Modification of hemoglobin A with dimethyl adipimidate. Contribution of individual reacted subunits to changes in oxygen affinity.

The effect of dimethyl adipimidate, a bifunctional imidoester, on the oxygen affinity of hemoglobin A has been studied. Treatment of human oxyhemoglobin with 5 mM dimethyl adipimidate at pH 8.5, room temperature is accompanied by an increase in oxygen affinity in the presence and absence of 2,3-diphosphoglyceric acid. Circular dichroism measurements in the ultraviolet region indicate that dimethyl adipimidate-treated hemoglobin exhibits a reduced conformational change upon deoxygenation. In order to study the contribution of reacted individual subunits, alpha and beta subunits of dimethyl adipimidate-treated and untreated hemoglobin have been separated and reconstituted to form hybrid tetramers containing either the alpha-treated (alpha t beta c) or the beta-treated subunits (alpha c beta t). Electrophoresis on sodium dodecyl sulfate polyacrylamide gels of isolated alpha and beta globin subunits as well as hybrid tetramers from dimethyl adipimidate-treated hemoglobin reveals that 20% of the globin subunits are cross-linked. In the absence of 2,3-diphosphoglyceric acid, modification of alpha subunits increases the oxygen affinity and reduces the conformational change of the tetramer upon deoxygenation whereas modification of beta subunits has no effect. However, treatment of beta subunits decreases the effect of 2,3-diphosphoglyceric acid on the oxygen affinity of the hybrids and reduces the 2,3-diphosphoglyceric acid-induced spectral changes in oxyhemoglobin. Therefore the interaction of dimethyl adipimidate with both the alpha and beta subunits contributes to regulating the oxygen affinity of human hemoglobin.