Oxidative Reactions of Normal and Abnormal Hemoglobins in the Presence of Phosphatidylserine Vesicles

Abstract

We have monitored a sequence of reactions involving hemoglobin (Hb) oxidation in the presence of phospholipid bilayers in carefully characterized buffer systems in order to delineate molecular events that may have cytotoxic effects, some of which may even lead to cellular death of erythrocytes. We followed reactions of subunit cross-linking, heme iron oxidation, heme destruction and iron release in normal (Hb A) and sickle [Hb S or beta6(A3)Glu-->Val] Hbs under different experimental conditions. Our results show that, in the presence of lipid surfaces, the reaction rates for heme Fe+2 to Fe+3 oxidation in Hb A and Hb S molecules were both enhanced by lipid surfaces. However, the extent of the enhancement in Hb A and Hb S was quite different in T5K6.5, a Tris buffer with low ionic strength and low pH. In T5K6.5, the rate constants were 0.8 h(-1) for Hb A and 4.3 h(-1) for Hb S, a 5-fold difference. This finding supports a published suggestion that sickle Hb exhibits abnormal heme oxidation in erythrocytes of sickle cell disease patients. We found that the rates were quite similar in P110N7.4, a phosphate buffer near physiological conditions. Even in T5K6.5, the lipid surface did not enhance Hb S reactions involving subunit cross-linking, heme destruction, or iron release. These findings suggest that heme oxidation and related reactions may not be responsible for detrimental cellular events in sickle erythrocytes under physiological conditions, as suggested in the literature.