Further Studies on the Activation of Acetic Thiokinase by Magnesium and Univalent Cations

Abstract

The activation of acetate by acetic thiokinase has been shown to proceed through 2 steps (2, 18): ATP + acetate =Adenyl acetate + PP1 I Adenyl acetate + CoA = Acetyl CoA + AMP II Von Korff (16) studied the over-all reaction and reported that univalent cations were required for the synthesis of acetyl CoA from ATP, acetate and CoA. K+, Rb+, and NH4+ at concentrations of 0.04 M gave maximum activity while Na+ and Li+ strongly inhibited the reaction. Hiatt and Evans (6) reported a similar effect of univalent cations upon the overall reaction catalyzed by acetic thiokinase from plants. The inhibitory effect of Na+ and Li+ was not due to competition between thes-e ions and activating univalent cations (6). Pyruvate kinase from animal tissues (8), formate activating enzyme from erythrocytes (4), yeast aldehyde dehydrogenase (5), and phosphotransacetylase (15) are other enzymes which are activated by K+ but are inhibited by Na+. Incorporation of amino acids into protein by cell-free extracts of pea roots is promoted by K+ and inhibited by Na+, Li+, NH4+, and Rb+ (17). The nature of the Na+ inhibition of these enzymes was not studied in detail. The overall acetic thiokinase reaction requires Mg++ (1,7,12) and Berg (2) reported that Mg+ + is required for only the first step in the reaction sequence. The effect of univalent cations on the individual steps in the reaction sequence has not been studied with either plant or animal acetic thiokinase. In this paper experimentation on the effect of cations on acetic thiokinase has been extended to study their effect on individual steps of the reaction sequence. It was considered that these studies would present a better understanding of the role of univalent cations in enzyme reactions and of the antagonism between Na+ and activating cations in certain enzyme systems.