A retrospective screening method for carbamate toxicant exposure based on butyrylcholinesterase adducts in human plasma with ultra-high performance liquid chromatography–tandem mass spectrometry

Carbamate pesticides are extensively used in agriculture for their inhibition to acetylcholinesterase and damages to the insects’ neural systems. Because of their toxicity, human poisoning incidents caused by carbamate pesticide exposure have occurred from time to time. What’s more, some lethally toxic carbamate toxicants known as carbamate nerve agents (CMNAs) have been supplemented in Schedule 1 of the Annex on Chemicals in the Chemical Weapons Convention (CWC) by Organisation of the Prohibition of Chemical Weapons (OPCW) from 2020. And some other carbamates, like physostigmine, have been used in clinical treatment as anticholinergic drugs and their misuse may also cause damages to the body. Similar to organophosphorus toxicants, carbamate toxicants would react with butyrylcholinesterase (BChE) in plasma when entering the human body, resulting in the BChE adducts, based on which the exposure of carbamate toxicants could be detected retrospectively. In this study, methylcarbamyl nonapeptide and dimethylcarbamyl nonapeptide from pepsin digestion of BChE adducts were identified with ultra-high performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) in product ion scan mode. Carbofuran was chosen as the target to establish the detection method of carbamate toxicant exposure based on methylcarbamyl nonapeptide digested from methylcarbamyl BChE. Procainamide-gel affinity purification, pepsin digestion and UHPLC–MS/MS analysis in multiple reaction monitoring (MRM) mode were applied. Under the optimized conditions of sample preparation and UHPLC–MS/MS MRM analysis, the limits of detection (LODs) reached 10.0 ng/mL of plasma exposed to carbofuran with satisfactory specificity. The quantitation approach was established with d3-carbofuran-exposed plasma as the internal standard (IS) and the linearity range was 30.0–1.00 × 103 nmol/L (R2 >0.998) with the accuracy of 95.6%–107% and precision of ≤9% relative standard deviation (RSD). The applicability was also evaluated by N,N-dimethyl-carbamates with the LODs of 30.0 nmol/L for pirimicarb-exposed plasma based on dimethylcarbamyl nonapeptide. Because most of carbamate toxicants has methylcarbamyl or dimethylcarbamyl groups, this approach could be applied on the retrospective screening of carbamate toxicant exposure including CMNAs, carbamate pesticides or carbamate drugs. This study could provide an effective means in the fields of CWC verification, toxicological mechanism investigation and down-selection of potential treatment options.