Inhibition by ethanol of rat liver plasma membrane (Na +, K +)ATPase: Protective effect of S-adenosyl- l-methionine, l-methionine, and N-acetylcysteine

Abstract

(Na +, K +)ATPase activity of rat liver plasma membranes was evaluated in female rats feeding an ethanol containing diet for 46 days (total ethanol ingested, 59.7 g/100 g body wt). Determinations were performed at the end of ethanol treatment or at various times after stopping treatment. (Na +, K +)ATPase and 5′-nucleotidase activities exhibited a 8- and 1.4-fold decrease, respectively, at the end of ethanol ingestion. In contrast no modifications of Mg 2+-ATPase activity were observed. There also occurred, in ethanol-treated rats, release of sorbitol dehydrogenase into the blood, fat accumulation in liver cells, and decrease in reduced glutathione (GSH) liver content. A decrease in (Na +, K +)ATPase activity was also found in plasma membranes isolated from hepatocyte suspensions after a 2-hr incubation with 50 m m ethanol or 1 m m acetaldehyde (ACA), in conditions that caused a great fall in hepatocyte GSH content but did not cause cell death. After the cessation of ethanol administration, there occurred a progressive recovery of (Na +, K +)ATPase activity, GSH and triacylglycerol content, and release of sorbitol dehydrogenase. These parameters reached control values 12 hr after ethanol withdrawal. S-Adenosyl- l-methionine (SAM), l-methionine, and N-acetylcysteine (NAC), given to rats during ethanol treatment, prevented the decrease in (Na +, K +)ATPase activity and GSH content. They also reduced steatosis and liver necrosis. The efficiency of these compounds decreased in this order: SAM, methionine, NAC. SAM accelerated the recovery of all parameters studied after ethanol withdrawal, and also protected (Na +, K +)ATPase activity and GSH content of isolated hepatocytes from the deleterious effect of ethanol. These SAM effects were prevented by 1-chloro-2,4-dinitrobenzene, a compound which depletes cell GSH. Treatment of isolated hepatocytes with [ 35S]SAM led to the synthesis of labeled GSH. The total amount and specific activity of labeled GSH underwent a significant increase, in the presence of 2 m m ethanol or 0.5 m m ACA, which indicates a marked stimulation of GSH synthesis by ethanol and ACA. These data indicate that ethanol intoxication may inhibit (Na +, K +)ATPase activity; an effect that does not seem to depend on cell necrosis. SAM, methionine, and NAC exert various degrees of protection toward ethanol-induced cell injury, which are related to the efficiency of these compounds in maintaining a high GSH pool.