1. Pig kidney alkaline phosphatase is inactivated by treatment with acid at 0°. 2. Inactivated enzyme can be partially reactivated by incubation at 30° in neutral or alkaline buffer. The amount of reactivation that occurs depends on the degree of acid treatment; enzyme that has been inactivated below pH3·3 shows very little reactivation. 3. Studies of the kinetics of reactivation indicate that the process is greatly accelerated by increasing temperature and proceeds by a unimolecular mechanism. The reactivated enzyme has electrophoretic and gel-filtration properties identical with those of non-treated enzyme. 4. The results can be best explained by assuming that a lowering of the pH causes a reversible conformational change of the alkaline phosphatase molecule to a form that is no longer enzymically active but is very susceptible to permanent denaturation by prolonged acid treatment. A reactivation mechanism involving sub-unit recombination seems unlikely.
