Thiol esters. I. Kinetic study of the formation of 2-dimethylaminoethanethiol propionate

The characterization of Lucilia cuprina acetylcholinesterase as a drug target, and the identification of novel inhibitors by high throughput screening

Acetylcholinesterase (AChE, EC3.1.1.7.) is the molecular target for the carbamate and organophosphate pesticides that are used to combat parasitic arthropods. In this paper we report the functional heterologous expression of AChE from Lucilia cuprina (the sheep blowfly) in HEK293 cells. We show that the expressed enzyme is cell-surface-exposed and possesses a glycosyl-phosphatidylinositol membrane anchor. The substrates acetyl-, propionyl- and butyrylthiocholine (AcTC, PropTC, ButTC), and also 11 further thiocholine and homo-thiocholine derivatives were chemically synthesized to evaluate and compare their substrate properties in L. cuprina AChE and recombinant human AChE. The Michaelis-Menten constants K(M) for AcTC, PropTC and ButTC were found to be 3-7-fold lower for the L. cuprina AChE than for the human AChE. Additionally, 2-methoxyacetyl-thiocholine and isobutyryl-thiocholine were better substrates for the insect enzyme than for the human AChE. The AcTC, PropTC and ButTC specificities and the Michaelis-Menten constants for recombinant L. cuprina AChE were similar to those determined for AChE extracted from L. cuprina heads, which are a particularly rich source of this enzyme. The median inhibition concentrations (IC(50) values) were determined for 21 organophosphates, 23 carbamates and also 9 known non-covalent AChE inhibitors. Interestingly, 11 compounds were 100- to >4000-fold more active on the insect enzyme than on the human enzyme. The substrate and inhibitor selectivity data collectively indicate that there are structural differences between L. cuprina and human AChE in or near the active sites, suggesting that it may be possible to identify novel, specific L. cuprina AChE inhibitors. To this end, a high throughput screen with 107,893 compounds was performed on the L. cuprina head AChE. This led to the identification of 195 non-carbamate, non-organophosphate inhibitors with IC(50) values below 10μM. Analysis of the most potent hit compounds identified 19 previously unknown inhibitors with IC(50) values below 200nM, which were up to 335-fold more potent on the L. cuprina enzyme than on the human AChE. Some of these compounds may serve as leads for lead optimization programs to generate fly-specific pesticides.

S-acylation of cysteine by O-acetylsalicylic anhydride: a possible mechanism for aspirin hypersensitivity?

In order to elucidate the possible reaction pathways for the acylation of protein by O-acetylsalicylic anhydride, the mechanism of the reaction between L-cysteine and O-acetylsalicylic anhydride was studied. O-Acetylsalicylic anhydride reacts with L-cysteine via a consecutive kinetic pathway. The thiol anion first reacts with the anhydride to form an intermediate thiol ester which then undergoes an intramolecular rearrangement to form the stable N-(O-acetylsalicyloyl)-2-amino-3-thiopropionic acid, 5. The importance of the free amino group in the intramolecular reaction was established by the observed stability of the S-(O-acetylsalicyloyl) derivative of N-acetylcysteine under similar reaction conditions. The formation of the thiol ester was pH dependent, suggesting that the thiol anion was the attacking species. The acyl transfer to the adjacent amino group was catalyzed by both phosphate and acetate buffers. The results suggest that the reaction of O-acetylsalicylic anhydride with the thiol-containing amino acids of a protein molecule may proceed via formation of an initial thio ester, followed by an S to N intramolecular acyl transfer to form an immunogenic amide.