Cross-linking of human placenta pi class glutathione S-transferase dimer by chlorambucil

Interaction of chlorambucil and the glutathione-depleted human placenta pi class glutathione S-transferase (pi GST) results in the formation of a complex between the drug and the protein at physiological pH. This complex is not formed in the presence of glutathione or S-hexylglutathione. Molecular mass measurement of the reaction product using matrix-assisted laser desorption mass spectrometry indicates that one molecule of chlorambucil cross-links two subunits of the homodimeric protein. A combination of enzymic proteolysis, high performance liquid chromatography, and mass spectrometry reveals that chlorambucil alkylation occurs at cysteine 47 of one subunit and cysteine 101 of the second subunit. This result supports the idea that conformational changes occur in glutathione-depleted pi GST, which allow the bifunctional tether of chlorambucil to cross-link the two subunits of the protein.