Redox Levels of Intravenously Administered [¹⁴C] Coenzyme Q₁₀ and Coenzyme Q₁₀-Reducing Activity in Subcellular Fractions of Guinea Pig Liver

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The blood level of [¹⁴C]coenzyme Q₁₀ and the redox levels of [¹⁴C]coenzyme Q₁₀ in the liver and heart were measured after in-travenous injection of [14^C]coenzyme Q₁₀ solubilized in multilamellar liposomes into guinea pigs. The blood level of radioactivity declined biexponentially with half-lives of 11.5 min and 15.6 h in the first and second phases, respectively. The levels of reduced [¹⁴C]coenzyme Q₁₀ in the liver and heart reached 55.8 and 46.4%, respectively, of the labeled compound in the tissues at 30min after the injection. Coenzyme Q₁₀-reducing activity in cytosol, microsomes and mitochondria was also investigated. This activity was found in all the fractions. The total activity was the highest in the liver cytosol. Moreover, the results of experiments using a purified enzyme suggested that one of the coenzyme Q₁₀-reducing enzymes was NAD(P)H : quinone oxidoreductase [EC 1.6.99.2, DT-di-aphorase]. These results are discussed in relation to the protective effect of reduced coenzyme Q₁₀ against lipid peroxidation in membranes.