flavohaemoglobin (Hmp) reduced purified mitochondrial cytochrome c aerobically in a reaction that was not substantially inhibited by superoxide dismutase, demonstrating that superoxide anion, the product of O reduction by Hmp, did not contribute markedly to cytochrome reduction. Cytochrome was reduced by Hmp even in the presence of 0.5 mM CO, when the haem B was locked in the ferrous, low-spin state, demonstrating that electron transfer to cytochrome from NADH was via FAD, not haem. Hmp also reduced the ferrisiderophore complex Fe(III)-hydroxamate K from bv. anaerobically in a CO-insensitive manner, but at low rates and with low affinity for this substrate. The NADH-cytochrome oxidoreductase activity of Hmp was slightly sensitive to the binding and reduction of O at the haem. The of cytochrome reduction fell from 7.1 sin the presence of 0.5 mM CO to 5.0 sin the presence of 100 μM Owith no significant change in K for cytochrome (6.8 to 7.3 μM, respectively). O at near-micromolar concentrations diminished cytochrome reduction to a similar extent as did 100 μM O Thus, Hmp acts as a reductase of broad specificity, apparently without involvement of electron transfer via the globin-like haem. These data are consistent with the hypothesis that Hmp could act as an intracellular sensor of O since, in the absence of O electron flux from FAD to other electron acceptors increases. However, the nature of such acceptors is not known and alternative models for O sensing are also considered.


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