Rheologic properties of mixed hemoglobin gels: deoxyhemoglobins S and A

To obtain new information concerning the behaviors, and in turn the structures, of gels formed from mixtures of S and A hemoglobins, their physical properties have been characterized by stress relaxation with a rotational rheometer. The variables manipulated were (1) initial total hemoglobin concentration, (2) mole fraction of hemoglobin S present in the mixture, (3) hemoglobin A as intact tetramer only or as both tetramer and hybridized hemoglobin AS, (4) annealing time, (5) shear history, (6) temperature, and (7) temperature and time of annealing. Characteristics monitored to gain information about the effect of these variables on gel properties were (1) lag time, (2) polymer mass, (3) polymer fraction, (4) polymer composition, (5) equilibrium total hemoglobin activity, and (6) solidity/total or hemoglobin S polymer mass (or total or hemoglobin S fraction). As expected, mixed hemoglobin SA gels were less solid than those of pure S of similar initial hemoglobin concentrations because of lower polymer mass, and gel properties were influenced by shear history, annealing time, temperature, and temperature and time of annealing. However, when solidities were compared on the basis of similar quantities of gel present, mixed hemoglobin SA gels were found to be more solid than those of pure S as the mole fraction of hemoglobin S decreased in the initial mixture. This is explained by the predominant influence on gel properties of high hemoglobin activity incurred by the volume exclusion effect of the total hemoglobin concentration. The presence of hemoglobin A with hemoglobin S results in polymers and gels that differ from those found in pure hemoglobin S. Pathophysiologic implications of these findings for sickle cell disorders are proposed.