Formation and Urinary Excretion of Arsenic Triglutathione and Methylarsenic Diglutathione

Abstract

Taking advantage of mice deficient in gamma-glutamyl transpeptidase that are unable to metabolize glutathione (GSH), we have identified two previously unrecognized urinary metabolites of arsenite: arsenic triglutathione and methylarsenic diglutathione. Following administration of sodium arsenite to these mice, approximately 60-70% of urinary arsenic is present as one of these GSH conjugates. We did not detect the dimethyl derivative, dimethyl arsenic GSH; however, dimethyl arsenic (DMAV) represented approximately 30% of urinary arsenic. Administration of buthionine sulfoximine, an inhibitor of GSH synthesis, to wild-type mice reduced urinary arsenic excretion by more than 50%, indicating the GSH dependence of arsenic metabolism, transport, or both. Rodents deficient in three known ABC family transporters (MRP1, MRP2, and MDR1a/1b) exhibited urinary arsenic levels similar or greater than those in wild-type rodents; however, administration of MK571, an MRP inhibitor, reduced urinary arsenic excretion by almost 50%. MK571-treated mice showed approximately 50% reduction of AsIII, MMAV, and AsV as compared to untreated wild-type controls, while DMAV levels were unchanged. These findings suggest that arsenic excretion is in part dependent on GSH and on an MRP transporter other than MRP1 or 2.

Arsenic Transport by the Human Multidrug Resistance Protein 1 (MRP1/ABCC1)

“…The authors suggested that the redundancy of transmembrane export pumps offers equivalent protection to the Mrp1(Ϫ/Ϫ) animals as the Mrp1(ϩ/ϩ) mice (62,63). An independent investigation of Mrp1(Ϫ/Ϫ) mice from the same colony revealed that 1 h after injection of As III there were no differences in urinary excretion of As III between Mrp1(ϩ/ϩ) and Mrp1(Ϫ/Ϫ) mice, suggesting that Mrp1 is not critical to urinary excretion of arsenic (28). In sharp contrast to the short exposure times and relatively high doses of arsenic used in these animal studies (28,62,63), human tumor formation is associated with long term exposure to comparatively low doses of arsenic (14).…”

“…An independent investigation of Mrp1(Ϫ/Ϫ) mice from the same colony revealed that 1 h after injection of As III there were no differences in urinary excretion of As III between Mrp1(ϩ/ϩ) and Mrp1(Ϫ/Ϫ) mice, suggesting that Mrp1 is not critical to urinary excretion of arsenic (28). In sharp contrast to the short exposure times and relatively high doses of arsenic used in these animal studies (28,62,63), human tumor formation is associated with long term exposure to comparatively low doses of arsenic (14). The high affinity low capacity transport of As(GS) 3 by MRP1 suggests that MRP1 would be better suited to protect tissues from low levels of arsenic and at higher concentrations may be saturated, and other protective mechanisms are required.…”

“…In addition to MRP1 and MRP2, seven other MRP isoforms (MRP3-7, ABCC11, and ABCC12) exist, several of which are known to transport conjugated organic anions and could be involved in the protection of tissues from the toxic effects of arsenic (3,28). The response of different individuals to chronic inorganic arsenic exposure is highly variable.…”

“…Thus, MRP1 could transport arsenic in a GSH-conjugated form. The major route of arsenic excretion is urinary (60 -80% of total arsenic ingested), and very recently arsenic-GSH conjugates have been isolated from the urine of mice lacking the gene for ␥-glutamyl transpeptidase, an enzyme responsible for GSH and GSH conjugate catabolism (28). Thus, ␥-glutamyl transpeptidase is implicated in the processing of arsenic-GSH conjugates at the kidney (28).…”

Uptake and translocation of inorganic and methylated arsenic species by plants

“…DMA III -peptide complexes, like the one that can be synthesised from glutathione as DMA III -GS, have not yet been observed in biological samples and are known to be very unstable. [25][26][27] It is, therefore, highly unlikely that DMA III -PC complexes are formed by plants, which might offer a possible explanation for the high TF of DMA V . Only in sulfur-rich plant species of the genus Brassicaceae it has been recently found that DMA V can be complexed by glutathione as a DMAS V -GS complex.…”