[Stimulation and inhibition induced by N-ethylmaleimide in hydrolysis of ATP in rat liver mitochondria].

: Data already reported have shown that in rat liver mitochondria N-ethylmaleimide (NEM) affects the energy dependent uptake of Ca2+ and K+ in a similar way as they do uncouplers and respiratory inhibitors. It was proposed that NEM could influence the energetic state of mitochondria, required to set up and maintain the gradient of cations across the inner membrane (1-3). In this paper the effect of NEM on the mitochondrial ATP-ase activity, has been tested. NEM added to intact mitochondria increases by more than 100% the hydrolysis of ATP supported by endogenous ATP-ase activity and inhibits the ATP-ase activity induced by couplers. Although the concentration of NEM is enought to inhibit the Pi-OH- exchange, in both cases all the Pi derived from ATP hydrolysis is found outside the mitochondria. The inhibition of uncoupler induced ATP-ase activity, the finding that the stimulatory effect of NEM is abolished by oligomycin and atractyloside and the observation that NEM does not influence the hydrolysis of ATP in sonic mitochondria, exclude the possibility of an unspecific damage of the inner membrane and of a direct effect on NEM on the ATP-ase system. The results obtained could be consistent with the possibility that in intact mitochondria nem may regulate the distribution across the inner membrane of H+ and/or K+. Concerning the problem of Pi transport our data are inconsistent with the existence of Pi-ADP cotransport (7) and with an involvement of dicarboxylate carrier.