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The pulse-radiolysis technique has been introduced because it permits a rapid reduction (in a few microseconds) of one heme group of the methemoglobin tetramer by hydrated electrons. The kinetics of the binding of oxygen to this particular valence intermediate (Hb3+) with one reduced alpha or beta subunit has been studied. It appears that the hydrated electrons preferentially reduce one type of subunit of methemoglobin at acid and neutral pH-values as is shown by the biphasic behaviour of Hb3+ on oxygenation. The second-order on-rate constants measured for the binding of oxygen to Hb3+ are 14 +/- 3 mM-1 ms-1 and 56 +/- 9 mM-1 ms-1, respectively. The relative contribution of the faster fraction is about 0.63 +/- 0.08 of the total oxygenation process. A comparison of the kinetic absorbance difference spectrum for the reduction of methemoglobin with the static difference spectrum of deoxyhemoglobin and methemoglobin in the Soret-region revealed a decreased absorbance of the unliganded subunit of Hb3+ at 430 nm. This fact suggests that Hb3+ is in the relaxed quaternary conformation, which is in agreement with the observed on-rate constants.
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