Isolation and characterization of a precursor form of lysosomal alpha-glucosidase from human urine

A two-step procedure is described for the isolation of lysosomal alpha-glucosidase from human urine. In the second step, affinity chromatography on Sephadex G-100, two fractions with acid alpha-glucosidase activity were obtained. Fraction I contained alpha-glucosidase of Mr 109000, whereas fraction II contained components of Mr 76000 and 70000. alpha-Glucosidase in fraction I had an Mr similar to that of the precursor of alpha-glucosidase detected in the medium of fibroblasts after labelling with [14C]leucine. The components in fraction II had Mr identical to those of the mature forms of alpha-glucosidase found in placenta or cultured human skin fibroblasts. alpha-Glucosidase in fraction I contained mannose 6-phosphate (3.5 mol/mol polypeptide). No mannose 6-phosphate was present in the components in fraction II. Fraction I, but not fraction II, was avidly endocytosed by alpha-glucosidase-deficient cultured human skin fibroblasts. Endocytosis of fraction I was inhibited by mannose 6-phosphate. The pH optimum and Km values for p-nitrophenyl alpha-glucoside, maltose and glycogen of fractions I and II alpha-glucosidase were almost identical. However, the activity with glycogen relative to that of either p-nitrophenyl alpha-glucoside or maltose was lower in fraction I than in fraction II. It is concluded that fraction I consists of the precursor form of alpha-glucosidase and fraction II of the mature forms of the enzyme. The importance of urine as a source of precursors of lysosomal enzymes is discussed.