Monomethylarsonic Acid Reductase and Monomethylarsonous Acid in Hamster Tissue

Abstract

The formation of monomethylarsonous acid (MMA(III)) by tissue homogenates of brain, bladder, spleen, liver, lung, heart, skin, kidney, or testis of male Golden Syrian hamsters was assessed using [(14)C]monomethylarsonic acid (MMA(V)) as the substrate for MMA(V) reductase. The mean +/- SEM of MMA(V) reductase specific activities (nanomoles of MMA(III) per milligram of protein per hour) were as follows: brain, 91.4 +/- 3.0; bladder, 61.8 +/- 3.7; spleen, 30.2 +/- 5.4; liver, 29.8 +/- 1.4; lung, 21.5 +/- 0.8; heart, 19.4 +/- 1.5; skin, 14.7 +/- 1.6; kidney, 10.6 +/- 0.4; and testis, 9.8 +/- 0.6. The concentrations of MMA(III) in male Golden Syrian hamster livers were determined 15 h after administration of a single intraperitoneal dose of 145 microCi of [(73)As]arsenate (2 mg of As/kg of body weight). Trivalent arsenic species (arsenite, MMA(III), and dimethylarsinous acid, DMA(III)) were extracted from liver homogenates using carbon tetrachloride (CCl(4)) and 20 mM diethylammonium salt of diethyldithiocarbamic acid (DDDC). Pentavalent arsenicals (arsenate, MMA(V), and dimethylarsinic acid, DMA(V)) remained in the aqueous phase. The organic and the aqueous phases then were analyzed by HPLC. Metabolites of inorganic arsenate present in hamster liver after 15 h were observed in the following concentrations (nanograms per gram of liver +/- SEM): MMA(III), 38.5 +/- 2.9; DMA(III), 49.9 +/- 10.2; arsenite, 35.5 +/- 3.0; arsenate, 118.2 +/- 8.7; MMA(V), 31.4 +/- 2.8; and DMA(V), 83.5 +/- 6.7. This first-time identification of MMA(III) and DMA(III) in liver after arsenate exposure indicates that the significance of arsenic species in mammalian tissue needs to be re-examined and re-evaluated with respect to their role in the toxicity and carcinogenicity of inorganic arsenic.

Methylated Arsenicals: The Implications of Metabolism and Carcinogenicity Studies in Rodents to Human Risk Assessment

“…The reduction of MMA V to MMA III by MMA-reductase was found to be the rate-limiting step in the methylation and metabolism of inorganic arsenic. The affinity of MMA-reductase for MMA V (K m = 2.16 × 10 −3 ) in homogenized rabbit liver is lower than the affinity of methyltransferases to either arsenite (K m = 5.5 × 10 −6 ) or MMA III (9.2 × 10 −6 ) by over two orders of magnitude (Sampayo-Reyes et al, 2000;Zakharyan et al, 1999b). Based on the K m values a concentration of MMA V in the millimolar range needs to be present before a significant amount of MMA V can be reduced to MMA III (Zakharyan et al, 1999b).…”

Evidence for induction of oxidative stress caused by chronic exposure of Chinese residents to arsenic contained in drinking water.

“…Either the trivalent organoarsenicals generated during biomethylation of arsenic or the arsenothiols formed in the cell by reaction with NPSH could alter the cellular GSH:GSSG ratio, decreasing the GSH level by the inhibition of GSH reductase. Recently, the concentrations of MMeAs(III) and MMeAs(V) in hamster liver after a single intraperitoneal dose of iAs(V) (2 mg As/kg of body weight) were 38.5 and 31.4 ng/g, respectively (MMeAs(III)/total MMeAs = 55.1%) (58). Although the method we used for determining blood arsenic in the present study could not separate MMeAs(III) and MMeAs(V), high levels of total MMeAs (49.2%) were observed in blood, suggesting the possibility that high levels of MMeAs(III) existed in other organs.…”

Determination of trivalent methylated arsenicals in rat urine by liquid chromatography–inductively coupled plasma mass spectrometry after solvent extraction

“…In addition, As(III) is most difficult to extract; As(III) was quantitatively extracted in a narrower pH range than MMA(III) and DMA(III), and in extraction experiment recovery of As(III) was lower than that of MMA(III) and DMA(III) [22,30]. Therefore, we selected sodium arsenite as a trivalent standard arsenical to use in optimizing the method of analysis for trivalent arsenicals.…”

“…Hasegawa et al [22] and Sampayo-Reyes et al [30] observed that trivalent arsenicals were partly oxidized after back-extraction. In our study, As(III) in aqueous phase after back-extraction was partially oxidized to arsenate (As(V)) ( Fig.…”