Secretion of a precursor form of lysosomal alpha-glucosidase from the brush border of human kidney proximal tubule cells

We have shown previously (R.P.J. Oude Elferink, E.M. Brouwer-Kelder, I. Surya, A. Strijland, M. Kroos, A.J.J. Reuser, J.M. Tager, Eur. J. Biochem. 139, 489-495 (1984)) that human urine contains considerable amounts of a precursor form of lysosomal alpha-glucosidase (about 50% of the total alpha-glucosidase activity present). We have now purified alpha-glucosidase from human kidney. Only about 5 to 10% of the total lysosomal alpha-glucosidase present in kidney comprises the precursor form of the enzyme. By means of immunocytochemistry using monoclonal antibodies, the precursor of alpha-glucosidase was detected in the brush border of the proximal tubule cells. Taking into account the amount of precursor alpha-glucosidase excreted daily into the urine and the amount present in the kidneys, we conclude that extensive secretion of precursor alpha-glucosidase occurs from the brush border of the proximal tubules