Katherine V. Darvesh

Carbamates with Differential Mechanism of Inhibition Toward Acetylcholinesterase and Butyrylcholinesterase

Most carbamates are pseudoirreversible inhibitors of cholinesterases. Phenothiazine carbamates exhibit this inhibition of acetylcholinesterase but produce reversible inhibition of butyrylcholinesterase, suggesting that they do not form a covalent bond with the catalytic serine. This atypical inhibition is attributable to pi-pi interaction of the phenothiazine moiety with F329 and Y332 in butyrylcholinesterase. These residues are in a helical segment, referred to here as the E-helix because it contains E325 of the catalytic triad. The involvement of the E-helix in phenothiazine carbamate reversible inhibition of butyrylcholinesterase is confirmed using mutants of this enzyme at A328, F329, or Y332 that show typical pseudoirreversible inhibition. Thus, in addition to various domains of the butyrylcholinesterase active site gorge, such as the peripheral anionic site and the pi-cationic site of the Omega-loop, the E-helix represents a domain that could be exploited for development of specific inhibitors to treat dementias.

Differential binding of phenothiazine urea derivatives to wild-type human cholinesterases and butyrylcholinesterase mutants.

A series of N-10 urea derivatives of phenothiazine was synthesized and each compound was evaluated for its ability to inhibit human cholinesterases. Most were specific inhibitors of BuChE. However, the potent inhibitory effects on both cholinesterases of one sub-class, the cationic aminoureas, provide an additional binding mechanism to cholinesterases for these compounds. The comparative effects of aminoureas on wild-type BuChE and several BuChE mutants indicate a binding process involving salt linkage with the aspartate of the cholinesterase peripheral anionic site. The effect of such compounds on cholinesterase activity at high substrate concentration supports ionic interaction of aminoureas at the peripheral anionic site.

The Role of Ylides in the Chlorination of Unsymmetrical Sulfides: An Ab Initio Molecular Orbital Study

Ab initio molecular orbital theory has been used to investigate a series of possible chlorosulfonium ylides to assess the potential for ylide intermediacy in the chlorination of organic sulfides. The majority of optimized structures are best viewed as thionium salts. Only ylide structures in which the putative carbanionic centre bears a powerful anion stabilizer (e.g. cyano or carbomethoxy) survive optimization.