| Abstract: | The stimulation-specific NADPH-dependent reduction of ubiquinone-1 (Q-1) in guinea-pig macrophages was studied. The activity was due neither to any modified product of the phagocytosis-specific NADPH oxidase nor to non-specific diaphorases of the cells, since the activity was measured in sonicated or detergent-disrupted cells by subtracting the activity in the resting cells from that in cells activated by phorbol 12-myristate 13-acetate. The activity was not mediated by superoxide anions, since strict anaerobic conditions were employed. The anaerobic reduction of Q-1 was NADPH-specific, like superoxide formation under aerobic conditions, and its maximal velocity was also essentially the same as that of superoxide formation. The oxidase does not directly reduce Q-1 under aerobic conditions [Nakamura, Murakami, Umei & Minakami (1985) FEBS Lett. 186, 215-218], and the electron transfer from NADPH to cytochrome c by the oxidase under aerobic conditions was not enhanced by the addition of Q-1. The observations indicate that the phagocytosis-specific NADPH oxidase reduces Q-1 and that oxygen competes with the reduction of Q-1. Q-1 seems to accept electrons not from the intermediary electron carriers of the oxidase but from the terminal oxygen-reducing site of the enzyme. |
