Kinetic properties of homogeneous preparations of pig kidney and pig muscle pyruvate kinases (EC 188.8.131.52) were studied. Both isozymes showed a hyperbolic relationship to ADP with an apparent Km of 0.3 mm. K+ and Mg2+ were necessary for the activity of both isozymes, and their dependences on these cations were similar. The muscle isozyme expressed Michaelis-Menten type of kinetics with respect to phosphoenolpyruvate, and the apparent Km was the same (0.03 mm) from pH 5.5 to pH 8.0. In contrast, the dependence on phosphoenolpyruvate changed with pH for the kidney isozyme. It showed similar properties to the muscle isozyme at pH 5.5-7.0 (apparent Km of 0.08 mm), while two apparent Km values for this substrate were present at pH 7.5-8.0, one low (0.1 mm) and one high (0.3-0.6 mm). At pH 7.5, fructose 1,6-bisphosphate converted the kidney isozyme to a kinetical form where only the lower apparent Km for phosphoenolpyruvate was detected. On the other hand, in the presence of alanine or phenylalanine the kidney pyruvate kinase showed only the higher Km for this substrate. At low phosphoenolpyruvate levels both isozymes were inhibited by phenylalanine, and half-maximal inhibition was found at 0.3 and 2.2 mm for the kidney and muscle isozymes, respectively. At a 5 mm concentration of the substrate only the kidney isozyme was inhibited, the apparent Ki being the same. Alanine inhibited the kidney isozyme (apparent Ki at 0.3 mm, irrespective of substrate concentration). No effect was seen on the muscle isozyme. Fructose 1,6-bisphosphate was an activator of the kidney isozyme at phosphoenolpyruvate concentrations below 1.0 mm It also counteracted the inhibition by alanine or phenylalanine of this isozyme. ATP inhibited both isozymes, and this inhibition was not counteracted by fructose 1,6-bisphosphate. The kidney isozyme showed both a high and a low apparent Km for phosphoenolpyruvate in the presence of ATP. The influence of the effectors on the activity of both isozymes varied markedly with pH, except for the action of ATP. At low substrate concentrations, however, the inhibitor action of ATP on the muscle enzyme was diminished around pH 7.5, in contrast to higher or lower pH values. Alanine or phenylalanine were more effective as inhibitors at higher pH values, and fructose 1,6-bisphosphate stimulated the kidney isozyme only at pH levels above pH 6.5. The influence of activators and inhibitors on the regulation of the kidney and muscle pyruvate kinases is discussed.
ASJC Scopus subject areas
- Molecular Biology