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The structural basis of the crystallizing tendencies of oxyHbC (beta 6Glu-->Lys), that produces haemolytic anaemia in homozygotes, is unknown. Using a fluorescent organic phosphate analogue (8-hydroxy-1,3,6-pyrenetrisulphonate), and conventional oxygen equilibrium studies, data suggest that the binding of inositolhexaphosphate (IHP) to oxyHbC differs from HbA, indicating perturbations of the oxyHbC central cavity, which was predicted from our earlier spectroscopic findings. To define the relationship between this conformational change in oxyHbC and its tendency to crystallize, the effect of four central cavity ligands on the crystallization rate was studied: a peptide containing 11 residues from the N-terminal portion of band 3, the full cytoplasmic domain of band 3, 2,3-diphosphoglycerate and IHP. OxyHbC crystallization was accelerated by all these central cavity ligands and not by the appropriate controls. These central cavity changes become an excellent candidate for the dramatic increase in the crystallization rate of oxyHbC.

Compound heterozygotes of variant haemoglobins (Hbs) with HbC, with or without novel phenotypic changes, have provided insight into the molecular basis of the interacting haemoglobins and information concerning the role of specific residues in the crystallization of oxy HbC. A high phosphate buffer system has proved useful for studying the effects of variant haemoglobins (naturally co-existing with HbC in the red cell) on the oxy HbC crystallization process and has led us to conclude that beta87 and beta73 are contact sites of the oxy HbC crystal. We now present investigations from two HbC compound heterozygotes which exhibit opposing effects upon HbC crystallization: HbC/Hb N-Baltimore (beta95 Lys-->Glu) and HbC/Hb Riyadh (beta120 Lys-->Asn). The latter inhibits the in vitro crystallization of HbC, explaining the lack of erythrocyte abnormalities (with the exception of microcytosis) in the doubly heterozygous infant. In contrast, Hb N-Baltimore accelerates the crystallization of HbC, contributing to multiple abnormalities in red cell morphology, albeit in the absence of morbidity. We conclude that (1) beta120 and beta95 are additional contact sites in the crystal, and (2) the HbC/Hb Riyadh haemoglobinopathy demonstrates that crystallization may not be required for the generation of the observed microcytosis and increased red cell density in HbC-containing red cells.

We report here that compound heterozygosity for hemoglobin Korle-Bu (HbKB) and HbC (beta 6 Glu-->Lys) is associated with moderate chronic hemolytic anemia with microcytosis. To understand the pathogenesis of this syndrome, we have studied the effect of Hb Korle-Bu (KB = beta 73 Asp-->Asn) on the crystallization of HbC. We have previously established that fetal Hb (HbF) inhibits the crystallization of HbC. In contrast, HbS accelerates crystallization affecting the pathogenesis of SC disease. We now report on in vitro crystallization of mixtures of HbKB, HbC, and various amounts of HbF and the native hemolysate of a child who is a compound heterozygote for HbKB and HbC. At 6 months of age, the propositus' hemolysate contained 55% HbKB, 39% HbC, and 6% HbF. Crystal formed within 2 minutes compared with 30 minutes for the mixture of 40% HbC:60% HbS and with 180 minutes for 40% HbC:60% HbA. The morphology of the crystals formed was cubic, in contrast with the tetragonal crystals observed in CC and SC disease. Early crystals did not exhibit "sharp edges" until 45 minutes. Purified HbKB formed aggregates but not crystals after 24 hours. Isopycnic gradients showed that the KB/C compound heterozygotes have red blood cell (RBC) densities intermediate between the AC and CC phenotype and similar to SC disease. The surface residue beta 73, known to participate in areas of interaction of the deoxy HbS polymer, can now be assigned to areas of contact in HbC containing crystals. The hemolysis observed in the HbKB/C compound heterozygote is likely to be secondary to the acceleration of Hb crystallization. The microcytosis and increased RBC density is clearly the consequence of the presence of HbC, but the basis of the increased RBC pathology compared with AC trait, despite the low proportion of HbC (35% to 40%), remains to be elucidated.

We previously reported that circulating hemoglobin (Hb) CC erythrocytes contain oxygenated HbC crystals with little or no HbF and that HbF inhibits in vitro crystallization of HbC. We now report that HbS accelerates in vitro crystallization of HbC. Crystals were formed in 1.8 mol/L potassium phosphate buffer, pH 7.4, at 30 degrees C and were counted in several time intervals with a hematocytometer. The hemoglobin composition of Millipore-isolated crystals and supernatant were also analyzed. Under the conditions selected, 100% HbS formed needle-shaped crystals only after two hours. Pure HbC does not form crystals within 15 minutes, whereas a ratio of 10% HbS:90% HbC forms 1,100 crystals/mm3, 20% HbS:80% HbC forms 370 crystals/mm3, and 30% HbS:70% HbC forms 5 crystals/mm3. Crystals formed in the presence of HbS are tetragonal, as are pure HbC crystals. As compared with 100% HbC, HbA or albumin mixed with HbC showed a decreased number of crystals as a result of dilution. Analysis of the Hb content of isolated crystals by citrate agar gel electrophoresis showed that HbS was rapidly incorporated into the crystal in the same ratio over time. These results demonstrate that HbS accelerates crystallization of HbC with respect to the rates of crystallization of any of these two Hbs separately, through a mechanism that involves cocrystallization. These results may be significant in understanding SC disease.

We have reported that circulating CC erythrocytes containing HbO2 C crystals exhibit little or no Hb F suggesting that Hb F may inhibit the crystallization of Hb C. We report now that Hb F inhibits in vitro crystallization of HbO2 and HbCO C when compared to the effect of Hb A in a wide range of mixture proportions. For example, while HbCO C solutions form tetragonal C crystals within 25 min, no crystals form within 2 h with 30% Hb F, whereas 550 crystals/mm3 form with 30% Hb A. Furthermore, an increase in the percent of Hb A is correlated with a greater number of orthorhombic crystal formation rather than the tetragonal morphology observed with 100% Hb C. We also report that Hb A2 (containing delta chains that exhibit 10 sequence differences with beta chains) and Hb Lepore Boston-Washington (a fusion mutant of delta and beta chains that contains only six of these differences) both inhibit Hb C crystallization. By comparing the sequences of the three inhibitory hemoglobins, we conclude that position Gln-87 in the gamma chains is, at least partially, the cause of the inhibitory effect of Hb F on the crystallization of Hb C.