Abstract

The reactivities of 5,5’-Dithiobis (2-nitrobenzoate) with the sulphydryl groups of major haemoglobin of duck (Anas platyrhinchos) have been carried out in the presence of inositol hexakisphosphate, inositol-P₆. The time course of the reaction is biphasic. The addition of inositol-P₆ changed the pH dependence profile of the apparent second-order rate constant of both oxy- and carbonmonoxyhaemoglobin for the reaction with CysF9(93)β. Inositol-P₆ reduced the rate of reaction with CysF9(93)β of the carbonmonoxy derivative by about 2 to 9-fold, depending on the pH. In contrast,the addition of inositol-P₆ increased the rates of reaction of the oxy- and aquomet derivatives. This difference might have risen from the fact that the salt bridge that is normally formed in human haemoglobins between HisHC3(146)β and AspFG(94)β was not formed in the presence of inositol-P₆ in liganded derivatives whose pH profiles were changed. Consequently, HisHC3(146)β comes sufficiently close to the CysF9(93)β sulphydryl group to interact with it electrostatically and so influences its reactivity directly. However, inositol-P₆ changed only that of carbonmonoxyhaemoglobin for CysB5(23)β. For the reaction with CysB5(23)β, inositol-P₆ reduced the rate in the carbonmonoxy derivative with increasing pH up to about 15-fold at pH 9.0, and increased the rates of reaction of the oxy- and aquomet derivatives by about 2 to 3-fold.

Key words: 5,5’-Dithiobis (2-nitrobenzoate), duck, haemoglobin, inositol-P₆, sulphydryl group.