The estimation of phospholipase A activity in aqueous systems.

1. Reduced haemoglobin combines successively with four molecules of nitrosobenzene and the nitrosotoluenes. 2. The rate constants for the combination of the first molecule of ligand at pH 9-1 and 200 are (M-1 sec.-'): nitrosobenzene, 9-2 x 104; o-nitrosotoluene, 1-7 x 104; m-nitrosotoluene, 10-5 x 104; p-nitrosotoluene, 10-5 x 104. 3. The fourth molecule to combine does so more rapidly than the first, the average increase in rate is sevenfold. The increase compared with the first molecule is about the same for all four compounds. 4. The rate of dissociation ofthe first molecule of the same compounds from fully saturated haemoglobin has been measured. For nitrosobenzene, and oand m-nitrosotoluene, the rates lie within the range 0-016-0-032 sec.-' at 200. The value for pnitrosotoluene is 0-2 sec.-'. 5. 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. 6. The rates of combination of the four 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. REFERENCES