Study of interactions between arsenicals and thioredoxins (human and E. coli) using mass spectrometry

Thioredoxin (Trx) plays an important role in achieving redox balances in cells and protecting the cells from oxidative damage. However, little is known about how arsenic affects Trx chemically. It is conceivable that trivalent arsenicals may bind to Trx, which has a highly conserved -CysGlyProCys- sequence. The objective of this study is to characterize the binding of seven arsenic species with Trx from E. coli and humans, using two mass spectrometry techniques. The arsenic-Trx complexes and the free arsenicals were well separated by size-exclusion liquid chromatography (LC) and detected with inductively coupled plasma mass spectrometry (ICPMS). The LC/ICPMS analyses showed that the trivalent arsenic species were able to form complexes with both human and E. coli Trx. Determination of binding constants indicated that affinity to Trx was higher for monomethylarsonous acid (MMA(III)) and phenylarsine oxide (PhAs(III)) than inorganic arsenite (iAs(III)) and dimethylarsinous acid (DMA(III)), probably because MMA(III) and PhAs(III) were able to form stable complexes by binding to two vicinal cysteines in the -CysGlyProCys- region of the Trx. The complexes of arsenicals with both human and E. coli Trx were further characterized by nano-electrospray tandem mass spectrometry. Binding stoichiometries for different arsenic species were consistent with the available cysteine residues in the Trx. Mass spectral evidence also suggests that the pentavalent arsenicals could be reduced by Trx. This study provides the first detailed chemical characterization of the interactions between Trx and arsenic species.