Further Studies on the Kinetics and Equilibria of the Reactions of Nitric Oxide with Haemoproteins

Reduced haemoglobin is known to react faster with NO than with either O2 or CO, and the reaction has now been studied in more detail to allow comparisons of the degree of haem-haem interaction with the different ligands. As a kinetic measure of the interaction, the observed ratio of to x'4 to x'1(x'1, x'4 being the respective velocity constants of the combination of the first and fourth ligand molecules) is divided by the value of the ratio to be expected on statistical principles, namely 0.25. It has been found that the extent of interaction, as determined in this way, is inversely related to the rate of combination of the first ligand molecule, and for CO, O2 and NO it is 80, 12 and 3.3 respectively. The rate constants for the combination of sheep haemoglobin with the first and fourth molecules of NO are 3 x 107 and 2.5 x 107M-1 s-1. Determinations of the rate of combination with, and dissociation of NO from, horse myoglobin (Mgb) at 22 °C give 1.7 x 107M-1 s-1 and 7 x 10-5 s-1 for the corresponding velocity constants. The expected concentration of free NO for half saturation is thus 4 x 10-12M corresponding to pNO of 1.5 x 10-6 mmHg. The competitive equilibrium of myoglobin between NO and CO has been directly measured and is about 9000 at 19 °C (in favour of NO). This value agrees fairly well with that calculated indirectly from the measured values of the velocity constants of the reactions CO+Mgb ⇌ COMgb and NO+Mgb ⇌ Mgb.