We reported previously that the distal(E7) histidine is replaced by glutamine in myoglobin from the shark, Galeorhinus japonicus. The amino-acid sequence of myoglobin from another shark, Heterodontus japonicus, has been determined. The myoglobin is composed of 148 residues, is acetylated at the N-terminus, and contains the distal(E7) histidine at position 59. Although the sequence homologies between G. japonicus, H. japonicus, and sperm-whale myoglobins were about 40-55%, their hydropathy profiles were very similar, indicating that they have a similar geometry in their globin folding. The autoxidation rates of the two shark oxymyoglobins were examined in 0.1 M buffer at 25 degrees C over pH range 4.5-11.5. The pH dependence for the autoxidation of H. japonicus myoglobin was very similar to that of sperm-whale myoglobin, although the rate was about 10-times higher over the pH range examined. In both myoglobins, autoxidation was largely accelerated by H+. On the other hand, the pH dependence of G. japonicus myoglobin, which has the distal glutamine in the place of histidine, was quite different from those of sperm-whale and H. japonicus myoglobins. One of the most remarkable features is the fact that the autoxidation rate is not enhanced with an increase in the concentration of H+ in the acidic range of pH, where the autoxidation of sperm-whale and H. japonicus myoglobins is most accelerated. This finding suggests that the distal(E7) histidine participates in the autoxidation reaction as a catalytic residue facilitating the movement of a catalytic proton.