Differences in the reaction sequences associated with drug-induced oxidation of hemoglobins E, S, A, and F

Comparisons were made of drug-induced oxidation of purified hemoglobins A, S, E, and F. Repetitive spectral scans of reaction mixtures containing menadione showed that Hb E was the most reactive and Hb F was the least reactive of the hemoglobins studied. Hb E oxidation was only slightly faster than normal, but it produced much larger relative quantities of low-spin ferric hemoglobin (hemichromes). Hb F oxidation was considerably slower than normal but produced normal amounts of hemichromes relative to methemoglobin. Precipitation occurred in the order E greater than S greater than A greater than F. The abnormally slow rate of Hb F oxidation was even more striking when the oxidant was acetylphenylhydrazine (APH), and the sensitivity of the reaction to catalase was severely diminished. These hemoglobins thus exhibit entirely different reaction profiles during drug-induced oxidation. The amino acid substitution in Hb E alters the globin tertiary structure, so that hemichromes can more readily form, whereas the decreased susceptibility to oxidative denaturation of Hb F appears related to the absence of a site that normally reacts with hydrogen peroxide to increase oxidation rate. Such Hb F stability is consistent with the mild phenotypic expression of doubly heterozygous beta-thalassemia/HPFH and Hb S/HPFH. The role of Hb E instability in altered red cell morphology relative to the thalassemia-like deficit of beta globin mRNA has not been entirely resolved. Nevertheless, the clinical repercussions of the abnormal properties of Hb E are mild, and they may be involved with the prevalence of this hemoglobin in Southeast Asia as a balanced polymorphism in which the advantage to heterozygotes is, as yet, unclear.