The flavohaemoglobin Hmp of Escherichia coli is inducible by nitric oxide (NO) and provides protection both aerobically and anaerobically from inhibition of growth by NO and agents that cause nitrosative stress. Here we report rapid kinetic studies of NO binding to Fe(III) Hmp with a second order rate constant of 7.5U10 S M 3I s 3I to generate a nitrosyl adduct that was stable anoxically but decayed in the presence of air to reform the Fe(III) protein. NO displaced CO bound to dithionitereduced Hmp but, remarkably, CO recombined after only 2 s at room temperature indicative of NO reduction and dissociation from the haem. Addition of NO to anoxic NADH-reduced Hmp also generated a nitrosyl species which persisted while NADH was oxidised. These results are consistent with direct demonstration by membrane-inlet mass spectrometry of NO consumption and nitrous oxide production during anoxic incubation of NADH-reduced Hmp. The results demonstrate a new mechanism by which Hmp may eliminate NO under anoxic growth conditions. z 1999 Federation of European Biochemical Societies.
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.
A method was established to estimate the pH change of several buffers solutions on freezing by using a combination of pH indicators. Among more than 30 buffers solutions examined, almost half exhibited a pH change in the temperature range between freezing point and 220 degrees K; the results were tabulated. Glycerol was found to suppress the pH changes because of its "salt buffer" effect.
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