SUMMARY
Oxygen equilibria of Hypostomus and Pterygoplichthys hemoglobins and their sensitivities to the erythrocytic nucleoside triphosphates (NTP), ATP and guanosine triphosphate (GTP) are studied to investigate the mechanisms by which blood adapts to air-and water-breathing. Hemoglobins of both species are heterogeneous. All hemoglobin fractions isolated by iso-electric focusing reveal a high sensitivity to NTP, but GTP depresses O2 affinity about twice as effectively as ATP. A cathodic hemoglobin component with a reversed Bohr effect was found in Pterygoplichthys but not in Hypostomus. The data are discussed in relation to the in vivo cofactor modulation of blood O2 affinity and the adaptive significance of functional heterogeneity of fish hemoglobins.