The reactions of some aromatic C-nitroso compounds with haemoglobin

Abstract

Reduced hemoglobin combines successively with 4 molecules of nitrosobenzene and the nitrosotoluenes. The rate constants for the combination of the first molecule of ligand at pH 9-1 and 20[degree] are (M-1 sec.-l): nitrosobenzene, 9.2 x 104; o-nitroso-toluene, 1.7 x 104; m-nitrosotoluene, 10.5 x 104; p-nitrosotoluene, 10.5 x 104. The 4th molecule to combine does so more rapidly than the first, the average increase in rate is 7-fold. The increase compared with the first molecule is about the same for all 4 compounds. The rate of dissociation of the first molecule of the same compounds from fully saturated hemoglobin has been measured. For nitrosobenzene, and o- and m-nitrosotoluene, the rates lie within the range 0.016-0.032 sec.-l at 20[degree]. The value for p-nitrosotoluene is 0.2 sec.-l. The activation energies for the combination of the first molecule of nitrosobenzene and the dissociation of the first molecule of nitrosobenzene give a normal value to the frequency factor in both cases. The rates of combination of the 4 nitroso compounds with myoglobin are numerically equal to those for the combination of the first molecule with reduced haemoglobin, with the exception of o-nitrosotoluene which combines with myoglobin almost as fast as do the other compounds.