title: Reexamination of the hyper thermodynamic stability of asymmetric cyanomet valency hybrid hemoglobin, (alpha+CN-beta+CN-)(alpha beta): no preferentially populating asymmetric hybrid at equilibrium.
abstract: It has been reported that hybridization of the equimolar mixture of cyanomethemoglobin and deoxyhemoglobin through dimer exchange reaction results in establishment of an approximately binomial (1:2:1) equilibrium distribution of these parental hemoglobins and their hybrid molecule, (alpha+CN-beta+CN-)(alpha beta), within several days under anaerobic conditions at pH 7.4, 21.5 degrees C, leading to a hyper (i.e., about 170 times) thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta) relative to the stability of the other diliganded species at pH 7.4, 21.5 degrees C [Daugherty, M. A., Shea, M. A., Johnson, J. A., LiCata, V. J., Turner, G. J., & Ackers, G. K. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 1110-1114]. To examine whether the published "binomiality" for this deoxy-cyanomet hybrid system really reflects the thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta), we used a binomial (1:2:1) equilibrium distribution of the equimolar mixture of cyanomethemoglobin and fully oxygenated hemoglobin as a starting condition, and then this system was rapidly deoxygenated. We found that the relative population of the hybrid was reduced to 8.6% of the total upon deoxygenation. It was also found that the hybridization experiment under anaerobic conditions was not allowed to continue for a long time due to a valency exchange reaction between deoxy and cyanomet derivatives. For instance, a 48 h incubation resulted in the oxidation of 44% of Fe2+ to Fe3+ hemes in the original deoxyhemoglobin and the reduction of 42% of Fe3+ to Fe2+ hemes in the original cyanomethemoglobin. These results suggest that a real distribution of the deoxy-cyanomet hybrid system at equilibrium is fairly far from 1:2:1 (binomial distribution), and the thermodynamic stability of (alpha+CN-beta+CN-)(alpha beta) is less than one-tenth of the hyperstability previously reported. In addition, most of the previous results on deoxy-cyanomet valency hybrids placed under long anaerobic conditions should be subject to reexamination due to possible valency exchange reactions.
authors.affiliation: Department of Physics, Jichi Medical School, Minamikawachi, Tochigi, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Shibayama
authors.forename: H
authors.initials: H
authors.lastname: Morimoto
authors.forename: S
authors.initials: S
authors.lastname: Saigo
mesh_terms: D000328:Adult; D005591:Chemical Fractionation; D006801:Humans; D007525:Isoelectric Focusing; D008024:Ligands; D008706:Methemoglobin / Q000031:analogs & derivatives* / Q000032:analysis / Q000737:chemistry; D010108:Oxyhemoglobins / Q000032:analysis / Q000737:chemistry*; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D008024:Ligands; D010108:Oxyhemoglobins; C007169:cyanomethemoglobin; D008706:Methemoglobin