytic activity.
D002478:Cells, Cultured; D004792:Enzyme Precursors / Q000378:metabolism; D005347:Fibroblasts / Q000201:enzymology; D006801:Humans; D007787:Lactose Intolerance / Q000378:metabolism*; D007976:Leupeptins / Q000494:pharmacology; D008247:Lysosomes / Q000378:metabolism; D008970:Molecular Weight; D009439:Neuraminidase / Q000172:deficiency* / Q000235:genetics; D001616:beta-Galactosidase / Q000235:genetics
D004792:Enzyme Precursors; D007976:Leupeptins; D009439:Neuraminidase; D001616:beta-Galactosidase; C032854:leupeptin