Glycosylasparaginase as a Marker Enzyme in the Detec-

Glycosylasparaginase (GA; EC 3.5.1.26) is a lysosomal enzyme that cleaves the N-glycosidic bond between as-paragine and N-acetylglucosamine residues in the degra-dation of glycoproteins (1). Deficient enzyme activity leads to the lysosomal storage disease aspartylglyco-saminuria (McKusick 208400). Human GA is synthesized as a single, enzymatically inactive precursor polypeptide. The single-chain precursor is thought to be activated on its way to lysosomes (2, 3) by cleavage into two N-glycosylated subunits of 24 kDA (a) and 18 kDA (b) and association of these subunits to form enzymatically active heterodimer or heterotetramer structures (4). GA is ac-tively transported into human lysosomes via mannose-6-phosphate receptor-mediated endocytosis (5). I-Cell disease (mucolipidosis II, McKusick 252500) an