Myoglobin as an oxygen indicator for measuring the oxygen binding characteristics of a modified myoglobin derivative containing covalently bound mesoheme

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Abstract
By measuring the visible spectrum of a mixture of [horse heart] myoglobin (Mb) and a modified derivative containing mesoheme in place of the normal protoheme, it is possible to evaluate the relative amounts of the oxidized, reduced and oxygenated forms of each type of Mb. If the O2 affinity of one Mb derivative is known, the O2 affinity of the other can be determined from measurements at various O2 partial pressures. In the absence of excess reducing agent, the rate of autoxidation can also be evaluated during the same experiment. The method described is suitable at very low O2 partial pressures, where most previous methods are inaccurate, and it is very convenient to use, since no time-consuming calibration procedures are required. Using protoheme Mb as an O2 indicator, the O2 pressure at half saturation (P1/2) of mesoheme Mb was shown to be 11% higher than the P1/2 of a modified Mb derivative containing covalently bound mesoheme. The autoxidation rate of the covalent derivative is faster than that of the noncovalent derivative, but it is less dependent on O2 pressure.