The structural basis of inhibitory effect of organic solvent dimethyl sulfoxide (DMSO) on human acetylcholinesterase (EC 3.1.1.7; hAChE) was inferred from the effect of DMSO on kinetics of reversible inhibition of uncharged, heterocyclic bis-oximes to hAChE, from DMSO effect on rates of reactivation of inactive organophosphate (OP)-hAChE conjugates by bis-oximes and by X-ray structures of bis-oxime and DMSO binding to hAChE. The reversible inhibition constant of DMSO for hAChE in 0.1 M phosphate buffer pH 7.4 at 22 °C, was Ki= (0.32 ± 0.04) % (or 45 ± 5 mM). The Ki of the bis-oxime LG-703 for hAChE was 3.2-fold larger in 1 % DMSO, consistent with direct competition between LG-703 and DMSO. The X-ray structure of the LG-703∗hAChE complex (PDB ID: 6U3P) shows DMSO and LG-703 bound to individual hAChE monomers, LG-703 in the chain A and DMSO in the chain B. In the co-crystallization both small molecules were present at a similar excess over their corresponding Ki values for hAChE (7.8-fold for DMSO and 6.5-fold for LG-703) and formation of two different complexes (DMSO∗hAChE and LG-703∗hAChE), in the same crystal, appears consistent with inhibition kinetics. Furthermore, rates of reactivation of paraoxon-inhibited hAChE (POX-hAChE) and of VX-hAChE by LG-703 and by a novel heterocyclic bis-oxime LG-1922 were reduced 2 - 3-fold in DMSO, consistent with observation of the active-center-bound DMSO molecules in the newly solved structure of the LG-1922∗POX-hAChE complex presented here and in our POX-hAChE structure (PDB ID: 8DT2) showing obstruction of the reactivator access to the conjugated P atom.

Department of Natural Sciences Tennessee Wesleyan University Athens TN 37303 USA

Department of Toxicology and Military Pharmacy Military Faculty of Medicine University of Defence 500 01 Hradec Kralove Czech Republic; Biomedical Research Center University Hospital Hradec Kralove 500 05 Hradec Kralove Czech Republic

Institute for Medical Research and Occupational Health HR 10001 Zagreb Croatia

Neutron Scattering Division Oak Ridge National Laboratory Oak Ridge TN 37831 USA

Skaggs School of Pharmacy and Pharmaceutical Sciences University of California San Diego La Jolla CA 92093 0650 USA

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