Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of titanium oxide-enriched peptides for detection of aged organophosphorus adducts on human butyrylcholinesterase

Exposure to nerve agents or organophosphorus (OP) pesticides can have life-threatening effects. Human plasma butyrylcholinesterase (BChE) inactivates these poisons by binding them to Ser198. After hours or days, these OP adducts acquire a negative charge by dealkylation in a process called aging. Our goal was to develop a method for enriching the aged adduct to facilitate detection of exposure. Human BChE inhibited by OP toxicants was incubated for 4 days to 6 years. Peptides produced by digestion with pepsin were enriched by binding to titanium oxide (TiO2) and analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. It was found that with two exceptions, all aged OP adducts in peptide FGES198AGAAS were enriched by binding to Titansphere tips. Cresyl saligenin phosphate yielded two types of aged adduct, cresylphosphate and phosphate, but only the phosphate adduct bound to Titansphere. The nerve agent VR yielded no aged adduct, supporting crystal structure findings that the VR adduct on BChE does not age. The irreversible nature of aged OP adducts was demonstrated by the finding that after 6 years at room temperature in sterile pH 7.0 buffer, the adducts were still detectable. It was concluded that TiO2 microcolumns can be used to enrich aged OP-modified BChE peptide.