THE EFFECT OF PREINCUBATION, pH, COPPER IONS, FRUCTOSE 1,6-DIPHOSPHATE AND DIETARY CHANGES ON ENZYME ACTIVITY By E. BAILEY, F. STIRPE* AND C. B. TAYLOR

1. Preincubation of partially purified rat liver L-type pyruvate kinase at 250 for 10min. causes amarked increase in co-operativity with respect to both the substrate, phosphoenolpyruvate, and the allosteric activator, fructose 1,6-diphosphate. 2. The results are consistent with the existence of two forms of liver L-type pyruvate kinase, designated forms LA and LB. It is postulated that form LA has a low Km for phosphoenolpyruvate (about 0 1mm) and is not allosterically activated, whereas form LB is allosterically activated by fructose 1,6-diphosphate, exhibiting in the absence of the activator sigmoidal kinetics with half-maximal activity at about 1 mM-phosphoenolpyruvate. In the presence of fructose 1,6-diphosphate, form LB gives Michaelis-Menten kinetics with Km less than 0 1mm. It is further postulated that preincubation converts form LA into form LB. 3. The influence of pH on the preincubation effect was studied. 4. The inhibition ofpyruvate kinase by Cu2+ was studied in detail. Though phosphoenolpyruvate and fructose 1,6diphosphate readily protect the enzyme against Cu2+ inhibition, little evidence of significant reversal of the inhibition by these compounds could be found. 5. The effects of starvation, fructose feeding and preincubation on the pyruvate kinase activity of crude homogenates of various tissues of the rat were also studied.