Phospholipid translocation from the outer to the inner leaflet of synaptic vesicle membranes isolated from the electric organ of Japanese electric ray Narke japonica

The phospholipid translocation from the outer to the inner leaflet of synaptic vesicles isolated from the electric organ of the Japanese electric ray, Narke japonica, was measured using fluorescent phospholipid probes. Phosphatidylcholine (PC), phosphatidylethanolamine (PE), or phosphatidylserine (PS) with a fluorescent NBD-labeled short acyl chain at the sn-2 position was mixed with purified synaptic vesicles and the probe in the outer leaflet of the membranes was reduced with dithionite to quench the fluorescence from time to time. The percentage of fluorescence remaining after the dithionite treatment served as an index for the phospholipid translocation. The results obtained indicated that about 30, 13, and 9% of NBD-PE, NBD-PS, and NBD-PC, respectively, were translocated from the outer to the inner leaflet in 3 h. Thus, the translocation activity in synaptic vesicle membranes was much higher for PE than for PS, in contrast to the previous results obtained with plasma membranes, including synaptosomal membranes. The percentages of the phospholipid in the inner leaflet at equilibrium were estimated to be 41, 31, and 14% for PE, PS, and PC, respectively. The translocation was inhibited by pretreatment with an SH reagent, iodoacetamide, indicating the involvement of a proteinaceous translocator. These data may provide a biochemical basis for elucidating the mechanisms of membrane fusion and exocytosis at nerve endings.