Ethanol effects on hepatic proteolytic systems

Excessive ethanol consumption causes liver injury, resulting in steatosis (fatty liver), alcoholic hepatitis, fibrosis, and ultimately cirrhosis. About 15% of alcoholics develop cirrhosis, this particular liver disease is normally irreversible. However, alcoholic fatty liver and, alcoholic hepatitis can be reversed. Ethanol-induced liver injury is, in part, caused by oxidative stress resulting from enhanced oxidant formation. One oxidant, peroxynitrite (PN), is a by-product of ethanol metabolism and is generated by the induction of nitric oxide synthase. This leads to the increased generation of nitric oxide which reacts with superoxide to form PN. PN causes the nitration of proteins by forming 3-nitrotyrosine adducts on their tyrosine residues. Three-nitrotyrosine adducts can alter the biological activities of proteins. Degradation of proteins also can be altered by the stress of oxidants on the lysosome and proteasome, the two principal proteolytic systems of the cells. The goal of this research project was to define how ethanol-induced oxidation and/or ethanol-elicited nitration of proteins affects the overall activity and susceptibility of these modified proteins to degradation, and to determine how these alterations may ultimately affect liver cell viability. We first hypothesized that nitration of proteins not only inactivates them but also alters their susceptibility to degradation by proteases. To test this hypothesis, we used liver cell proteins and a hen egg lysozyme, to assess the effects of nitration by PN and/or PN donors on protein function, and susceptibility to proteasomal degradation in an isolated system.;Next, we hypothesized that the lack of cellular Cu/Zn-SOD, an enzyme that prevents cellular damage from superoxide, would induce increased generation of oxidants, thereby causing damage to hepatic proteins, and the hepatic proteolytic system. We used SOD1 null and wild type mice to determine how ethanol-induced oxidation and nitration altered hepatic proteins and how hepatic proteolytic systems are effected by ethanol-induced oxidative stress in vivo. An understanding of the mechanism by which ethanol-induced oxidative stress affects hepatic proteolytic systems, may lead to the development of experimental strategies to facilitate the repair or removal of damaged proteins in the cell, possibly minimize liver injury resulting from ethanol consumption as well as other hepatotoxins.