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 speci fic activity of (Na+-K+)-ATPase of NIDDM membranes, both in the absenc e and presence of digoxin were lower than the specific activity of nor mal enzymes (83.6 percent and 74.0 percent of the normal enzyme respec tively). Addition of digoxin decreased the activity of this enzyme (38 .0 percent in NIDDM and 30.0 percent in normal enzyme). Although the a ffinity of the pump for ATP was similar in both membranes of NIDDM and normal humans (K-m for ATP = 19.9 +/- 0.24 mu M ATP and 20.0 +/- 0.21 mu M ATP respectively), the V-max of NIDDM membranes was more than 20 percent lower than that of the normal enzyme. The specific activity o f Mg2+-dependent Ca2+-pumping ATPase (Ca2+-Mg2+)-ATPase) of NIDDM memb rane 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 membr anes of NIDDM (K-m for ATP = 50.0 +/- 4.3 mu M ATP) in comparison to n ormal membranes (K-m for ATP = 63.1 +/- 38 mu M ATP), the V-max of NID DM membranes was similar to the normal enzyme. Altogether, these findi ngs suggest that both the (Na+-K+)-ATPase and Ca2+-pumping ATPase of N IDDM membranes are less functional than the enzymes in normal erythroc ytes.