Functional characterization of hemoglobins from South Brazilian fresh water teleosts--II. Three cichlids (Crenicichla lepidota, Aequidens port alegrensis and Geophagus brasiliensis).

Oxygen equilibria of the multiple hemoglobin (Hb) from the South American cichlid fishes Crenicichla lepidota, Aequidens portalegrensis, and Geophagus brasiliensis were measured at three different temperatures (10, 20 and 30 degrees C). The cofactor-free whole hemolysates exhibited extremely high O2 affinities (half-saturation oxygen tension, P50 approximately 1 mm Hg at pH 7.2 and 20 degrees C), exceptionally large Bohr effects (phi = delta log P50/delta pH = -1.1 to -1.3 at pH 7.2-6.2 and 20 degrees C), low cooperativity, and a low sensitivities to ATP. The O2 affinities showed high thermal sensitivity (delta H for the overall oxygenation reactions were -71 to -79 kJ mol-1 at pH 8.0). Nine Hb components separated from Geophagus showed similar, if not identical, functional properties. The oxygenation properties of the Hb systems, particularly the high thermal and pH sensitivities, are interpreted as adaptions to large diurnal variations in ambient temperature, oxygen tension and activity patterns of the fish in nature.