Characterization of the hemoglobins of the Australian lungfish Neoceratodus forsteri (Krefft)

We examined for the first time the hemoglobin components of the blood of the Australian lungfish, Neoceratodus forsteri and their functional responses to pH and the allosteric modulators adenosine triphosphate (ATP), guanosine triphosphate (GTP), 2,3-bisphosphoglyceric acid (BPG) and inositol hexaphosphate (IHP) at 25 °C. Lysates prepared from stripped, unfractionated hemolysate produced sigmoidal oxygen equilibrium curves with high oxygen affinity (oxygen partial pressure required for 50% hemoglobin saturation, p₅₀ = 5.3 mmHg) and a Hill coefficient of 1.9 at pH 7.5. p₅₀ was 8.3 and 4.5 mmHg at pH 6 and 8, respectively, which corresponded to a modest Bohr coefficient (Δlog p₅₀/ΔpH) of − 0.13. GTP increased the pH sensitivity of oxygen binding more than ATP, such that the Bohr coefficient was − 0.77 in the presence of 2 mmol L^− 1 GTP. GTP was the most potent regulator of hemoglobin affinity, with concentrations of 5 mmol L^− 1 causing an increase in p₅₀ from 5 to 19 mm Hg at pH 7.5, while the order of potency of the other phosphates was IHP > ATP > BPG. Three hemoglobin isoforms were present and each contained both α and β chains with distinct molecular weights. Oxygen affinity and pH-dependence of isoforms I and II were essentially identical, while isoform III had a lower affinity and increased pH-dependence. The functional properties of the hemoglobin system of Neoceratodus appeared consistent with an active aquatic breather adapted for periodic hypoxic episodes.