Publisher Summary Quinone oxidoreductases of the plasma membrane relate functionally to the operation of a cell surface redox chain, where cytosolic NAD(P)H is oxidized. Plasma membrane quinones serve as lipid-soluble transmembrane shuttles to transfer the 2H+ + 2e- from NAD(P)H to 1/2 O2 to form water. The reduction of 1/2 O2 is at the expense of hydroquinone catalyzed by cell surface hydroquinone oxidases. The first demonstration of a redox-related plasma membrane enzyme was that of an NADH-ferricyanide reductase observed with purified fractions of plasma membranes isolated from rat liver. A plasma membrane location was subsequently confirmed by electron microscope cytochemistry. Involvement in plasma membrane electron transport was inferred from observations in which ferricyanide and other impermeant oxidants were reduced by intact cells. Quinone oxidoreductases, of the plasma membrane, have physiological significance as components in support of plasma membrane electron transport. Among these are the ECTO-NOX proteins that comprise a family of NAD(P)H oxidases of plants and animals that exhibit both oxidative and protein disulfide isomerase-like activities that alternate and serve as terminal oxidases for plasma membrane electron transport. A unique feature of the ECTO-NOX proteins is that the two enzymatic activities they catalyze, hydroquinone (NADH) oxidation and disulfide–thiol interchange, alternate within a 24-min period.