Erythrocyte membrane digoxin-sensitive (Na(+)-K+)-ATPase of non-insulin dependent diabetic humans

Erythrocyte plasma membranes of non-insulin dependent diabetic humans (NIDDM) and healthy humans were prepared by hypotonic lysis. The specific activity of (Na(+)-K+)-ATPase of NIDDM membranes, both in the absence and presence of digoxin were lower than the specific activity of normal enzymes (83.6 percent and 74.0 percent of the normal enzyme respectively). Addition of digoxin decreased the activity of this enzyme (38.0 percent in NIDDM and 30.0 percent in normal enzyme). Although the affinity of the pump for ATP was similar in both membranes of NIDDM and normal humans (K(m) for ATP = 19.9 +/- 0.24 microM ATP and 20.0 +/- 0.21 microM ATP respectively), the Vmax of NIDDM membranes was more than 20 percent lower than that of the normal enzyme. The specific activity of Mg(2+)-dependent Ca(2+)-pumping ATPase (Ca(2+)-Mg(2+)-ATPase) of NIDDM membrane was lower than 80 percent of the specific activity of the normal enzymes. While the affinity of the pump for ATP was lower in the membranes of NIDDM (Km for ATP = 50.0 +/- 4.3 microM ATP) in comparison to normal membranes (Km for ATP = 63.1 +/- 38 microM ATP), the Vmax of NIDDM membranes was similar to the normal enzyme. Altogether, these findings suggest that both the (Na(+)-K+)-ATPase and Ca(2+)-pumping ATPase of NIDDM membranes are less functional than the enzymes in normal erythrocytes.