Abstract
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 Q 10 (Q 10H 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 10H 2 oxidation was 33 μM. Activities were inhibited competitively by the cancer cell-specific NADH oxidase inhibitors, capsaicin and the antitumor sulfonylurea N-(4-methylphenylsulfonyl)- 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 0H 2 forms. With the mixture, a rate of Q 0H 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 10H 2 oxidation was not stimulated by addition of equal amounts of Q 10 and Q 10H 2. However, addition of Q 0 to the Q 10H 2 did stimulate. The oxidation of Q 10H 2 proceeded with what appeared to be a two-electron transfer. The oxidation of Q 0H 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 acceptors at the cell surface.
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