Aspartate aminotransferase catalyzed oxygen exchange with solvent from oxygen-18-enriched .alpha.-ketoglutarate: evidence for slow exchange of enzyme-bound water

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Abstract
Partitioning of the ketimine (or ketimine + quinonoid) intermediate(s) in the mitochondrial aspartate aminotransferase reactions was investigated by following the rates of loss of 18O from carbonyl-18O-enriched .alpha.-ketoglutarate together with the rate of L-glutamate formation. The ratio of these rate constants was found to equal 1 at 10.degree. C, implying that the above intermediate(s) face(s) equal barriers with respect to the forward and reverse reactions. This partition ratio of 1 together with that measured from the .alpha.-amino acid side of the reaction [Julin, D. A., Wiesinger, H., Toney, M. D., and Kirsch, J. F. (1989) Biochemistry (preceding paper in this issue)] suggests that the rate constant for exchange of .alpha.-keto-glutarate-derived H218O from the ketimine (or ketimine + quinonoid) form(s) of the enzyme with solvent is comparable with that for kcat.

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