Post-translational control of human hemoglobin synthesis: the role of the differential affinity between globin chains in the control of mutated globin gene expression

Hb G-San Josè variant levels correlate with alpha-thalassemia genotypes

Hb G-San Josè or beta7(A4)Glu-->Gly has been reported in Southern Italian or Mexican families. We have studied four families from Sicily and Campania, Southern Italy. In six carriers, the hemoglobin variant level ranged from 32 to 38%. In four double heterozygotes for Hb G-San Josè and alpha-thalassemia the variant level showed a strong correlation with the alpha-thalassemia genotype. In fact, the variant level was 15% when interacting with the - (alpha)20.5/alphaalpha, 19.6% with the alphaalpha/alphaPoly Aalpha, and 24.8% with alphaalpha/alpha(-5) ntalpha genotypes. In two double heterozygotes for Hb G-San Josè and beta+ -IVS-I-6 (T-->C) the hemoglobin variant level was 67%. These data show that the reduced synthesis of alpha chains causes drastic reduction of probability to form Hb G-San Josè in favor of the formation of Hb A. Moreover, this reduction, (i) correlates with the type of alpha-thalassemia genotype and with the degree of the alpha chain deficiency, and (ii) is, most probably, more marked than the degree of alpha chain reduction. The minor affinity of the beta chain variant for the alpha chains associated with the reduced synthesis of the alpha chains is probably the principal cause of the variant hemoglobin reduction. Moreover, the rapid removal of the abnormal chains by proteolytic enzymes must have an essential role in order to reduce the chain variant pool. These conclusions are in agreement with the results obtained in reticulocyte and in vitro recombination experiments.

Differences in affinity of beta and delta hemoglobin chains for alpha chains. A possible explanation for the variation in the percentages of hemoglobin A2 in thalassemia and other disorders

Individual human alpha, beta and delta hemoglobin chain types were isolated in a native form so that the hemoglobin tetramers Hb A and Hb A2 could be reconstituted. In order to measure the relative affinity of beta and delta chains for alpha chains, an equimolar mixture of beta and delta chains was incubated with decreasing concentrations of alpha chains. When alpha chains were present in an amount equal to or greater than that of the mixture of beta and delta chains, the amount of Hb A2 was similar to that of Hb A. When alpha chains were in limiting concentrations, more Hb A than A2 was always formed with gradually decreasing amounts of Hb A2. The data as interpreted indicate that alpha chains have an affinity for beta chains at least 7.6-times greater than that for delta chains. These results also suggest that the differential affinity between beta and delta chains may be a major post-translational mechanism, which explains the increased level of Hb A2 in beta-thalassemia, and its reduced amounts in alpha-thalassemia and in acquired disorders with deficient alpha-chain synthesis.

The effect of pH on the rate of dissociation of the oxygenated beta chain tetramer of Hb A

The effect of pH on the overall assembly of oxyhemoglobin A following mixing of equivalent concentrations of alpha and beta heme subunits has been studied in 0.1 M potassium phosphate buffers at 20 degrees C. The resultant kinetic profiles monitored at 582.5 nm (the maximum of the oxyhemoglobin - oxy chain difference spectrum) were homogeneous and appeared to be first order. The rate of these exponential time courses, reflecting the rate of dissociation of the beta chain tetramer, increased from 0.013 min-1 at pH 6.4 to 0.30 min-1 at pH 8.0 and 1.0 min-1 at pH 8.5. Concurrent with this increased rate was a decrease in the overall color yield from the reaction. The absorbance changes, which involve a significant contribution from the beta chain tetramer to monomer dissociation step, changed three fold over the pH range studied. The findings indicate that protons enhance the stability of the beta chain tetramer.