Abstract

Myoglobin (Mb) is reported in biochemistry and physiology textbooks to act as an O₂ reservoir and to facilitate O₂ diffusion from capillaries to mitochondria, to sustain cellular respiration. Recently, it has been proposed that Mb is an intracellular scavenger of bioactive nitric oxide (NO), regulating its level in the skeletal and cardiac muscle and thereby protecting mitochondrial respiration, which is impaired by NO. This novel function of Mb is based on the rapid and irreversible reaction of ferrous oxygenated Mb (MbO₂) with NO yielding ferric oxidized Mb (metMb) and nitrate (NO₃^-). The efficiency of this process, which is postulated to depend on the superoxide (O₂^-) character acquired by O₂ once bound to the heme iron, may be enhanced by intramolecular diffusion of NO trapped momentarily into cavities of the protein matrix. O₂ can also react with ferrous nitrosylated Mb (MbNO), albeit very slowly, leading to metMb and NO₃^-. The O₂-dependent NO-detoxification process may be considered to be pseudo-enzymatic given that metMb obtained by the primary reaction of MbO₂ with NO is reduced back to ferrous Mb by a specific metMb-reductase, and can therefore repeat a cycle of NO conversion to harmless nitrate.

Abbreviations: Hb: Hemoglobin; HbO₂: Ferrous oxygenated hemoglobin; Mb: Myoglobin; DeoxyMb: Ferrous deoxygenated Mb; MbO₂: Ferrous oxygenated Mb; MbNO: Ferrous nitrosylated Mb; MetMb: Ferric oxidized Mb; MetMbNO: Nitrosylated metMb; NO: Nitric oxide

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