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, <em>Neoceratodus forsteri</em> 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, <em>p</em>₅₀ = 5.3 mmHg) and a Hill coefficient of 1.9 at pH 7.5. <em>p</em>₅₀ 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 <em>Neoceratodus</em> appeared consistent with an active aquatic breather adapted for periodic hypoxic episodes.