Blood-oxygen affinities and hemoglobin compositions in two Taiwanese toads with different altitudinal distribution

The Taiwanese toads, Bufo bankorensis, show wide altitudinal distribution from lowland to over 3000 m. Bufo melanostictus, on the other hand, are restricted to area below 500 m. We compared blood-oxygen affinities and hemoglobin compositions in these two species to understand whether the difference in altitudinal distribution can be attributed to physiological variations. Blood-oxygen equilibrium curves were determined by mixing technique. At 25 °C, the half saturation pressures (P₅₀) were 47.96 and 41.43 torr (Hill's n were 2.72 and 2.70) in Bufo bankorensis from the high and the low altitudes, respectively, and was 47.31 torr (Hill's n was 3.12) in Bufo melanostictus. At 15°C, P₅₀ were 29.92 and 25.90 torr (Hill's n were 2.47 and 2.54) in Bufo bankorensis from the high and the low altitudes, respectively, and was 34.58 torr (Hill's n was 2.53) in Bufo melanostictus. Tricine SDS-PAGE and isoelectric focusing showed that hemoglobins in both species were composed of three polypeptides, however, the molecular weight and pI were different between the two species. These results suggest that the hemoglobin composition may be responsible for the increased blood-oxygen affinity in Bufo bankorensis for adaptation to altitude hypoxia.