Glycohydrolases in diabetes: characterization of acid α-glucosidase from liver and neutral α-glucosidase from sera of diabetic patients and controls

Abstract

Acid alpha-glucosidase activity measured in the supernatant fraction of liver homogenates obtained from adult-onset diabetic patients is significantly decreased when compared to controls (2.86 +/- 1.18 and 5.79 +/- 1.82 nmoles/min/mg protein +/- S.D., respectively). The biochemical properties (Km values, thermostability, pH optimum, isoelectric focusing profiles) of acid alpha-glucosidase obtained from the livers of diabetic patients were similar to those of acid alpha-glucosidase obtained from the livers of controls. Mixing studies gave additivity of acid alpha-glucosidase activity suggesting that neither inhibitors nor activators are present (in diabetic and control livers, respectively). Neutral alpha-glucosidase activity measured in the sera of diabetic patients was significantly increased when compared to controls (4.35 +/- 1.82 and 2.44 +/- 1.05 nmoles/h/ml +/- S.D., respectively). Neutral alpha-glucosidase in the sera of diabetic and control patients has a similar pH optimum but the enzyme in the serum of diabetics has a slightly lower apparent Km value for the 4-methylumbelliferyl substrate (0.6 vs. 0.9 mmol/L) and slightly increased thermostability. Experiments involving dialysis of patient serum, addition of glucose to patient serum and mixing of control and diabetic patient sera all suggest that glucose exhibits only slight inhibition of serum neutral alpha-glucosidase activity. Isoelectric focusing indicates that neutral alpha-glucosidase activity in the sera of diabetic patients is consistently different from the enzyme in the sera of control patients in that a significantly smaller percentage of activity is found in the acidic region with pI values less than 4.8.