A multifunctional hydroquinone oxidase of the external cell surface and sera

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Abstract
A multifunctional cell surface protein with NADH oxidase (NOX) activity and capable of oxidizing hydroquinones is located at the exterior of the cell and is shed in soluble form into sera. The oxidase appears to function as a terminal oxidase of a trans plasma membrane electron transport chain consisting of a NAD(P)H‐ubiquinone reductase at the cytosolic membrane surface, possibly a b‐type cytochrome, ubiquinone and the oxidase. Hyperactivity or conditions that interrupt ordered 2H+ + 2e− transport from NAD(P)H or hydroquinone to molecular oxygen and other acceptors at the external cell surface may result in the generation of superoxide. The latter may serve to propagate aging‐related redox changes both to adjacent cells and circulating blood components. A circulating NOX activity form associated with aging and the reduction of cytochrome c by sera of aged patients that is partially inhibited by ubiquinone are described.