Polyphasic linkage between protein solubility and ligand binding in the hemoglobin-polyethylene glycol system.

Dilute hemoglobin A, hemoglobin S, and horse hemoglobins can be made to precipitate by addition of polyethylene glycol (PEG). PEG itself has no effect on the oxygen affinity of the soluble hemoglobin. In the resulting two-phase system the amount of precipitation is dependent on the oxygen saturation and, conversely, thw two-phase mixtures exhibit altered oxygen affinity such as is seen with gelled hemoglobin S. A decreased oxygen affinity results when the deoxy form is the less soluble as in the case of hemoglobin A or S. When the oxy form is the less soluble, as in the case of horse hemoglobin, an affinity increase results. These reciprocal shifts in solubility and oxygen affinity are seen as thermodynamically linked processes, independent of solid phase structure. However, the structure, as seen by electron microscopy, reveals that deoxyhemoglobin S gelled in the absence of PEG is identical with deoxyhemoglobin S solid phase formed in the presence of PEG.