Effect of Potassium & Other Univalent Cations on Activity of Pyruvate Kinase in Pisum sativum

Potassium is a major component of both plant and animal organisms, yet information on the role of this element in metabolism is limited. A compilation (1) of the inorganic composition of many plant species reveals that a major portion of normal leaves contain from 1 to 4 % K+ on a dry weight basis. Potassium deficiency symptoms are noted when the K+ contents of leaves range from 0.1 to 1.3 % of their dry matter. There are many general statements (3) to the effect that K+, Na+, and other ions that are present in biological materials in high concentrations function in the osmotic regulation of tissue fluids. Undoubtedly this is an important role of the so-called bulk ions, but relatively recent biochemical investigations (3) have provided convincing evidence that a series of enzyme systems require K+, NH4+ or Rb+ for activity and that the concentration of these ions needed for maximum reaction velocities is in the same range as the concentration of K+ in normal biological materials. Pyruvate kinase was one of the first enzyme systems shown to require a univalent cation. Initial experiments by Boyer, et al., (2) and further investigations by Lardy and Ziegler (6) and Kachmar and Boyer (5) provided evidence that either K+, NH4+ or Rb + was essential for the activity of pyruvate kinase from animal sources. Sodium ions activated the enzyme slightly and Li+ and Ca++ were inhibitory. Miller and Evans (9) demonstrated that pyruvate kinase from a series of higher plants also require K+, NH4+ or Rb+ and that a concentration near 0.04 N was necessary for maximum reaction velocity. Several attempts have been made by the author to determine the effect of K+ deficiency in higher plants on pyruvate kinase in tissue extracts. In all cases, crude extracts of the species investigated contained an active phosphatase which catalyzed the hydrolysis of phosphoenolpyruvate (PEP). The enzymic cleavage of the substrate for pyruvate kinase has prevented an accurate determination of the activity of this enzyme. McCollum, et al. (8) also have observed appreciable phosphatase activity in their investigations of the pyruvate kinase in higher plant species. A possible means of inhibiting certain plant phosphatases is suggested by the work of Spencer (10) who found that low concentrations of molybdate inhibited the hydrolysis of a series of phosphate esters by an acid phosphatase from tomato. Phosphoenolpyruvate was not used as a substrate for the phosphatase in Spencer's investigations. The purpose of the experiments reported here was to develop procedures that will allow valid pyruvate kinase assays in crude extracts of higher plants and to determine the effect of a range of K + and other univalent cation concentrations in nutrient media on the pyruvate kinase activity of cell-free extracts of the pea plant.