Celecoxib non-competitively inhibits acetylcholinesterase

Abstract

Isocitrate lyase from the mycelium of Phycomyces blakesleeanus was inactivated with thiol-reactive reagents, 5,5′-dithiobis-(2-nitrobenzoic)acid, p-hydroxymercuribenzoic acid, N-ethylmaleimide or iodoacetate, at pH 6.8 and 25°C. In all cases the inactivation is characterized by a biphasic kinetic profile. The rapid initial phase of inactivation does not increase linearly with increasing reagent concentration, but exhibits an apparent saturation effect, suggesting the formation of a reversible complex between the enzyme and the reagent prior to the inactivation step. Re-activation of the enzyme was observed under thiol excess treatment. The pH dependence of the initial phase of inactivation suggests that a group on the enzyme with pKa = 6.8 is being modified. The effect of ligands was tested on the inactivation reaction. Mg2+-Ds-isocitrate and Ds-isocitrate provided total protection, whereas Mg2+ ions, succinate and oxalate provided only partial protection of the enzyme against inactivation. On the basis of these results, we would suggest that the thiol-reactive reagents modify at least one thiol group crucial for the enzymatic activity and probably located in the interface between succinate and glyoxylate subsite.