The stabilizing effect of ADP and the binding of ADP by the threonine dehydrase of

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Abstract Evidence has been presented earlier (Hayaishi, et al ., 1963) that ADP stimulates the deamination of threonine to alpha ketobutyrate by threonine dehydrase obtained from extracts of Clostridium tetanomorphum . The effect of ADP is relatively specific and is much more pronounced at low concentrations of threonine than at high concentrations. ADP does not participate directly in the reaction but markedly increases the affinity of the enzyme for its substrate (Hayaishi, et al ., 1963), i.e., ADP may be called an “allosteric effector” (Monod and Jacob, 1961; Monod, et al ., 1963). However, the mechanism by which ADP exerts its effect is unknown. The present communication describes two additional observations which may lead to an understanding of the mode of action of some allosteric effectors: 1) ADP protects threonine dehydrase activity against inactivation by dilution and heat, and 2) ADP is bound by a partially purified enzyme fraction.

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