Electrogenic mechanism in Characeae plasma membrane

The chemiosmotic hypothesis has brought new aspects in the plant membrane research. The H+-pump of the plasma membrane generates the electrochemical potential gradient of H+ which is available for transport of ions, sugars and amino acids. Further, the H+ pump works electrogenic. This article summarizes the results on the electrogenic mechanism and its regulation in Characeae cells which were revealed by the intracellular perfusion study. 1) The electrogenic pump is fueled by Mg·ATP. 2) The H+-efflux is dependent on ATP, evidence that the ion species carried by the pump is H+. 3) The electrogenic activity of the pump almost saturated at 100μM ATP. 4) The pump is inhibited by vanadate applied internally and half inhibition concentration is about 5μM. 5) ADP inhibits the pump in the presence of ATP. 6) The pump is activated by light via photosynthesis. Activiation is caused by the factor (s) other than the substrates, H+ and ATP. 7) Anoxia inhibits the pump. However, the extent of ATP decrease caused by anoxia is not enough to account for the inhibition of the membrane potential. Some part of the inhibition is caused by increased ADP. 8) In excitation of the plasma membrane, the pump component of the membrane potential also changes together with the diffusion potential. Advantages of the intracellular perfusion for studying electrogenesis were discussed.