Effects of paraquat on Mg(2+)-ATPase activity in rat liver.

Intraperitoneal injection of paraquat (70 mg/kg) elicited a significant decrease of 20% in activity of Mg(2+)-ATPase in hepatic mitochondria, which is always surrounded by a high concentration of oxygen, in rats. The decrease of mitochondrial Mg(2+)-ATPase activity was completely abolished by pretreatment with vitamin E, which is a scavenger of oxygen radicals. On the other hand, paraquat administration did not change the Mg(2+)-ATPase activity in endoplasmic reticulum, which exists in an anaerobic condition in living cells. When liver microsomes were incubated with 1 mM paraquat under aerobic conditions, the Mg(2+)-ATPase activity was decreased by 42%. The decrease of Mg(2+)-ATPase activity was completely eliminated by pretreatment with vitamin E. Furthermore, lipid peroxidation in microsomes was tremendously increased by the addition of 1 mM paraquat under aerobic conditions. The increase of lipid peroxidation was completely abolished by preadministration of vitamin E in rats. The results suggest that the inhibition of Mg(2+)-ATPase activity induced by paraquat may be mediated by active oxygen, which is produced by the reaction of paraquat radicals; molecular oxygen may be involved in the induction of hepatic cell injury.