Determination of in vivo adducts of disulfiram with mitochondrial aldehyde dehydrogenase

Extensive use for disulfiram (DSF) has been found in the aversion therapy treatment of recovering alcoholics. Although it is known to irreversibly inhibit hepatic aldehyde dehydrogenase (ALDH), the specific mechanism of in vivo inhibition of the enzyme by the drug has not been determined yet. We have demonstrated in this report a novel, but simple and rapid method for structurally characterizing in vivo derived protein–drug adducts by linking on-line sample processing to HPLC–electrospray ionization mass spectrometry (HPLC–MS) and HPLC–tandem mass spectrometry (HPLC–MS/MS). Employing this approach, rats were administered DSF, and their liver mitochondria were isolated and solubilized. Both native and in vivo DSF-treated mitochondrial ALDH (mALDH) were purified in one step with an affinity cartridge. The in vivo DSF-treated mALDH showed 77% inhibition in enzyme activity as compared with that of the control. Subsequently, the control and DSF-inhibited mALDH were both subjected to HPLC–MS analyses. We were able to detect two adducts on DSF-inhibited mALDH, as indicated by the mass increases of ∼71 and ∼100 Da. To unequivocally determine the site and structure of these adducts, on-line pepsin digestion-HPLC–MS and HPLC–MS/MS were performed. We observed two new peptides at MH + = 973.7 and MH + = 1001.8 in the pepsin digestion of DSF-inhibited enzyme. These two peptides were subsequently subjected to HPLC–MS/MS for sequence determination. Both peptides possessed the sequence FNQGQC 301C 302C 303, derived from the enzyme active site region, and were modified at Cys 302 by N-ethylcarbamoyl (+71 Da) and N-diethylcarbamoyl (+99 Da) adducts. These findings indicated that N-dealkylation may be an important step in DSF metabolism, and that the inhibition of ALDH occurred by carbamoylation caused by one of the DSF metabolites, most likely S-methyl- N,N-diethylthiocarbamoyl sulfoxide (MeDTC-SO). Finally, there was no evidence of the presence of an intramolecule disulfide bridge modification on the peptide FNQGQCCC.