Atypical effect of some spin trapping agents: Reversible inhibition of acetylcholinesterase

N-tert-butyl-alpha-phenylnitrone (PBN), a widely used nitrone-based free ra dical trap was recently shown to prevent acetylcholinesterase (AChE) inhibi tors induced muscle fasciculations and brain seizures while being ineffecti ve against glutamergic or cholinergic receptor agonist induced seizures. In the present study we compared the effects on AChE activity of four free ra dical spin traps PEN, alpha-(4-pyridil-1)-N-tert-butyl nitrone (POBN), N-te rt-butyl-alpha-(2-sulfophenyl)-nitrone (S-PBN) and 5-diethoxyphosphoryl-5-m ethyl-1-pyrroline-N-oxide (DEPMPO). The kinetics of AChE inhibition were st udied in vitro using a spectrophotometric kinetic assay with AChE from rat brain, diaphragm, electric eel and mouse brain. Spin trapping compounds S-P BN and DEPMPO, in concentrations up to 3 mM did not inhibit hydrolysis of A Ch, while PEN and POBN inhibited hydrolysis of ACh in a reversible and conc entration-dependent manner. Double reciprocal plots of the reaction velocit y against varying ACh concentrations at each inhibitor concentration were l inear and generally indicated mixed type inhibition. PEN was the most poten t inhibitor of mouse AChE with Ki and Ki' of 0.58 and 2.99 mM, respectively , and the weakest inhibitor of electric eel AChE. In contrast, POBN showed the highest affinity for electric eel enzyme, with Ki and Ki' values of 1.0 65 and 3.15 mM, respectively. These findings suggest that the effect of PEN and POBN on AChE activity does not depend on trapping of damaging reactive oxygen and that in addition to their antioxidant action other pharmacologi cal effects of these compounds should be considered when neuroprotective ac tions of PEN or POBN are investigated. (C) 2000 Elsevier Science Inc.