NADH oxidase activity of HeLa plasma membranes inhibited by the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl) urea (LY181984) at an external site.

NADH oxidase activity from HeLa plasma membranes was inhibited by the antitumor sulfonylurea N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984). With sealed right side-out vesicles, the drug inhibited half maximally at about 30 nM and the inhibition was nearly complete. A closely related but growth-inactive sulfonylurea, N-(4-methylphenylsulfonyl)-N'-(phenyl)urea (LY181985), did not inhibit the activity. With plasma membranes first solubilized with 2% Triton X-100, activity also was inhibited by LY181984 and not by LY181985 but the maximum inhibition at 10 microM LY181984 was only 50%. When sealed right side-out plasma membrane vesicles were frozen and thawed repeatedly to evert some of the vesicles into an inside-out configuration, the NADH oxidase activity again was only about 50% inhibited by 1 microM LY181984. In such preparations, the right side-out vesicles exhibited an electrophoretic mobility greater than that of the inside-out vesicles. Sidedness was confiremd by measurements of ATPase latency and binding of immunogold-labeled concanavalin A. When the two vesicle populations were resolved by preparative free-flow electrophoresis, the active antitumor sulfonylurea LY181984 inhibited only the NADH oxidase activity of the right side-out vesicles. These findings suggested two NADH sites or activity isoforms for the plasma membrane NADH oxidase. One activity, inhibited by LY181984, appeared to be accessible to external NADH only with sealed right side-out vesicles. The other, not inhibited by LY181984, was accessible to NADH only with inside-out vesicles or after membrane disruption by Triton X-100. The findings demonstrate that the NADH oxidation site inhibited as a result of binding the active antitumor sulfonylurea LY181984 is at the external cell surface. Plasma membrane vesicles from HeLa cells are able to oxidize NADH supplied to either membrane surface but only with inside-out vesicles is NADH oxidation sensitive to inhibition by the antitumor sulfonylurea.