Abstract
We prepared and characterized subcellular membrane fractions from the CNS of Aplysia californica that are enriched in isolated nerve terminals (synaptosomes). Ganglia were homogenized in 1.1 M sucrose and fractionated on a 2-step sucrose gradient, yielding 50 micrograms protein/animal in the synaptosomal fraction (P3), which was enriched 3- fold in plasma membrane as compared with the initial homogenate. Quantitative morphometry of electron micrographs revealed that P3 contained 25% intact synaptosomes, a 5-fold enrichment over the homogenate. Although fractionation on a 5-step sucrose gradient reduced the yield of protein in the synaptosomal fraction to 40 micrograms/animal, this fraction (the 0.35 M/0.75 M interface) was more enriched in plasma membrane than P3 and was less contaminated by lysosomes and free mitochondria. By electron microscopy, the 0.35 M/0.75 M interface contained up to 50% synaptosomes. Synaptosomal fractions contained cAMP-, Ca2+/calmodulin-, and Ca2+/phospholipid- dependent protein kinase activities and were enriched in a Mr 40,000 pertussis toxin substrate, Gi/o. In the accompanying paper, we show that these synaptosomes retain the ability to release transmitters.