Regulation of rat liver pyruvate kinase. The effect of preincubation, pH, copper ions, fructose 1,6-diphosphate and dietary changes on enzyme activity

[摘要]

1. Preincubation of partially purified rat liver L-type pyruvate kinase at 25° for 10min. causes a marked 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 L_A and L_B. It is postulated that form L_A has a low K_m for phosphoenolpyruvate (about 0·1mm) and is not allosterically activated, whereas form L_B is allosterically activated by fructose 1,6-diphosphate, exhibiting in the absence of the activator sigmoidal kinetics with half-maximal activity at about 1mm-phosphoenolpyruvate. In the presence of fructose 1,6-diphosphate, form L_B gives Michaelis–Menten kinetics with K_m less than 0·1mm. It is further postulated that preincubation converts form L_A into form L_B. 3. The influence of pH on the preincubation effect was studied. 4. The inhibition of pyruvate kinase by Cu²⁺ was studied in detail. Though phosphoenolpyruvate and fructose 1,6-diphosphate 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.