Choline acetyltransferase in mammalian synaptosomes: Evidence for an integral membrane-bound form

Choline acetyltransferase activity was detected in extensively washed membranes prepared from rat and guinea pig synaptosomes. When these preparations were treated with the non-ionic detergent Triton X-114 and heated to 37 degrees C to cause phase separation, a significant percentage was found to associate with the detergent-rich phase, indicating that the enzyme might be an integral membrane-bound protein. In guinea pigs receiving septal lesions, a large reduction in both total and in Triton X-114-extractable choline acetyltransferase in hippocampal synaptosomes was observed indicating that the detergent-extracted form is associated with cholinergic nerve terminals. When membrane-bound choline acetyltransferase from lysed, washed synaptosomes was incubated in Triton X-114, 30% of the membrane-associated enzyme could be extracted into the detergent-rich phase. This extraction could be improved by reducing the chloride content of the extraction medium. When the chloride content of synaptosomes, prepared from rat cerebral cortex, was manipulated, by either exposure to ?-aminobutyric acid, muscimol or to a medium containing reduced levels of chloride, the ability of antibodies against choline acetyltransferase to specifically immunolyse (in the presence of complement) the cholinergic synaptosome population was enhanced. These results suggest that the choline acetyltransferase found in the nerve terminal region exists in at least two forms (a soluble and a lipophilic form) which are partially interconvertible. The conversion between the two forms can be influenced by chloride ions.