Covalent modification of lysine residues by allyl isothiocyanate in physiological conditions: Plausible transformation of isothiocyanate from thiol to amine

We investigated the reactivity of allyl isothiocyanate (AITC) with amino groups under physiological conditions. First, the chemical reaction of AITC with bovine serum albumin (BSA) was investigated. When BSA was incubated with AITC in a phosphate buffer (pH 7.4), the loss of Lys residues was observed. Second, the Lys residue N^α-benzoyl-glycyl-L-lysine (BGK) was reacted with AITC in the buffer, and a novel peak was detected using high performance liquid chromatography (HPLC). The peak was purified and identified as AITC-modified BGK with a N^£-thiocarbamoyl linkage. However, a thiol residue is known to be a predominant target of an isothiocyanate (ITC). Although AITC may react with a thiol moiety in vivo, a thiocarbamoyl linkage between ITC and thiol is unstable, and an AITC molecule may be regenerated. To prove the plausible transformation of ITC from thiol to amine, synthetic AITC-conjugated N^α-acetyl-L-cysteine (NAC) was incubated with BGK at 37 °C in physiological buffer, and the generation of AITC-Lys was analyzed. The loss of the AITC-NAC adduct corresponded to the formation of the AITC-BGK adduct. Furthermore, using a novel monoclonal antibody (A4C7mAb) specific for AITC-Lys, we found that the AITC-Lys residue was generated from the reaction between AITC-NAC and BSA. Although AITC preferentially reacts with thiol rather than with Lys, AITC can be liberated from thiols and can then react with amino groups. The ITC-Lys adduct may be a useful marker for ITC target molecules.