Antioxidant role of cellular reduced coenzyme Q homologs and alpha-tocopherol in free radical-induced injury of hepatocytes isolated from rats fed diets with different vitamin E contents.

Reduced coenzyme Q9 (CoQ9H2) and reduced coenzyme Q10 (CoQ10H2) as well as alpha-tocopherol (alpha-Toc) are known to be potent lipid-soluble antioxidants in mammalian tissues. Reduced coenzyme Q homolog (CoQnH2) appears to show antioxidant activity independent of that of alpha-Toc (Matsura, T., Yamada, K. and Kawasaki, T. (1992) Biochim. Biophys. Acta 1123, 309-315). To further confirm this, we have studied the antioxidant role of cellular CoQnH2 and alpha-Toc using hepatocytes isolated from rats fed diets containing deficient, sufficient, and excess amounts of vitamin E (VE). Cellular damage was induced with a hydrophilic radical initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). The concentration of alpha-Toc in VE-deficient hepatocytes was approximately 1/12 that in VE-sufficient hepatocytes, whereas the concentration of alpha-Toc in VE-excess hepatocytes was approximately 7-fold that in VE-sufficient hepatocytes. The molar ratios of alpha-Toc to CoQnH2 (CoQ9H2 plus CoQ10H2) in VE-deficient, sufficient and excess cells were 0.03, 0.33 and 2, respectively. In the hepatocytes in these three dietary groups, alpha-Toc status had little effect on the concentration of CoQ homologs. These hepatocytes were incubated with 50 mM AAPH for 4 h. The cell viability in all groups of hepatocytes decreased rapidly after 3 h of AAPH treatment, and was associated with the increase of lipid peroxides. The loss of cell viability and the increase of lipid peroxidation in VE-deficient cells were more pronounced than those in the hepatocytes of the other two groups. The endogenous CoQ9H2 content of each group of hepatocytes decreased linearly with a reciprocal increase in oxidized CoQ9 after addition of AAPH, whereas the decrease of endogenous CoQ10H2 in each group during AAPH treatment was much less than that of endogenous CoQ9H2. alpha-Toc in the three VE dietary groups of hepatocytes was also consumed without a time lag after addition of AAPH, and it was not spared by CoQnH2, even in VE-deficient cells where the CoQnH2 concentration was 38-fold that of alpha-Toc. These results indicate that CoQnH2, especially. CoQ9H2, is a lipid-soluble antioxidant, which is as effective as alpha-Toc in rat hepatocytes under the conditions employed in this study, and acts independently of alpha-Toc to inhibit lipid peroxidation.