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

By measuring the visible spectrum of a mixture of myoglobin 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 myoglobin. If the oxygen affinity of one myoglobin derivative is known, the oxygen affinity of the other can be determined from measurements at various oxygen 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 oxygen 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 myoglobin as an oxygen indicator, the oxygen pressure at half saturation (P 1/2) of mesoheme myoglobin was shown to be 11% higher than the P 1/2 of a modified myoglobin 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 oxygen pressure.