By observing the chemical shifts of the proton and carbon-13 nuclei of reduced glutathione, the interactions of arsenate, arsenite and dimethylarsinate with this tripeptide have been characterized. These spectral studies show the reduction and complexation of arsenic to be a two-step process. Initially, the oxidation of 2 mol of glutathione reduces arsenate to arsenite. Then, 3 mol of glutathione are consumed in the formation of a glutathione-arsenite complex. Similar experiments with arsenite identified a (glutathione)3-arsenite complex; however, no oxidized glutathione was detected. The arsenite binding site in the glutathione-arsenite complex is the cysteinyl sulfhydryl. The glutathione-arsenite complex is stable over the pH range from 1.5 to 7.0-7.5. At higher pH, dissociation occurs releasing reduced glutathione. For a glutathione to dimethylarsinate ratio of 3, oxidized glutathione is also coupled with a reduction to trivalent dimethylarsinous acid, prior to the formation of a 1:1 glutathione-dimethylarsinite complex. The role of reduced glutathione in the metabolism of arsenic is consistent with the previously described effects of this agent on the organismic toxicity of arsenic. (Copyright (c) 1994 Elsevier Science Ireland Ltd.)