Formation of malondialdehyde adducts in livers of rats exposed to ethanol:Role in ethanol-mediated inhibition of cytochrome c oxidase

Background: Previous studies in our laboratory demonstrated that short-term ethanol consumption by maternal rats increased the hepatic levels of 4-hyd roxynonenal (HNE) in both the adult and the fetus. Additionally, HNE inhibi ted cytochrome c oxidase (COX) by forming adducts with the enzyme subunits. The present study examined modification of COX by another major aldehydic lipid peroxidation product, malondialdehyde (MDA), and its role in COX inhi bition by ethanol. Methods and Results: It is demonstrated in vitro that MDA inhibits the acti vity of purified COX while forming adducts with the enzyme. Compared with H NE, MDA is a more potent inhibitor of COX Overnight incubation at room temp erature caused an 80% decrease in COX activity by MDA versus a 67% decrease by HNE. MDA produced marked inhibition of COX activity at physiologically relevant concentrations, e.g., 43% inhibition at 10 mu M. Although our prev ious studies documented that HNE formed adducts primarily with subunit IV o f COX via histidine residues, the current report showed that MDA forms addu cts with both subunit IV and subunit V via lysine residues. Furthermore, bo th aldehydes induce carbonyl formation in subunit IV. The in vivo role of M DA in the impairment of COX by ethanol is assessed in both adult and fetal Liver after maternal ethanol consumption. Conclusions: The results showed that: (1) there are significant increases i n MDA levels in liver homogenate as well as mitochondria in both adult and fetal livers after ethanol exposure; (2) these MDA levels are in the nanomo le/mg protein range, in contrast to picomole/mg protein range of HNE in ide ntical setting; and (3) ethanol-induced production of MDA is accompanied by enhanced formation of MDA adducts with COX. These findings suggest that MD A may play at least as equally an important role as MNE in ethanol-induced inhibition of COX.