[Effect of chlorpropamide and of phenformin on isolated liver plasmatic membranes]

Previous observations from this laboratory suggest that chlorpropamide and phenformin, two hypoglycemic drugs belonging to sulfonylureas and biguanides respectively, act on liver plasma membranes altering the activity of enzymes bound to plasma membrane as (Na+-K+)-ATPase. This enzyme, an integral membrane protein which shows a cooperative behavior, has been used as an allosteric probe able to give information on plasma membrane. Our experiments show that (Na+-K+)-ATPase has a positive cooperative behaviour, as suggested from Hill coefficient n > l. The two hypoglycemic drugs decrease the Hill coefficient; in addition both chlorpropamide and phenformin inhibit (Na+-K+)-dependent but not Mg2+-dependent ATPase activity.