The kinetics of dissociation of the first ligand molecule from fully saturated carboxyhaemoglobin and nitric oxide haemoglobin in sheep blood solutions

When solutions of COHb are mixed with NO, the replacement of CO by NO is conditioned by the velocity of the reaction Hb4(CO)4 → Hb4(CO)3+CO (velocity constant l4) owing to the far greater affinity of Hb for NO than for CO. Measurements based on this principle have yielded values of l4 more conveniently than those based on the previous procedure of mixing Hb4(CO)4 with an excess of O2. Values of l4 by the NO and O2 methods are in good agreement, thus supporting strongly the view that the kinetics of reactions of ligand molecules and haemoglobin molecules in which three or four of the gas binding groups in the haemoglobin molecule are combined with ligand molecules, are subject to simple statis­tical principles. The effects of pH and temperature on l4 have been determined by the NO method. When COHb solutions are shaken with CO containing small percentages of NO, the CO is replaced at a rate equal to n' [NO] [COHb]/[CO]. Theoretically 4n'/j4 should equal N where j4 is the velocity constant of the reaction Hb4(NO)4→Hb4(NO)3+NO, and N is the partitition equilibrium constant of haemoglobin between NO and CO. This equation has been confirmed both at pH 6⋅8 and pH 9⋅0. The absolute values of j4, k4 (the velocity constant of the reaction Hb4(O2)4→ Hb4(O2)3+O2) and l4 are related to their respective energies of activation in the way expected on the simple collison theory of reactions in solution.