Thioredoxin alkylation by a dihaloethane-glutathione conjugate

Glutathione is a thiol-containing tripeptide which functions to protect cellular constituents from endogenous and xenobiotic electrophiles via conjugation and eventual excretion. In the case of compounds such as 1,2-dihaloethanes, however, conjugate formation results in bioactivation of the species rather than detoxification. The conjugate can then act as an alkylating agent toward cellular constituents including DNA, proteins, or lipids. Alkylation of protein thiols in cells exposed to dihaloethane may contribute substantially to the toxicity produced by these compounds. We examined the reactivity of the conjugate S-(2-chloroethyl)-glutathione (CEG) toward the model protein Escherichia coli thioredoxin. At physiological pH, treatment of thioredoxin by CEG resulted in the production of several bands visible on isoelectric focusing, which were determined by matrix-assisted laser desorption ionization (MALDI) mass spectrometry to be mono-, di-, tri-, and tetra-alkylated forms of thioredoxin. A concomitant loss of in vitro enzymatic activity was observed. These products were also observed when reaction was allowed to take place at pH 11.4. Treatment at pH 4.4 resulted in lesser alkylation of thioredoxin, with only the mono- and di-alkylated forms detected. Iodoacetic acid treatment of CEG-alkylated thioredoxin revealed that the iodoacetic acid-susceptible Cys32 was not carboxymethylated, suggesting that this is one of the sites alkylated by CEG.