Effect of monovalent cations on <i>Vc</i>IPK (A) and <i>Vc</i>IIPK (B) activities.

<p><i>Vc</i>IPK (closed symbols) and <i>Vc</i>IIPK (open symbols) activities in the presence of K⁺ (■,□), Rb⁺ (▲,△), NH₄⁺ (●,○), Cs⁺ (▼,▽), Na⁺ (◆, ◇) or Li⁺ (◀,◁) are shown. The reaction mixtures contained 25 mM HEPES pH 7.0, 0.2 mM NADH, 8 μg/ml LDH and TMACl in order to maintain the ionic strength at 300 and 450 mM in <i>Vc</i>IPK and <i>Vc</i>IIPK, respectively. In (A) the concentrations of PEP^3-, ADP-Mg and Mg²⁺_free were 16 mM, 6.5 mM and 2 mM, respectively. In (B) the experiments were carried out with 30.8 mM PEP^3-, 9 mM ADP-Mg and 15 mM Mg²⁺_free. Assays were performed at 25°C and the reactions were started by the addition of PK. Amounts of PK ranged from 0.1 to 0.5 μg/ml. The data of figure 2A were fitted to the Hill equation. Standard deviation bars of three experiments are shown.