Cytolethality of Glutathione Conjugates with Monomethylarsenic or Dimethylarsenic Compounds

Abstract

Arsenic is a metalloid element that is widely distributed in the environment as a natural component of soil and in water as inorganic trivalent (arsenite) or pentavalent (arsenate) forms, 1) and its toxicity has been known since ancient times. In Asia and the Americas, chronic arsenic poisoning has occurred as a result of the consumption of high levels of arsenic-contaminated well water. Epidemiological studies have provided clear evidence that inorganic arsenicals are a human carcinogen with target sites including liver, skin, lung, kidney and urinary bladder.2) However, the mechanism of arsenic-induced impediment is not clear. On the other hand, inorganic arsenite has emerged as a potent chemotherapeutic agent with remarkable efficacy for certain human cancers, such as acute promyelocytic leukemia.3,4) It would thus appear that environmental and iatrogenic exposure to arsenic will continue to be common.In humans and numerous experimental animals, pentavalent arsenate is rapidly reduced to trivalent arsenite. 5) Subsequently, it is enzymatically methylated into organic arsenicals, such as monomethylarsonic acid (MMAs V ) and dimethylarsinic acid (DMAs V ).6) MMAs V and DMAs V are the major organic pentavalent arsenic metabolites in human urine after exposure to inorganic arsenicals. 5,6) It is believed that methylation of inorganic arsenicals results in a reduction in general toxicity, as indicated by their increased in vivo lethal dose in 50% of a population (LD 50 ) and in vitro lethal concentration in 50% of a population (LC 50 ). 7,8) However, recent studies have increasingly suggested that the methylation of inorganic arsenicals is not a universal detoxification mechanism. Some researchers have reported that trivalent methyl arsenicals, such as monomethylarsonous acid (MMAs III ) and dimethylarsinous acid (DMAs III ), were found in urine collected from people who had been exposed to high concentrations of inorganic arsenicals, 9,10) and that synthetic trivalent methyl arsenicals, such as monomethylarsine oxide (MMAs III O) and iododimethylarsine (DMAs III I), are more cytotoxic in vitro than inorganic arsenicals and pentavalent methyl arsenicals.11) Trivalent methyl arsenicals are thought to be generated as arsenical-glutathione conjugates, such as mono-methylarsonous diglutathione (MMAs III DG) and dimethylarsinous glutathione (DMAs III G), in the human body. [12][13][14] It was also reported that reduced glutathione (GSH) nonenzymatically reduces pentavalent methyl arsenicals to trivalent methyl arsenicals in water, thus resulting in the formation of MMAs III DG and DMAs III G in vitro.

Effects of exogenous cysteine on inorganic and organic arsenicals‐induced cytolethality

“…28 We previously demonstrated that arsenicals and GSH concentrations greatly affected the formation and cytolethality of trivalent arsenicals. 17 In the human body, other thiol reagent such as Cys may also reduce pentavalent methylated arsenicals to trivalent methylated arsenicals. Some studies have suggested that Cys may enhance arsenic cytolethality; 13 -15 however, not much is known concerning the effects of exogenous Cys on the cytolethality of arsenicals.…”

“…The TLC method have been authorized for separating and detecting of arsenic compounds. 8,17,24 As shown in Fig. 3(A We also examined the production of the DMAs-Cys conjugate from DMAs V and Cys using the same HPTLC system; 1 mM DMAs V was incubated with or without 1, 3, 5 or 10 mM Cys in distilled water for 1 h at 37…”

Speciation, Metabolism, Toxicity, and Protein-binding of Different Arsenic Species in Human Cells

“…They were shown to be toxic by dissociating into trivalent species and glutathione. 17,18 On the other hand, GSH conjugation promotes the elimination of these species and flags them for removal from the cells, thus reducing intracellular arsenic concentrations.…”

“…Since a number of As metabolites could exist in human cells and some of which (e.g., trivalent arsenicals) are redox sensitive, an efficient separation is required for their determination to maintain integrity of the analytes. I first compared three HPLC columns, namely ion exchange, C 18 , and C 8 , and found that C 8 column was the most promising for further optimization to achieve better separation of a variety of arsenicals. I then performed further experiments to optimize the C 8 column-based HPLC-ICP-MS method, focusing on mobile phase and nebulizer gas as I observed they are critical in the performance of the method.…”

“…8,17,18,104,130,131 Recently, it was shown that in fact S-adenosylmethionine (SAM), the enzyme responsible for the methylation of arsenite, prefers the glutathionylated arsenicals (arsino-glutathione (As(GS) 3 ) and monomethylarsinoglutathione (MMA(GS) 2 )) as substrates for methylation compared to As III and MMA III . 132 Arsino-glutathione and monomethylarsino-glutathione have been identified in rat bile and urine following intravenous injection, while DMA III (GS) was not.…”