Glycosylasparaginase as a Marker Enzyme in the Detection of I-Cell Disease

Abstract

Glycosylasparaginase (GA; EC 3.5.1.26) is a lysosomal enzyme that cleaves the N-glycosidic bond between asparagine and N -acetylglucosamine residues in the degradation of glycoproteins (1). Deficient enzyme activity leads to the lysosomal storage disease aspartylglycosaminuria (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 (α) and 18 kDA (β) and association of these subunits to form enzymatically active heterodimer or heterotetramer structures (4). GA is actively transported into human lysosomes via mannose-6-phosphate receptor-mediated endocytosis (5). I-Cell disease (mucolipidosis II, McKusick 252500) and a clinically milder, form pseudo-Hurler polydystrophy (mucolipidosis III, McKusick 252600), are autosomal, recessively inherited lysosomal storage diseases in which the transport of newly synthesized lysosomal enzymes into lysosomes is affected (6). The defect is in the enzyme, UDP- N -acetylglucosamine:lysosomal enzyme N -acetylglucosamine-1-phosphotransferase (EC 2.7.8.17), which adds the phosphate trafficking signal to lysosomal enzymes. The activity of phosphotransferase can be measured using radioactive UDP- N -acetylglucosamine as the donor substrate and endogenous lysosomal enzymes or artificial carbohydrate structures as the assay acceptors (7). The defective synthesis of mannose-6-phosphate recognition markers on the carbohydrate moieties …