A comparative study of the enolization of pyruvate and the reversible dehydration of pyruvate hydrate

Article abstract of DOI:10.1021/ja00034a049

The enolization of pyruvate and the reversible dehydration of pyruvate hydrate were studied at 25.0°C using spectrophotometric methods. The enolization of pyruvate was followed at 353 nm by monitoring the rate of uptake of triiodide ion. The dehydration of pyruvate hydrate was initiated by introducing small quantities of preacidified solutions of pyruvic acid containing, at the kinetic zero, ca. 60% of the hydrate into buffer solutions. A decrease in absorbance at 325 nm took place as the reaction progressed to a final solution composition of 6% hydrate. The reactions were studied in acetate, MES, phosphate, arsenate, imidazole, 1-methylimidazole, HEPES, Tris, and borate buffers. The dehydration of pyruvate hydrate was found to be sensitive toward general-acid and general-base catalysis, while the enolization of pyruvate was catalyzed only by the basic components of the buffers studied. The corresponding rate coefficients were determined for the acidic and basic catalysts, and taking into account the appropriate statistical correction factors associated with the capacity of the catalysts to donate and accept protons, Br?nsted plots were constructed. Br?nsted coefficients were determined for enolization (β = 0.47) and for dehydration (α = 0.54, β= 0.52). While relatively normal catalytic behavior was observed for the enolization of pyruvate, deviations for the dehydration of hydrated pyruvate were noted. Analysis of these deviations, in light of a comparison of the relative magnitude of the catalytic rate coefficients for the reversible hydrations of other carbonyl compounds, suggests the possible contribution of a general-base catalytic path involving the intramolecular participation of the carboxylate group of hydrated pyruvate. The data are also considered in terms of the possible roles the rates of interconversion and positions of equilibria between keto, enol, and hydrated species may play in the physiological reactions of pyruvate. Finally, the Br?nsted analysis provides the necessary basis for a comparison of the relative susceptibilities of the many substrates of carbonic anhydrase II including pyruvate hydrate.