Effect of Atebrin on Bacterial Membrane Adenosine Triphosphatases in Relation to the Divalent Cation Used as Substrate and/or Activator

The action of atebrin on purified adenosine triphosphatase (ATPase) from Micrococcus lysodeikticus was studied as well as on the membrane-bound and soluble ATPases from Escherichia coli and Bacillus megaterium. Atebrin inhibited the Ca(2+)-dependent activity of all these enzymes, and the inhibition was reversed by an excess of Ca(2+) ions. Kinetic studies carried out with the purified enzyme from M. lysodeikticus showed that the inhibition by atebrin was strongly cooperative, suggesting the complex nature of the process. On the other hand, atebrin stimulated the Mg(2+)ATPase activity of the M. lysodeikticus enzyme, displacing its adenosine 5'-triphosphate (ATP)/Mg(2+) optimum ratios, but inhibited the Mg(2+)-ATPase activity of E. coli provided that ATP was in excess over Mg(2+), i.e., that the ATP/Mg(2+) ratio was higher than its optimum. These results suggest that divalent cations influence the bacterial ATPases in different ways depending on the type of divalent ion and/or enzyme. The effect of atebrin on bacterial ATPases may reflect those differences, and its complex mechanism of action might be related to the existence of more than one site for divalent cations and/or distinct conformational states in these enzymes.