Gelling properties of apohemoglobin S alone and in mixtures with hemoglobin S

Apohemoglobin S formed a gel in the cold (5° C) with a protein concentration in the supernatants after centrifugation of the gels (C(sat)) near 27 g/dl, in 0.02 M phosphate buffer at pH 7.2. Under the same experimental conditions in mixtures of apohemoglobin S and deoxyhemoglobin S the solubility of hemoglobin S in the cold was decreased from C(sat) > 40 g/dl in the absence to about 18 g/dl in the presence of apohemoglobin S. Conversely, in the same mixture, C(sat) of apohemoglobin S was decreased to about 5 g/dl. Also, gelling occurred in mixtures of oxyhemoglobin S and its apoderivative. Apohemoglobin A alone did not form gels; however, it induced fiber formation in deoxyhemoglobin S in the cold; unlike apohemoglobin S, it was not included in the precipitate. Gels of apohemoglobin S were not birefringent, and inspection at the electron microscope failed to show the presence of organized structures. Excluded volume effects were probably at the origin of the decreased solubility of hemoglobin S and apohemoglobin S in the presence of each other.