The oxygen equilibrium properties of blood and of solutions of haemoglobin from Xenopus laevis are reported. At pH 7.6 the oxygen affinity of the blood, expressed as half saturation oxygen tension (P50) amounts to 27.0 mm and 13.7 mmHg (3.60 and 1.83 kPa) when measured at 25 and 10 °C, while the Bohr factor (Δlog P50/ΔpH) was −0.40, and the Hill's cooperativity coefficient, n, averaged 2.1. These data reflect an overall heat of oxygenation, ΔH, of −7.9 kcal. mol−1, which decreased to −6.3 kcal. mol−1 when the live animals were acclimated to each measuring temperature. Xenopus blood showed a high O2 capacity (15 vol.%) compared to that of other amphibians.
Acclimation to water of increased salinity (12%0), and aestivation, raised blood O2 affinity; at 25 °C and pH 7.6, P50 decreased to 21.1 and 25.2 mmHg (2.81 and 3.36 kPa), respectively. These changes were concomitant with increases in the blood levels of urea. In contrast to NaCl and ATP, urea increased O2 affinity of the purified haemoglobin, suggesting that oxygenationlinked binding to haemoglobin is involved in the modulations of the blood O2 affinity during aestivation and acclimation to salt water.
Xenopus haemoglobin consists of two components. The major component is electrophoretically anodal, and has O2 binding properties similar to those of the haemolysate; the minor component is cathodal, and shows extremely low P50, pH sensitivity and cooperativity.