Biophysical and biochemical studies of isolated cholinergic vesicles from Torpedo marmorata

Recent work on the structure and function of cholinergic synaptic vesicles isolated from the electrometer nerve terminals of Torpedo marmorata, in which lipid and protein analysis, water space and density measurements, proton and 31P NMR spectroscopy and uptake were used, is reviewed. A consistent model emerges of a vesicle with a highly hydrated core, containing acetylcholine in free solution at an approximate concentration of 0.5 M, ATP and Ca2+ at an approximate concentration of 0.2 M, and a pH of about 6.7 enclosed within a lipoprotein membrane with a fairly high (approximately 30%) water content and about five specific protein constituents. One of these is actin; another, resembling the ADP/ATP exchange carrier of mitochondria, is thought to be the ATP carrier; and a third may be a Ca2+,Mg2+-activated ATPase known to be associated with these vesicles. The vesicular ATP carrier is less specific than the mitochondrial carrier. Small amounts of glucosaminoglycan are present, the negative charges of which may have a role in binding Ca2+ and acetylcholine. Actin is the only common constituent of the vesicle and presynaptic plasma membrane, which shows that the former retains its identity through one or more cycles of exo- and endocytosis. Glycerol water space measurements show that the small, dense synaptic vesicles generated by stimulation-induced cycles of exo- and endocytosis contain less water than vesicles from unstimulated tissue. It is suggested that these changes are secondary to osmotic pressure changes in the vesicle core accompanying discharge and repletion of transmitter and ATP.