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Lack of correlation between cardiac myoglobin concentration and in vitro metmyoglobin reductase activity.

Myoglobin (Mb) is an intracellular protein found in red skeletal muscle and heart. It binds oxygen reversibly and enhances oxygen consumption, especially under conditions of low oxygen availability (Bailey et al. 1990; Wittenberg and Wittenberg, 1989). A variety of biochemical and biophysical experiments reveals that Mb functions by facilitating the diffusion of oxygen from the extracellular space to the mitochondria (Wittenberg and Wittenberg, 1989). Each molecule of Mb contains a single iron atom which may bind to one molecule of O2 when the iron is in the ferrous state (i.e. Mb-Fe). Mb-Fe undergoes spontaneous oxidation in vitro to the ferric state (Mb-Fe), known as metmyoglobin (metMb), which cannot bind O2. Autoxidation occurs for myoglobin isolated from numerous sources with a half-time of the order of hours under conditions approximating in vivo pH, ionic strength and temperature (e.g. Kitahara et al. 1990; Tajima and Shikama, 1987; Livingstone et al. 1986). It is generally accepted that spontaneous autoxidation must occur in vivo; however, metmyoglobin accumulation under physiological conditions has never been reported. Since the turnover time for Mb may exceed several months (Hickson and Rosenkoetter, 1981), a mechanism must exist to reconvert metMb to Mb and any failure in this mechanism could impair oxygen delivery to the mitochondria. Reduction of metMb has been observed in isolated, perfused rat and sea raven hearts following treatment with NaNC)2, which oxidizes Mb (Tamura et al. 1980; Bailey and Driedzic, 1988). Two fundamentally different enzyme mechanisms have been proposed to account for metMb reduction in vivo. On the basis of studies with isolated enzymes, the redox state of Mb is considered to be maintained by the enzyme metmyoglobin reductase, which has been characterized as NADH-cytochrome bs oxidoreductase. The postulated mechanism assumes a rapid reduction of the enzyme by NADH, followed by enzymatic reduction of cytochrome bs Fe to cytochrome bs Fe. The ferrous cytochrome then nonenzymatically reduces metMb (Livingstone et al. 1985). The enzyme has been isolated from mammalian heart (Hagler et al. 1979) and fish skeletal muscle (Levy et al. 1985) and has a relative