Studies on Hemoglobin Tryptophany1 Contact Residues in the Haptoglobin · Hemoglobin Complex

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Abstract
Hemoglobin and apohemoglobin bind in the same molar ratio. Some properties of the haptoglobin.cntdot.apohemoglobin [human] complex, prepared by isoelectric focusing in the presence of an excess of haptoglobin are described. This complex does not exhibit the irreversibility of complexes obtained with Hb in identical experimental conditions. The freezing of the conformation of apohemoglobin upon binding to haptoglobin was studied by fluorescence quenching experiments carried out in the presence of 8 M acrylamide. Changes in conformation of haptoglobin upon binding to apohemoglobin were detected by titration of the exposed tryptophans using N-bromosuccimide. Comparison of the additivity of exposed tryptophans in the complexes reveal that 2 tryptophans become inaccessible in the complex formation of haptoglobin with Hb but not with apohemoglobin. These tryptophans, probably located on the .alpha.1.beta.2 contact interface of Hb, were tentatively identified as Trp-C3(37).beta.