Interactions of nicotinamide adenine dinucleotides with varied states and forms of hemoglobin.
Spectrofluorometric techniques were used to quantify NADPH-hemoglobin interactions based on the quenching of NADPH fluorescence upon binding to hemoglobin. Fluorometric titrations were carried out with hemoglobin in varied states and with hemoglobins in which the beta-chain anion site is altered. At pH 6.5 in 0.05 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer, NADPH binds with high affinity, Kd = 1.03 microM, to deoxy human hemoglobin tetramers. Lower affinity binding of NADPH occurs as the beta-chain anion-binding site is discharged by increasing the pH. Moreover, the cofactor binds in a 1:1 ratio to deoxy tetramers, inositol hexaphosphate binds competitively, and binding is decreased in hemoglobins whose structural alterations result in decreased effects of 2,3-diphosphoglycerate. The cofactor binds to oxidized (met) hemoglobin with an estimated Kd of 33.3 microM but has little or no affinity for the oxy form. These results indicate that NADPH binds at the beta-chain anion-binding site and can be considered as a fluorescent analog of 2,3-diphosphoglycerate. Fluorescence measurements gave no indication of NADPH binding to deoxygenated ferrous or ferric myoglobin. Reductive processes within the erythrocyte, such as reduction of met hemoglobin and hemoglobin-catalyzed enzymatic reactions, may be affected by the significant binding of the reduced cofactor to both deoxygenated and oxidized hemoglobin. Cofactor-hemoglobin interactions predict a shift in redox potential as red cells become oxygenated, which may account for unexplained oxygen-linked shifts in red cell metabolism.
Ogo, S; Focesi, A; Cashon, R; Bonaventura, J; Bonaventura, C
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