KINETICS OF CATION-INDUCED AGGREGATION OF TORPEDO ELECTRIC ORGAN SYNAPTIC VESICLES

Synaptic vesicles from the Torpedo ray can be induced to aggregate in the presence of Ca2+ and K+ in the 4 mM and 50 mM range, respectively. The reactions are strikingly similar to those of chromaffin granule membranes reported previously (Morris, S.J., Chiu, V.C.K. and Haynes, D.H. (1979) Membrane Biochem. 2, 163-202). The Ca2+-induced reaction includes dimerization and higher order aggregation, and is shown to be due to electrostatic screening interactions and binding to negatively-charged groups on the membrane surface. The K+-induced reaction includes only dimerization and is shown to be due to screening interactions alone. The kinetics of the dimerization reactions were studied using the stopped-flow rapid mixing technique. The Ca2+-induced reaction has a 'bimolecular' rate constant of 4.77 · 108 M-1 · s-1 while the value for the K+-induced reaction is 7.05 · 109 M-1 · s-1. These values are close to the limit of diffusion control (8.03 · 109 M-1 · s-1), indicating that no large energy barriers or structural barriers to aggregation exist. Arrhenius plots for the Ca2+-induced aggregation showed a break at 5°C. Above this temperature, the activation energy is low (+0.65 kcal/mol), consistent with the above. Below this temperature, the activation energy is high, consistent with a membrane structure change increasing the energetic and structural barriers. This information, and the observation of a high stability constant of the complex, were taken as evidence for the involvement of 'recognition sites' on the membrane surface. © 1979.