Non-quantal acetylcholine release after cholinesterase inhibition in vivo.

After anticholinesterase treatment in vivo, depolarization of the postsynaptic muscle fibre membrane by about 4 mV develops due to non-quantally released acetylcholine from the motor nerve terminal. This conclusion was supported by experiments with the curarization of diaphragm slices from anticholinesterase treated mice during intracellular microelectrode recordings.     

Dibucaine inhibition of serum cholinesterase

The dibucaine number (DN) was determined for serum cholinesterase (EC 3.1.1.8, SChE) in plasma samples. The ones with a DN of 79-82 were used, because they had the "usual" SChE variant. The enzyme was assayed colorimetrically by the reaction of 5,5'-dithiobis-[2-nitrobenzoic acid] (DTNB) with the free sulfhydryl groups of thiocholine that were produced by the enzyme reaction with butrylthiocholine (BuTch) or acetylthiocholine (AcTch) substrates, and measured at 412 nm. Dibucaine, a quaternary ammonium compound, inhibited SChE to a minimum within 2 min in a reversible manner. The inhibition was very potent. It had an IC(50) of 5.3 microM with BuTch or 3.8 microM with AcTch. The inhibition was competitive with respect to BuTch with a K(i) of 1.3 microM and a linear-mixed type (competitive/noncompetitive) with respect to AcTch with inhibition constants, K(i) and K(I) of 0.66 and 2.5 microM, respectively. Dibucaine possesses a butoxy side chain that is similar to the butryl group of BuTch and longer by an ethylene group from AcTch. This may account for the difference in inhibition behavior. It may also suggest the existence of an additional binding site, other than the anionic binding site, and of a hydrophobic nature.     

The kinetics of inhibition of erythrocyte cholinesterase by monomethylcarbamates

1. The kinetics of the interaction of erythrocyte cholinesterase with 1-naphthyl N-methylcarbamate, 2-isopropoxyphenyl N-methylcarbamate and phenyl N-methylcarbamate were studied. Rate constants for inhibition and rate constants for spontaneous reactivation were determined. The calculated rate constants for spontaneous reactivation agreed well with those obtained experimentally. 2. The degree of inhibition obtained after preincubation of enzyme and inhibitor was found to be independent of both the substrate concentration and the dilution of the inhibited enzyme. 3. The reaction between the enzyme and the inhibitor was consistent with carbamates being regarded as poor substrates of cholinesterases. There was no evidence for the formation of a reversible complex between the enzyme and the carbamate.