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

Abstract

The role of metals in the activation of acetic thiokinase from spinach (Spinacia oleracea) was studied. Adenyl acetate, an intermediate in the 2-step reaction, was synthesized and the individual steps were assayed by measuring the conversion of adenyl acetate to ATP and acetyl CoA, respectively. The reversible conversion of ATP and acetate to adenyl acetate and pyrophosphate exhibited an absolute requirement for Mg ions. Mg ions were without effect on the subsequent conversion of adenyl acetate to acetyl CoA. K ions did not stimulate the 1st step of the reaction. The synthesis of acetyl CoA from adenyl acetate and CoA, however, required the presence of univalent cations. K or Rb ions satisfied this requirement but Na ions were without effect on the reaction. The optimum concentration of K and Rb was 0.04 [image]. Univa-lent cations inhibited the conversion of adenyl acetate and pyrophosphate to ATP. This inhibition was competitive with respect to Mg suggesting that univalent cations were occupying sites normally occupied by Mg ions in the active complex.