1. Pig heart pyruvate dehydrogenase phosphate complex in which all three sites of phosphorylation were completely phosphorylated was re-activated at a slower rate by phosphatase than complex predominantly phosphorylated in site 1. The ratio of initial rates of re-activation was approx. 1:5 with a comparatively crude preparation of phosphatase and with phosphatase purified by gel filtration and ion-exchange chromatography. 2. The ratio of apparent first-order rate constants during dephosphorylation of fully phosphorylated complex averaged 1/3.8/1.3 for site 1/site 2/site 3. Only site-1 dephosphorylation was linearly correlated with re-activation of the complex throughout dephosphorylation. Dephosphorylation of site 3 was linearly correlated with re-activation after an initial burst of dephosphorylation. 3. Because dephosphorylation of site 1 was always associated with dephosphorylation of site 2, it is concluded that dephosphorylation cannot be purely random. 4. The ratio of apparent first-order rate constants for dephosphorylation of site 1 (partially/fully phosphorylated complexes) averaged 1.72. This ratio is smaller than the ratio of approx. 5 for the initial rates of re-activation. Possible mechanisms involved in the diminished rate of re-activation of fully phosphorylated complex are discussed.

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