The effect of urea on sickling.

The effect of urea on the oxygen affinity of sickle cells and normal cells was studied up to a concentration of about 1.0 M. Besides the increase in oxygen affinity found in both normal and sickle cells there was a further increase found only in the sickle cells. This specific increase was caused by the direct inhibition of the polymerization of deoxygenated Hb-S by the urea and was used to measure the extent of this inhibition. Even at concentrations of 1.0 M the urea did not fully inhibit the polymerization. At the urea concentrations recommended for treatment of sickle cell crises and for the oral treatment of the disease there was only a very slight inhibition of polymerization. The small increase in oxygen affinity brought about by these concentrations of urea (equivalent to a log P50 change of 0.016) will cause some additional minimal inhibition of sickling at physiological partial pressures of oxygen. Oxygen dissociation measurements of Hb-A in dilute solution showed that 0.95 M urea had no effect on the interaction of 2,3-DPG and chloride with haemoglobin nor on the haem-haem interactions. This implies that the quaternary 'deoxy' form of the haemoglobin may still form and that the action of urea in inhibiting the polymerization is through interference with the intermolecular bonds rather than by inducing a conformational change which disrupts the bonding. Unless urea at the concentrations recommended for treatment inhibits the sickling of the cells by some mechanism other than its effect on the haemoglobin (such as by an effect on the membrane), these results suggest that it is of little use for the treatment of sickle cell disease.