Immediate reduction of cytochrome c by photoexcited NADH: reaction mechanism as revealed by flow-flash and rapid-scan studies.

Upon exposure of an aqueous solution of NADH and cytochrome c to a laser pulse at 355 nm under anaerobic conditions, cytochrome c is reduced within 1-2 ms to a maximal extent of 90%. The reduction proceeds in two phases: rapid reduction by hydrated electrons followed by bimolecular electron transfer from the NAD radical to ferric cytochrome c. In addition, a third reduction phase emerges in the presence of an appropriate concentration of molecular oxygen, where the superoxide anion is a reductant. As the oxygen concentration approaches 20% saturation, the cytochrome c reduction by NAD is abolished first and then the reduction by hydrated electrons, since molecular oxygen competes with cytochrome c for NAD and hydrated electrons. At 20% oxygen, cytochrome c is reduced almost exclusively by the superoxide anion, but the amount reduced on a single laser pulse is only one-fourth that reduced under anaerobic conditions. The second-order rate constants for the reduction of cytochrome c at pH 7.4 and 20 degrees C by NAD and the superoxide anion are 2.0 x 10(9) and 4.0 x 10(6) M-1 s-1, respectively.