Exposure of purified acetylcholinesterase from Torpedo california to a system generating oxygen radicals (viz. ascorbic acid/Fe(EDTA)2/H2O2) inactivated the enzyme. The enzyme retained its native dimeric form, but electrophoresis under denaturing conditions showed some cleavage of peptide bonds. Spectroscopic characterization revealed a shift to the red in the intrinsic fluorescence emission peak, a large decrease in molar ellipticity in the near UV with a much smaller decrease in the far UV, and increased binding of the amphiphilic probe, 1-anilino-8-naphthalene sulfonate, all relative to native enzyme. The treated enzyme was also highly susceptible to proteolysis. These data show that oxygen radical treatment converts acetylcholinesterase to a partially unfolded state, which retains most of its secondary structure but lacks substantial tertiary structure, thus resembling a 'molten globule' state. This model system may offer a mechanism for explaining the consequences of oxidative stress in vivo: partially unfolded proteins generated by oxidative stress may interact with molecular chaperons of the heat shock family, thus activating the heat-shock factor and, thereby, activating heat-shock genes.