Pyruvate carboxylase from chicken liver mitochondria is rapidly inactivated by exposure to low temperatures and loss of catalytic activity is accompanied by an apparent dissociation of a tetrameric form of the enzyme into four protomers. Both inactivation and dissociation can be reversed almost completely under appropriate conditions by rewarming. The results of a more detailed study of these processes are consistent with a rapid and reversible dissociation reaction which involves the intermediate formation of an inactive tetramer. The results also suggest that on longer exposure to low temperatures, further changes occur in the protomer which prevent reactivation by rewarming although association to tetramers or aggregates may still occur. Inactivation and dissociation effects similar to those observed at low temperatures can occur at room temperature in the presence of low concentrations of urea and to some extent after adjustment to mildly alkaline pH ranges. Acetyl coenzyme A affords partial or complete protection against the inactivation and dissociation caused by cold, urea, or pH changes. The concentration range, high degree of concentration dependence, and specificity of the protective effects of acyl coenzyme A are very similar to those observed for the activation of the catalytic reaction of this enzyme by this class of compounds. The enzyme is also protected against dissociation and to some extent against inactivation by adenosine triphosphate and several related substances. The presence of adenosine triphosphate considerably enhances the reactivation process in terms of recovery of catalytic activity and restoration of the original sedimentation pattern. The presence of a number of other substances including inorganic phosphate, methanol, several polyols, high concentrations of KCl, and oxalacetate, protect pyruvate carboxylase against inactivation by low temperatures. © 1969, American Chemical Society. All rights reserved.
