Abstract
Abstract The binding of thiosulfate, bromide, and iodide to human methemoglobin A is examined. Visible and electron paramagnetic spectra of the substituted methemoglobins suggest that the bromide and iodide complexes are predominantly low spin, and the thiosulfate, high spin. Thiosulfate binds hyperbolically while iodide and bromide show apparent sigmoidal behavior. The binding behavior of the halide ions is not due to cooperative interaction, but rather to a protein, structural perturbation induced by neutral salts.
Highlights
The last two ligands appear to bind sigmoidally, but, as we discuss, these halides do not bind with cooperativity
Protein was equilibrated with the ligand until equilibrium was reached at 22.0 f O.l”, and the absorbance was measured with a Gary model 16 spectrophotometer
The binding of thiosulfate, iodide, and bromide to methemoglobin iron was determined by measuring absorbance changes in the visible region of the protein spectrum
Summary
The binding of thiosulfate, bromide, and iodide to human methemoglobin A is examined. Visible and electron paramagnetic spectra of the substituted methemoglobins suggest that the bromide and iodide complexes are predominantly low spin, and the thiosulfate, high spin. Thiosulfate binds hyperbolically while iodide and bromide show apparent sigmoidal behavior. The binding behavior of the halide ions is not due to cooperative interaction, but rather to a protein, structural perturbation induced by neutral salts. (Cyanide has been reported to bind sigmoidally [1] to methemoglobin, but more recently [6] hyperbolic binding was found.) This difference in binding characteristics may be a reflection of a basic difference in the heme-protein interaction. Regardless of the lack of similarity in binding behavior, knowledge of the manner in which apoprotein interacts with the prosthetic group of methemoglobin to control the magnitude of the ligand-Fe(II1) association constant is of interest, and may be relevant to a discussion of the sigmoidal oxygenation of deoxyhemoglobin. The last two ligands appear to bind sigmoidally, but, as we discuss, these halides do not bind with cooperativity
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