Physical and chemical properties of opossum hemoglobin. A hemoglobin containing glutamine at position 58 (E7) in the α subunit

It has recently been proposed that the α subunit of the hemoglobin isolated from opossum (Didelphius marsupialus) blood contains 40 amino acid differences from the α subunit of human hemoglobin A. In the heme-binding region at Position E7, a glutamine residue is found in place of the distal histidine. Opossum hemoglobin is electrophoretically similar to hemoglobin A and has a sedimentation coefficient similar to that of hemoglobin A, even though it is found to contain six mercurialreactive sulfhydryl residues. On the other hand, opossum hemoglobin is more alkaline resistant and much more readily oxidized by Fe(CN)₆^3- than hemoglobin A. The optical absorption properties of opossum hemoglobin are similar to those of hemoglobin A except in the visible region near 600 nm where opossum methemoglobin resembles methemoglobin M_Boston. Unlike mutant human hemoglobins containing amino acid substitutions at Position E7, the O₂-binding properties of opossum hemoglobin resemble those of hemoglobin A. Electron spin resonance studies at -174°C show that the high spin signal of opossum methemoglobin consists of a doublet, one peak representing α subunits and one representing β subunits. These two peaks are found to have different temperature dependencies. Inside the opossum crythrocyte only a small amount of methemoglobin can be detected but upon lysing the cells this amount increases greatly with time. The combination of amino acids which exist in opossum hemoglobin are such that a distal histidine is not required in the α subunit for normal transport of O₂.