The transition in hemoglobin proton-binding characteristics within the basal actinopterygian fishes
H+ titration studies on oxygenated and deoxygenated haemoglobins from carp, rainbow trout, spiny dogfish and pig are reported, and compared with Hb-H+ equilibria in other species and structural information deduced from amino acid sequences. The buffer values of oxygenated and deoxygenated teleost haemoglobins are low in comparison with elasmobranch and mammalian haemoglobins. This correlates with a much lower content of histidine residues and alpha-amino groups in teleost haemoglobins, than that in elasmobranch, dipnoan, amphibian, reptilian, avian and mammalian haemoglobins. The low total histidine content in teleost haemoglobins is paralleled by a reduced number of titratable histidine residues compared with that in mammals. An inverse relationship is observed between the magnitude of buffer values and the magnitude of fixed-acid Haldane effects. The largest Haldane effect and smallest buffer values are seen in carp, followed by trout, whereas the smallest Haldane effect and largest buffer values are seen in dogfish. The H+ equilibria of pig haemoglobin are intermediate between those of teleost and elasmobranch haemoglobins.