Delay time of hemoglobin S polymerization prevents most cells from sickling in vivo

A laser photolysis technique has been developed to assess the quantitative significance of the delay time of hemoglobin S gelation to the pathophysiology of sickle cell disease. Changes in the saturation of hemoglobin S with carbon monoxide produced by varying the intensity of a photolytic laser beam were used to simulate changes in the saturation of oxyhemoglobin S produced by variations in oxygen pressure. The presence of polymer at steady-state saturation with carbon monoxide was determined by measurement of the kinetics of gelation after complete photodissociation. The kinetics are a very sensitive probe for polymer since small amounts of polymerized hemoglobin increase the rate of nucleation sufficiently to eliminate the delay period. First, the equilibrium gelation properties of partially photodissociated carbonmonoxyhemoglobin S were shown to be the same as partially oxygenated hemoglobin S, and the method was then used to determine the effect of saturation on the formation and disappearance of polymers in individual sickle cells. The saturation at which polymers first formed upon deoxygenation was much lower than the saturation at which polymers disappeared upon reoxygenation. The results indicate that at venous saturations with oxygen, gelation takes place in most cells at equilibrium, but is prevented from occurring in vivo because the delay times are sufficiently long that most cells return to the lungs and are reoxygenated before polymerization has begun.