Kinetic and spectroscopic properties of the cyanide complexes of ferrous haemoglobins I and IV from trout blood

The cyanide ion is a ligand of ferrous as well as ferric haemoproteins and this study presents a kinetic characterization of the dissociation of its complexes with the two main haemoglobin components from trout blood. Both these haemoglobins bind oxygen co-operatively at neutral or alkaline pH values but one of them is insensitive to pH and allosteric effectors (haemoglobin I, HbI) while the other (haemoglobin IV, HbIV) is strongly sensitive and shows the so-called Root effect (i.e. the incomplete oxygen saturation in air-equilibrated solutions at pH values of < 6.5). Comparison of the kinetics of dissociation of cyanide from ferrous forms of HbI and HbIV reveals that: (i) cyanide dissociates in both cases by a complex reaction, and, at least in the case of HbIV, this may be attributed to functional differences between the α and β subunits; (ii) the reaction is only scarcely co-operative in HbI and not at all so in HbIV; and (iii) the Bohr and Root effects are not manifested in this reaction. The functional heterogeneity of ferrous α and β chains of trout HbI has not been observed for any other ligand; moreover, the observation that co-operativity for cyanide dissociation is expressed by human haemoglobin but not by trout HbIV is surprising.