Extrecellular reduction of ubiquinone-1 and -10 by human Hep G2 and blood cells

Abstract

Ubiquinol-10 (CoQ 10H 2) is present in human low density lipoproteins (LDL) where it contributes significantly to the antioxidant defenses against radical-mediated oxidative damage. As CoQ 10H 2 becomes oxidized to ubiquinone-10 (CoQ 10) during the earliest stages of in vitro oxidation of LDL, we investigated a possible cellular recycling of oxidized CoQ 10H 2, adding CoQ 10 or its ambiphilic, short-chain analogue ubiquinone-1 (CoQ 1), to cells that are exposed to LDL in vivo. Whole blood, isolated red blood cells and human hepatoma Hep G2 cells (used as a model of hepatocytes) rapidly and efficiently reduced added CoQ 1 to ubiquinol-1 (CoQ 1H 2) detectable outside the cells. In whole blood the same steady-state level of CoQ 1H 2 was reached whether an equimolar amount of CoQ 1 or CoQ 1H 2 was added. Red cell membranes also showed some reducing activity, whereas CoQ 1 added to human blood plasma remained largely in its oxidized form. Cell- and membrane-mediated reduction of CoQ 1 was enhanced by NADH, FAD, or human plasma. In comparison to this rapid reduction of extracellular CoQ 1, formation of CoQ 10H 2 from CoQ 10 incorporated into human LDL by red blood and Hep G2 cells was slow. Our results show that although human blood cells and Hep G2 cells are endowed with a highly reducing activity for CoQ 1, the natural CoQ 10 does not appear to represent an efficient substrate for this activity.