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Dimerization and Other Chemical Changes in Amphibian Hemoglobins during Metamorphosis

From the ¹ From the Department of Chemistry and the Institute of Molecular Biophysics, Florida State University, Talahassee, Florida

Electrophoretic separations and amino acid analyses of the multiple hemoglobins of Rana grylio tadpoles and frogs are reported which show that major changes in the primary structures of these hemoglobins are taking place during metamorphosis. Amino acid compositions of adult Rana pipiens and Rana catesbeiana hemoglobins are also presented.

Studies in the ultracentrifuge showed that the hemoglobins of two bullfrog species dimerize in vitro to the extent of 70 to 80%. The ability to form dimers is a function of the stage of metamorphosis. Mercaptoethanol was the only reagent tested that reconverted all of the dimer molecules to monomers. The molecular weight of the dimer was found to be 130,000.

The mechanism of dimer formation involves the disappearance of reactive sulfhydryl groups in frog hemoglobins after hemolysis. The rate at which these —SH groups were lost was related to the rate at which the dimer was formed in the ultracentrifuge. The rate of —SH disappearance in the dimerizing systems was significantly faster than in the nondimerizing hemoglobins, and corresponded to a loss of two —SH groups per 65,000 g of hemoglobin. Two assay methods were used for measuring sulfhydryl content, p-chloromercuribenzoate titrations, and mercury orange determinations. A model for dimer formation was also proposed.

The oxygen binding properties of hemoglobin monomer and dimer were determined with the use of fractions isolated by starch block electrophoresis.

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