AH5183 and Cetiedil: Two Potent Inhibitors of Acetylcholine Uptake into Isolated Synaptic Vesicles from Torpedo marmorata

Abstract

Synaptic vesicles purified on a sucrose-KLl sedimentation gradient were tested for their ability to accumulate [l-14C]acetylcholine ([l-14C]ACh) in the absence and in the presence of AH5183 and cetiedil. Kinetic studies of ACh transport showed that it was time dependent and saturable as a function of ACh concentration, with a KT of 1.2 mM. The protein-modifying agents N-ethylmaleimide and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole were powerful inhibitors of ACh uptake. In agreement with other studies. AH5183 was found to be a potent inhibitor of ACh uptake by synaptic vesicles. Inhibition was of the mixed noncompetitive type, and the inhibition constant was 45.2 .+-. 3.4 nM. Cetiedil, a drug that resembles ACh, was previously shown on intact nerve endings to inhibit the translocation of newly synthesized ACh into the synaptic vesicle compartment, and we demonstrate here that cetiedil is indeed an efficient blocker of ACh uptake by isolated synaptic vesicles. It acted as a competitive inhibitor, with a Ki of 118.5 .+-. 9.5 nM. Neither ATP-dependent calcium uptake nor Mg2+-ATPase activity was affected by the drugs, a finding showing their specificity toward the ACh uptake process. the binding of L-[3H]AH5183 to intact vesicles was characterized in the absence or the presence of ACh or cetiedil. Saturation experiments showed a total binding capacity of .apprx. 126 pmol/mg of vesicular protein and a dissociation constant of 19.9 .+-. 4.1 nM under control conditions. Whereas the specific binding of L-[3H]AH5183 remained unaffected by ACh, even at high concentrations, cetiedil was shown to act as a competitive inhibitor with a Ki of 4.6 .mu.M. We conclude that both AH5183 and cetiedil are efficient inhibitors of ACh transport by isolated synaptic vesicles but that they are likely to act at different sites.