Glutathione transferase P1-1 as an arsenic drug-sequestering enzyme

Arsenic-based compounds are paradoxically both poisons and drugs. Glutathione transferase (GSTP1-1) is a major factor in resistance to such drugs. Here we describe using crystallography, X-ray absorption spectroscopy, mutagenesis, mass spectrometry, and kinetic studies how GSTP1-1 recognizes the drug phenylarsine oxide (PAO). In conditions of cellular stress where glutathione (GSH) levels are low, PAO crosslinks C47 to C101 of the opposing monomer, a distance of 19.9 Å, and causes a dramatic widening of the dimer interface by approximately 10 Å. The GSH conjugate of PAO, which forms rapidly in cancerous cells, is a potent inhibitor (K_i  = 90 nM) and binds as a di-GSH complex in the active site forming part of a continuous network of interactions from one active site to the other. In summary, GSTP1-1 can detoxify arsenic-based drugs by sequestration at the active site and at the dimer interface, in situations where there is a plentiful supply of GSH, and at the reactive cysteines in conditions of low GSH.