Acid α-glucosidase deficiency in cultured fibroblasts with phenotype 2 of acid α-glucosidase

The enzyme acid α-glucosidase shows a polymorphism determined by two alleles, namely phenotypes 1, 2-1 and 2. We report that the activity of acid α-glucosidase, measured in fibroblasts homozygous for isozyme 2 is significantly reduced. In fibroblasts harvested at early confluency the activity in the strain with phenotype 2 of acid α-glucosidase was 23% and 5% compared to the levels in normal phenotype 1 strains using maltose and glycogen as substrate, respectively. At late confluency, the same fibroblasts showed an increase in activity to 60% and 15% of normal, respectively. At both early and late confluency, two fibroblast strains with the adult form of acid α-glucosidase deficiency had activity of approximately 7% of the activity in normal phenotype 1 fibroblasts with both substrates. In general, an increased amount of glycogen was measured in deficient fibroblasts, including the strain with phenotype 2, but overlapping was observed with normal cells. These findings show that fibroblasts homozygous for the isozyme 2 allele have a reduced ability to hydrolyse the natural substrate, glycogen.