Open AccessHeterogeneity in the Kinetics of Oxygen Binding to Partially Reduced Human Methemoglobin
Author(s) -
RAAP Adriaan,
LEEUWEN Johan W.,
ECKSCHOUTEN Tony,
ROLLEMA Harry S.,
BRUIN Simon H.
Publication year - 1977
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1977.tb11989.x
Subject(s) - methemoglobin , chemistry , tetramer , kinetics , absorbance , oxygen , reaction rate constant , photochemistry , dithionite , hemoglobin , radiolysis , inorganic chemistry , radical , organic chemistry , chromatography , enzyme , physics , quantum mechanics
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 (Hb 3+ ) with one reduced α or β 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 Hb 3+ on oxygenation. The second‐order on‐rate constants measured for the binding of oxygen to Hb 3+ 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 Hb 3+ at 430 nm. This fact suggests that Hb 3+ is in the relaxed quaternary conformation, which is in agreement with the observed on‐rate constants.