African elephant myoglobin with an unusual autoxidation behavior: comparison with the H64Q mutant of sperm whale myoglobin.

Elephant myoglobins both from Asian and African species have a glutamine in place of the usual distal (E7) histidine at position 64. We have isolated native oxymyoglobin directly from the skeletal muscle of African elephant (Loxodonta africana), and examined the autoxidation rate of oxymyoglobin (MbO2) to metmyoglobin (metMb) as a function of pH in 0.1 M buffer at 25 degreesC. As a result, African elephant MbO2 was found to be equally resistant to autoxidation as sperm whale myoglobin. However, the elephant myoglobin exhibited a distinct rate saturation below pH 6. Kinetic analysis of the pH profiles for the autoxidation rate has disclosed that African elephant MbO2 does not show any proton-catalyzed process, such as the one that can play a dominant role in the autoxidation reaction of sperm whale myoglobin by involving the distal histidine as its catalytic residue. Such a greater stability of African elephant MbO2 at low pH could be explained almost completely by the single H64Q mutation of sperm whale myoglobin. In African elephant aqua-metmyoglobin the Soret band was considerably broadened so as to produce another peak in the pentacoordinate 395 nm region. This unique spectral feature was therefore analyzed to show that the myoglobin is in equilibrium between two species, depending upon the presence or absence of a water molecule at the sixth coordinate position.