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The Taiwanese toads, Bufo bankorensis, show wide altitudinal
distributionfrom lowland to over 3000 m. Bufo melanostictus, on
the other hand, are restricted to area below 500 m. I compared
blood-oxygen capacity and hemoglobin compositions in these two
species to understand whether the difference in altitudinal
distribution can be attributed to physiological variations. I
also examine the deoxygenated aggregation of hemoglobins in
these two toads. Blood-oxygen equilibrium curves were measured
by mixing technique. At 25 oC, the half saturation pressures
(P50) were 47.96 and 41.43 torr in Bufo bankorensis from high
and low altitudes, respectively, and was 47.31 torr in Bufo
melanostictus. Cooperativity of oxygen binding, evaluated as
Hill's n, exceeded 4 in both species between 75% and 90%
saturation at 25 oC. At 15 oC, P50 were 29.92 and 25.90 torr in
Bufo bankorensis from high and low altitudes, respectively, and
was 34.58 torr in Bufo melanostictus. At 15 oC, Hill's n in both
species within any saturation range never exceeded 4.
Isoelectric focusing showed that Bufo bankorensis has two major
hemoglobins, and Bufo melanostictus has three. Tricine SDS-PAGE
showed that hemoglobins of both toad species were composed of at
least three polypeptides. HPLC analysis also showed similar
results with electrophoresis. In native-PAGE, hemoglobins under
nitrogen treatment had lower relative mobility than in
oxygenated treatment, indicating formation of hemoglobin
aggregations under deoxygenation. Besides deoxygenated
aggregation, hemoglobins of these toads also formed disulfide
bond linkage between tetramers. These results suggest that
blood-oxygen affinity of Bufo bankorensis was higher than Bufo
melanostictus, and may contribute to the adaptation of Bufo
bankorensis to altitudinal hypoxia. Dissociation of hemoglobin
aggregates during oxygenation may enhance oxygen-binding
coopreativity, resulting in rapid loading of oxygen near
saturation.
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