Impaired proteolytic processing of lysosomal N-acetyl-β-Hexosaminidase in cultured fibroblasts from patients with infantile generalized N-acetylneuraminic acid storage disease

Cultured skin fibroblasts from patients suffering with infantile generalized N-acetylneuraminic acid (NeuAc) storage disease accumulate free NeuAc in a population of lysosomes less dense than those observed in normal fibroblasts (1.035 vs. >1.07 mean density), as assessed by the distribution of lysosomal enzyme activities and NeuAc on Percoll gradients after subcellular fractionation. In the present study, normal and affected fibroblasts were labeled with [ 35 S]methionine, and cell homogenates or subcellular fractions from Percoll gradients were immunoprecipitated with polyclonal antibodies to lysosomal N-acetyl-β-hexosaminidase (Hex); immunoprecipitated polypeptides were analyzed by SDS-polyacrylamide gel electrophoresis. The synthesis and initial processing of Hex polypeptides were comparable in normal and affected fibroblasts, but mature polypeptides were quantitatively localized in “buoyant” lysosomes of affected cells, along with Hex activity; moreover, mature α-chain of Hex was ∼2 kDa larger than that observed in normal cells. The molecular weight difference was apparently due to impaired proteolytic processing of α-chain in affected fibroblasts, since treatment of immunoprecipitated α-chain from normal and affected cells with neuraminidase and endo-β-N-acetylglucosaminidase H failed to resolve the molecular weight difference. The impaired processing was observed to be persistent (after a chase of up to 200 h), but had no apparent effect on the turnover or activity of Hex in affected fibroblasts. The observed proteolytic processing defect may be primary or secondary in infantile NeuAc storage disease.