Carbon dioxide governs the oxygen affinity of crocodile blood

OXYGEN affinity of haemoglobin inside red blood cells is decreased to a level where oxygen unloading can proceed at sufficiently high partial pressures in the tissue capillaries. This adaptation of oxygen affinity is accomplished mainly by the interaction of haemoglobin with intraerythrocytic phosphates ¹. These comprise 2,3-diphosphoglycerate (DPG) in most mammals, ATP and GTP in fish², and myo-inositol pentaphosphate (IP⁵) in mature birds^3-5. No haemoglobin system has been described in higher vertebrates which is devoid of functional interaction with the respective intracellular phosphates. Recently, however, Sullivan⁶ has questioned the role of phosphates for the control of oxygen affinity in crocodilered cells (ATP and inorganic phosphate have been identified^3,7,8). He remarks that 'the substance regulating oxygen affinity in alligators is very efficient, but unknown'. We show here that the oxygen affinity of haemoglobin from a member of the order crocodiles (Crocodylus porosus) is not affected by any of the regulating allosteric cofactors known with the exception of molecular CO ₂ and protons.