On the mechanism of the dissociation of haemoglobin

Hemoglobinopathic erythrocytosis due to a new electrophoretically silent variant, hemoglobin San Diego (beta109 (G11)val--met)

Examination of 13 members of a Filipino family revealed that 6 had erythrocytosis inherited as a simple autosomal dominant trait. Application of several electrophoretic and chromatographic tests failed to reveal the presence of an abnormal hemoglobin in hemolysates from affected individuals. However, measurements of oxygen dissociation curves using whole bloods, dialyzed hemolysates, and 2,3-diphosphoglyceric acid-stripped hemolysates clearly showed that affected persons had an abnormal hemoglobin characterized by a high affinity for oxygen. Compositional analyses of all tryptic peptides from the beta-chains of the proband revealed a valyl-methionyl ambiguity in betaT12a. Blockage of lysyl residues and subsequent tryptic hydrolysis at arginyl residues permitted the isolation of fragments containing residues 105 through 146. Automatic sequence analysis of the fragments demonstrated the presence of both valine and methionine in nearly equal proportions at position beta109. This new hemoglobin variant is designated Hb San Diego (beta109(G11) Val-->Met).

Hemoglobin Abraham Lincoln, beta32 (B14) leucine leads to proline. An unstable variant producing severe hemolytic disease

An unstable hemoglobin variant was identified in a Negro woman with hemolytic anemia since infancy. A splenectomy had been performed when the patient was a child. The anemia was accompanied by erythrocyte inclusion bodies and excretion of darkly pigmented urine. Neither parent of the proposita demonstrated any hematologic abnormality, and it appeared that this hemoglobin variant arose as a new mutation. Erythrocyte survival in the patient was greatly reduced: the erythrocyte t(1/2) using radiochromium as a tag was 2.4 days, and a reticulocyte survival study performed after labeling the cells with L-[(14)C]leucine indicated a t(1/2) of 7.2 days. When stroma-free hemolysates were heated at 50 degrees C, 16-20% of the hemoglobin precipitated. The thermolability was prevented by the addition of hemin, carbon monoxide, or dithionite, suggesting an abnormality of heme binding. An increased rate of methemoglobin formation was also observed after incubation of erythrocytes at 37 degrees C. The abnormal hemoglobin could not be separated from hemoglobin A by electrophoresis or chromatography, but it was possible to isolate the variant beta-chain by precipitation with p-hydroxymercuribenzoate. Purification of the beta-chain by column chromatography followed by peptide mapping and amino acid analysis demonstrated a substitution of proline for beta32 leucine. It appears likely that a major effect of this substitution is a disruption of the normal orientation of the adjacent leucine residue at beta31 to impair heme stabilization.

Hemoglobin olympia ( 20 valine leads to methionine): an electrophoretically silent variant associated with high oxygen affinity and erythrocytosis

In a family with erythrocytosis, electrophoretic and chromatographic studies failed to demonstrate a hemoglobin variant. However, the oxygen dissociation curves of affected individuals were shifted to the left of normal and this shift persisted when oxygen equilibria were studied in 2.3-diphosphoglycerate-stripped hemolysates. A mutant hemoglobin was evidently present in the red blood cells of the affected persons and was responsible for the increased oxygen affinity and erythrocytosis. Specific staining of tryptic peptide maps of beta-chains from the propositus showed that peptide betaT(3) was positive for a sulfur-containing amino acid. Amino acid analysis yielded a composition identical to that of normal betaT(3), except that there were 2.6 residues of valine and 0.4 residues of methionine (normal composition: Val = 3.0, Met = 0). This suggested that the beta-chains of affected individuals consisted of a mixture of two kinds of chains, 40% of which had a methionyl residue in betaT(3). Structural studies of isolated cyanogen bromide fragments demonstrated unequivocally that, in the abnormal beta-chains, valine in position 20 is replaced by methionine. The new hemoglobin mutant is designated hemoglobin Olympia (beta20 (B2) valine --> methionine).

Noncooperativity of the dimer in the reaction of hemoglobin with oxygen (human-dissociation-equilibrium-sulfhydryl-absorption-x-ray analysis)

The theory that the alphabeta dimer is the functional unit of cooperativity in hemoglobin has been tested by determination of the oxygen equilibrium curve of stable deoxy dimers, obtained by the addition of 0.9 M MgCl(2) to human des-Arg 141alpha-hemoglobin. Cooperativity was absent in this medium, but was regained on transfer of the hemoglobin to a dilute phosphate buffer, where tetramers reformed. X-ray analysis of crystals of oxy- and deoxy-des-Arg hemoglobins showed that the removal of Arg 141alpha would leave the structure of alphabeta dimers unchanged. Nonreactivity of the sulfhydryl groups at 112beta proved that the subunits in deoxy dimers form the same contact as in oxy dimers, namely alpha(1)beta(1), and that no significant dissociation into free subunits occurs in 0.9 M MgCl(2). The absorption spectrum of the deoxy dimers corresponded to the sum of the spectra of the free deoxy alpha and beta subunits, and was different from that of the deoxy tetramer, showing the constraining salt bridges formed by the C-terminal residues in the tetramer to be necessary for the spectral changes normally observed on association of the deoxy subunits.

Hemoglobin Louisville (beta-42 (CD1) phe-leu): an unstable variant causing mild hemolytic anemia

An unstable hemoglobin variant termed Hb Louisville, was found in four members of a Caucasian family, who were suffering from a mild hemolytic anemia. The variant showed a decreased stability upon warming at 65 degrees C and an increased tendency to dissociate in the presence of sulfhydryl group-blocking agents. The structural abnormality was identified as a replacement of phenylalanyl residue in position 42 (CD1) by a leucyl residue. Substitution of this phenylalanyl residue, which participates in the contact with heme, by a nonpolar leucyl residue has apparently less severe consequences than a replacement of the same residue by a polar seryl residue as in Hb Hammersmith. Oxygen equilibrium studies of total hemolysate from one Hb Louisville heterozygote indicated a decreased oxygen affinity, a marked decrease in heme-heme interaction, and a normal Bohr effect. Studies with isolated Hb Louisville were not made because it was not possible to separate the variant from normal Hb A.

Hemoglobin Savannah (B6(24) beta-glycine is greater than valine): an unstable variant causing anemia with inclusion bodies

An abnormal hemoglobin, termed Hb Savannah, was found in red cell hemolysate of a young Caucasian girl with severe hemolytic anemia. The presence of this unstable variant became evident when inclusion bodies appeared rapidly upon exposure of red cells to redox dyes and a large percentage of hemoglobin in hemolysate precipitated on warming to 65 degrees C. Treatment of the hemoglobin with p-hydroxymercuribenzoate (PMB) caused a rapid dissociation into monomers; starch-gel electrophoresis of PMB-treated hemoglobin showed the presence of abnormal beta-chains. Data from structural studies of isolated beta-chains indicated substitution of a valyl residue for the normally occurring glycyl residue at position 24, which corresponds to helical residue B6. A similar substitution but with an arginine replacing the glycyl residue has been observed in Hb Riverdale-Bronx. The glycine to valine substitution will change the relationship of the B and the E helices which results in extensive conformational changes in the beta-chain. This change presumably causes an increased dissociation of the hemoglobin molecule into dimers and probably monomers, and a decreased stability of the alphabeta-dimers. The hemoglobin abnormality may be the result of a fresh mutation because the abnormality is not present in the parents nor in any of the seven siblings.