Calorimetric studies of oxyhemoglobin dissociation. I. Reaction of sodium dithionite with oxygen

Calorimetric studies of the reduction of free oxygen in solution by sodium dithionite are in agreement with a stoichiometry of 2 moles Na2S2O4 per mole of oxygen. The reaction is biphasic with ΔHt - 118±7 kcal mol−1 (−494 ± 29 kJ mol−1). The initial phase of the reaction proceeds with an enthalpy change of ca −20 kcal (−84 kJ) and occurs when 0.5 moles of dithionite have been added per mole dioxygen present. This could be interpreted as the enthalpy change for the addition of a single electron to form the superoxide anion. Further reduction of the oxygen to water by one or more additional steps is accompanied by an enthalpy change of ca −100 kcal (−418. 5 kJ). Neither of these reductive phases is consistent with the formation of hydrogen peroxide as an intermediate. The reduction of hydrogen peroxide by dithionite in 0.1 M phosphate buffer, pH 7.15, is a much slower process and with an enthalpy change of ca − 74 kcal mol−1 (−314 kJ mol−1). Dissociation of oxyhemoglobin induced by the reduction of free oxygen tension with dithionite also shows a stoichiometry of 2 moles dithionite per mole oxygen present and an enthalpy change of ca. −101 ±9 kcal mol−1 (−423± 38 kJ mol−1). The difference in the observed enthalpies (reduction of dioxygen vs. oxyhemoglobin) has been attributed to the dissociation of oxyhemoglobin, which is 17 kcal mol−1 (71 kJ mol−1).