Identification of histidine-122alpha in human haemoglobin as one of the unknown alkaline Bohr groups by hydrogen--tritium exchange.

Human carbonmonoxy- and deoxy-haemoglobins were incubated at 37 degrees C in 3H2O at various pH values to measure the pH-dependent hydrogen--tritium exchange at the C-2 position of the imidazole ring of histidine-122alpha. To obtain the pseudo-first-order rate constants for the exchange, k, the two peptides containing histidine-122alpha were isolated and the amounts of tritium incorporated were determined. The rate constants gave pK values for the histidine of 6.1 in carbonmonoxyhaemoglobin and 6.6 in deoxyhaemoglobin, showing that it contributes about 20% to the total alkaline Bohr effect and about 10% at pH7.4.