Kinetics of reaction of 5,5′-dithiobis(2-nitrobenzenzoate) with CysF9[93]β sulphydryl group of sheep haemoglobins at neutral and physiological pH ranges

The kinetics of the reaction of 5,5′-dithiobis(2-nitrobenzoate) (DTNB) with CysF9[93]β sulphydryl group of the sheep haemoglobins were studied at neutral and physiological pH (6.8 ⩽ pH ⩽ 7.6) ranges under pseudo first order conditions. Each reaction is of first order with respect to the DTNB concentration. The pH dependence of the observed rate constant of the reaction at neutral and physiological ranges gave a complex trend. The observed rate shows that at neutral pH, the presence of inositol hexakisphossphate (IHP) leads to a relative lowering of the pseudo first order rate constant. Overall, the rate varied among the derivatives as aquomet > oxy > carbomonoxy and the major haemoglobin show a higher reactivity than the minor haemoglobin.