Reduced coenzyme Q
10 (CoQ
10H
2) is known as a potent antioxidant in biological systems. However, it is not yet known whether CoQ
9H
2 could act as an antioxidant in human cells. The aim of this study is to assess whether exogenously added CoQ
9 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). CoQ
9-enriched cells were obtained by treatment of HepG2 cells with 10 μM CoQ
9 liposomes for 24 h. CoQ
9-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 CoQ
9-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 CoQ
9-enriched cells. The incubation of CoQ
9-enriched cells with AAPH or AMVN led to a decrease in cellular CoQ
9H
2 and reciprocal increase in cellular CoQ
9 resulting from its antioxidant function. Taken together, it was demonstrated for the first time that exogenously added CoQ
9 could prevent oxidative stress-mediated damage to human cells by virtue of its antioxidant activity.
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