Effects of secretagogues on ATP levels and protein carboxyl methylation in rat brain synaptosomes.

The influence of various substances which are known to alter free intracellular calcium concentrations on protein carboxyl methyltransferase (PCM) activity was investigated in rat brain synaptosomes. The synaptosomes were labeled with L-[3H]methionine and the 3H-methyl esters of proteins were formed from the methyl donor S-[3H]adenosyl-L-methionine ([3H]AdoMet). The calcium ionophore A23187 and ouabain decreased PCM activity and the decrease produced by A23187 was antagonized by ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid and MnCl2. On the other hand, ruthenium red, an inhibitor of calcium uptake, stimulated PCM activity. These data suggest that PCM activity is inversely related to the free cytoplasmic calcium concentration. Veratridine, A23187 and elevated potassium ions decreased the levels of ATP and [3H]AdoMet. The A23187-mediated decrease in ATP levels and the reduced [3H]AdoMet formation was antagonized by ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid and MnCl2. Inhibition of metabolic activity of the synaptosomes by NaCN led to: decreased ATP levels; inhibition of [3H]AdoMet formation; and inhibition of PCM activity. These data suggest that the decrease in protein methylation produced by secretagogues is associated with an increase in the concentration of free intracellular calcium which results in a decrease in the metabolically active pool of ATP. This leads to a decreased rate of AdoMet formation, a cosubstrate for PCM and a resultant decrease in PCM activity.