The plasma membrane NADH oxidase activity partially purified from the surface of HeLa cells exhibited hydroquinone oxidase activity. The preparations completely lacked NADH:ubiquinone reductase activity. However, in the absence of NADH, reduced coenzyme Q10 (Q(10)H(2) = ubiquinol) was oxidized at a rate of 15 +/- 6 nmol min(-1) mg protein(-1) depending on degree of purification. The apparent K-m, for Q(10)H(2) oxidation was 33 mu M. Activities were inhibited competitively by the cancer cell-specific NADH oxidase inhibitors, capsaicin and the antitumor sulfonylurea N-(4-methylphenylsulfony)-N'-(4-chlorophenyl)urea (LY181984). With coenzyme Q(0), where the preparations were unable to carry out either NADH:quinone reduction or reduced quinone oxidation, quinol oxidation was observed with an equal mixture of the Q(0) and Q(0)H(2) forms. With the mixture, a rate of Q(0)H(2) oxidation of 8-17 nmol min(-1) mg protein(-1) was observed with an apparent K-m, of 0.22 mM. The rate of Q(10)H(2) oxidation was not stimulated by addition of equal amounts of Q(10) and Q(10)H(2). However, addition of Q(0) to the Q(10)H(2) did stimulate. The o.uidation of Q(10)H(2) proceeded with what appeared to be a two-electron transfer. The oxidation of Q(0)H(2) may involve Q(0) but the mechanism was not clear. The findings suggest the potential participation of the plasma membrane NADH oxidase as a terminal oxidase of plasma membrane electron transport from cytosolic NAD(P)H via naturally occurring hydroquinones to accepters at the cell surface. (C) 1999 Elsevier Science B.V. All rights reserved.
