Targeted oxidation of Torpedo californica acetylcholinesterase by singlet oxygen: identification of N-formylkynurenine tryptophan derivatives within the active-site gorge of its complex with the photosensitizer Methylene Blue

The principal role of AChE (acetylcholinesterase) is termination of impulse transmission at cholinergic synapses by rapid hydrolysis of the neurotransmitter acetylcholine. The active site of AChE is near the bottom of a long and narrow gorge lined with aromatic residues. It contains a CAS (catalytic ‘anionic’ subsite) and a second PAS (peripheral ‘anionic’ site), the gorge mouth, both of which bind acetylcholine via π-cation interactions, primarily with two conserved tryptophan residues. It was shown previously that generation of ¹O₂ by illumination of MB (Methylene Blue) causes irreversible inactivation of TcAChE (Torpedo californica AChE), and suggested that photo-oxidation of tryptophan residues might be responsible. In the present study, structural modification of the TcAChE tryptophan residues induced by MB-sensitized oxidation was investigated using anti-N-formylkynurenine antibodies and MS. From these analyses, we determined that N-formylkynurenine derivatives were specifically produced from Trp⁸⁴ and Trp²⁷⁹, present at the CAS and PAS respectively. Peptides containing these two oxidized tryptophan residues were not detected when the competitive inhibitors, edrophonium and propidium (which should displace MB from the gorge) were present during illumination, in agreement with their efficient protection against the MB-induced photo-inactivation. Thus the bound MB elicited selective action of ¹O₂ on the tryptophan residues facing on to the water-filled active-site gorge. The findings of the present study thus demonstrate the localized action and high specificity of MB-sensitized photo-oxidation of TcAChE, as well as the value of this enzyme as a model system for studying the mechanism of action and specificity of photosensitizing agents.