Alteration of kidney brush border membrane maltase in aging rats

The specific activities of membrane-bound maltase (alpha-d-glucoside glucohydrolase, EC 3.2.1.20) in renal cortex homogenates and isolated brush border membranes of senescent rats decreased about 30% compared to the specific activities of the enzyme from young adult animals. The decline was gradual and concomitant with the aging process. When the enzymes from rats of 25 and 6 months of age were solubilized and purified to homogeneity the same decrement with age was found, 32.5 and 46.1 units/mg of protein, respectively. This finding suggests that the decrease in maltase activity with age results from an alteration in the enzyme per se, rather than from a change in the enzyme's membrane environment, which was reflected secondarily as a loss in activity. Recoveries of enzyme activity and protein and fold-purification were similar for young and old maltase, indicating that the age-related difference in specific activities of the pure enzymes was not due to the selective purification of an altered species of enzyme. The age-associated difference in activity was not attributable to the presence of proteolytic activity in the homogenate nor to the presence of an activator in the young or an inhibitor in the old kidney. The pure enzymes from young adult and aged animals did not differ in molecular weight, electrophoretic mobility, amino acid composition and Km value. Circular dichroism spectra revealed that both the young and old enzymes contained beta-structure. However, the old enzyme had more helical structure than did the young enzyme, suggesting a conformational alteration with age.