Partial characterization of the in vitro activation of inactive pyruvate, pi dikinase from darkened maize leaves

In vitro activation of dark-inactivated pyruvate, orthophosphate dikinase extracted from maize (Zea mays L. cv. Golden Cross Bantam T51) leaves was examined. The inactive form of the enzyme and orthophosphate behaved kinetically as substrates for the reaction, which was catalyzed by an activating factor. This factor was bound by Blue Dextran Sepharose 4B and could be eluted by KCl at a concentration of 0.5m. The molecular weight of the maize leaf activating factor was about 88,000. Cibacron Blue 3G-A, a reactive moiety of Blue Dextran, inhibited the factor competitively with respect to the concentration of the inactive dikinase with a K(i) of 4.6 micromolar. Adenosine diphosphate and pyrophosphate were also found to be competitive inhibitors of activation, with respect to the inactive dikinase, giving K(i) values of 90 and 140 micromolar, respectively. Adenosine, other nucleotide diphosphates, and dinucleotides gave little or no inhibition of activation. These results suggest the association of a nucleotide, presumably nucleotide diphosphate, with the inactive form of pyruvate, orthophosphate dikinase.