Ethylcholine mustard aziridinium (ECMA) inhibits choline transport in synaptosomes at a half-maximal concentration of about 20 microM. The rate of inhibition falls off rapidly after 10 min and the concentration dependency reaches a plateau at about 100 microM. The inhibition is not removed by washing the synaptosomes, and choline and hemicholinium-3 protect the carrier against attack by the mustard. Choline efflux, particularly that stimulated by choline in the medium (transactivation) is also inhibited by the aziridinium compound. Similarly choline influx activated by preloaded internal choline is inhibited by ECMA. The mustard can enter the synaptosomes in an active form but most of the carrier is alkylated when facing the outside. Prior depolarization of the synaptosomes causes an increase in the rate of inhibition by ECMA which is proportionally about the same as the increase in choline influx also caused by depolarization. At low ECMA concentrations the rate of inhibition is that of a first-order reaction with the carrier but at high ECMA concentrations the translocation of the carrier to the outward-facing conformation controls the rate of inhibition. Using a model of choline transport with some simplifying assumptions it is possible to estimate the amount of carrier; cholinergic synaptosomes carry about six times the concentration of carrier found in noncholinergic ones. In noncholinergic synaptosomes the carrier faces predominately out, the reverse in cholinergic ones. The rate constant of carrier translocation is increased by combination with choline some six- to sevenfold to about 3.5 min-1. The rate constant of ECMA attack on the carrier is about 440 M-1 sec-1
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