Recent interest in therapies for sickle cell anemia based on elevating fetal Hb has made accurate estimates of the sparing effect offetal Hb (Hb F) and other non-sickle Hbs on sickle Hb (Hb S) polymerization essential. We have developed a technique, using HbCO as surrogate for HbO2, that enables us to assess the solubility of Hb S as a function of ligand saturation under conditions that mimic those of the sickling disorders. Equimolar mixtures of unliganded Hb S with Hb F or normal Hb A2 were isosoluble. Solubilities for equimolar mixtures with normal (Hb A) or abnormal (Hb C) Hbs were also identical but were lower than in the prior case. Thus, the sparing effect of both Hb F and Hb A2 should be considered in therapeutic strategies designed to modify Hb S polymerization. Hemolysates, stripped of 2,3-bisphosphoglycerate, from sickle cell disease patients with Hb (F + A2) levels varying from 6 to 25%, as well as from a sickle trait individual, were used to evaluate equilibrium solubiity as a function of ligand saturation over the range of pathophysiologic interest (25-70%). Our results show that the sparing effect of Hb (F + A2) increases relative to that of Hb A as ligand saturation increases, and that in the absence of ligand, '30% Hb (F + A2) is essentially isosoluble with the 60% Hb A of sickle trait. Although detailed knowledge of expected therapeutic benefits is confounded by the heterogeneity of Hb F distribution and other variables, these data should provide a framework for estimating likely clinical benefit from pharmacologic efforts to modulate globin gene expression.Sickle cell anemia exists in individuals who are homozygous for a point mutation (GAG --GTG) at codon 6 of the 3-globin gene, which results in a Glu-6 -* Val substitution in P-globin. are, thus, said to have a "sparing" effect on intracellular Hb S polymerization (3, 4). For Hbs F and A2, this solubilization can be described well with a model that assumes complete exclusion of either the homotetramer (a2-y2 or a2&2) or the hybrid heterotetramer (a238y or a2138) from the nascent polymer (5-7). For Hbs A and C, the data are compatible with exclusion of only homotetramers (a2I3A and a2K) from the polymer. Hybrid heterotetrameric forms (a2f3Sf3A and a2138S,c) appear to be incorporated into the polymer, but with a copolymerization probability roughly one-half that of Hb S (8-10). These copolymerization tendencies explain why the solubility enhancement induced by Hb F or A2 in admixture with Hb S is greater than that for Hb A or C.The solubilizing properties of various non-S Hbs have been established by use of a chemical reducing agent (dithionite) to completely deoxygenate mixtures of oxy-Hb S and the oxy form of other hemoglobins (Hb X), followed by centrifugation to fractionate the resulting semisolid gel into a tightly packed polymer pellet and a supernatant consisting of soluble Hb S and Hb X. The concentration of all Hb species in the supernatant is by definition the saturation concentration (csat), or equilibrium solubility, under g...

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