Photoinactivation of acetylcholinesterase by erythrosin B and related compounds

Abstract

Acetylcholinesterase was rapidly inactivated when exposed to light in the presence of xanthene dyes. Photosensitizing efficiency paralleled the dye triplet state quantum yields, increasing in the order fluorescein < eosin B < eosin Y < erythrosin B < rose bengal. The observed first-order rate constants of photoinactivation increased hyperbolically with dye concentration. Evidence for the formation of a dye–enzyme complex prior to inactivation was obtained from spectrophotometric and protein fluorescence quenching methods. The latter technique allowed estimates of the dye–enzyme dissociation constants for rose bengal (20 μM) and erythrosin B (30 μM). After photoinactivation, a portion of the dye became covalently bound to the enzyme. The photoinactivation reaction occurs in both aerobic (air saturated) and anaerobic (argon saturated) solution, with the rates of photoinactivation being about three to five times greater under the latter conditions. The aerobic reaction exhibits a large deuterium isotope enhancement effect and is largely (but not completely) quenched by 10−2 M azide. The anaerobic reaction is unaffected by azide and exhibits only a small deuterium isotope effect. These results indicate that the photoinactivation reaction proceeds mainly by a type II (singlet oxygen mediated) pathway under aerobic conditions and by a type I (radical) pathway under anaerobic conditions. The enzyme was protected from inactivation by edrophonium, a competitive inhibitor, but not by d-tubocurarine, a peripheral-site ligand, indicating that destruction of a crucial residue at or near the catalytic site is an important component of the inactivation process. Extensive destruction of tryptophan undoubtedly occurs, at least under aerobic conditions. The destruction of smaller amounts of other photosensitive residues cannot be ruled out. The pH dependence of the photoinactivation reaction appears to eliminate histidine as a primary target under both aerobic and anaerobic conditions; an additional group which appears to titrate in the region of pH 7–9 is observed under anaerobic but not aerobic conditions.