The plasma membrane NADH oxidase of HeLa cells has hydroquinone oxidase activity

The plasma membrane NADH oxidase activity partially purified from the surfa ce of HeLa cells exhibited hydroquinone oxidase activity. The preparations completely lacked NADH:ubiquinone reductase activity. However, in the absen ce 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 purific ation. The apparent K-m, for Q(10)H(2) oxidation was 33 mu M. Activities we re inhibited competitively by the cancer cell-specific NADH oxidase inhibit ors, 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 amo unts 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 membran e electron transport from cytosolic NAD(P)H via naturally occurring hydroqu inones to accepters at the cell surface. (C) 1999 Elsevier Science B.V. All rights reserved.