Protection by Exogenously Added Coenzyme Q9 against Free Radical-Induced Injuries in Human Liver Cells

Reduced coenzyme Q10 (CoQ10H2) is known as a potent antioxidant in biological systems. However, it is not yet known whether CoQ9H2 could act as an antioxidant in human cells. The aim of this study is to assess whether exogenously added CoQ9 can protect human liver cells against injuries induced by a water-soluble radical initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) and a lipid-soluble radical initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). CoQ9-enriched cells were obtained by treatment of HepG2 cells with 10 µM CoQ9 liposomes for 24 h. CoQ9-enriched cells were exposed to 10 mM AAPH and 500 µM AMVN over 4 h and 24 h, respectively. The loss of viability after treatment with AAPH or AMVN was much less in CoQ9-enriched cells than in naive HepG2 cells. The decrease in glutathione and the increase in thiobarbituric acid-reactive substance after treatment with AAPH or AMVN were also suppressed in CoQ9-enriched cells. The incubation of CoQ9-enriched cells with AAPH or AMVN led to a decrease in cellular CoQ9H2 and reciprocal increase in cellular CoQ9 resulting from its antioxidant function. Taken together, it was demonstrated for the first time that exogenously added CoQ9 could prevent oxidative stress-mediated damage to human cells by virtue of its antioxidant activity.