Decreased enzyme activity of hepatic thioredoxin reductase and glutathione reductase in rabbits by prolonged exposure to inorganic arsenate.

Chronic exposure of humans to inorganic arsenic, mainly pentavalent arsenate (iAsV), results in drinking water-induced oxidative stress (Pi et al., 2002). Thioredoxin reductase (TR) and glutathione reductase (GR) are the two critical enzymes in the response to oxidative stress in vivo. In the present study we examined alterations in enzyme activities of hepatic TR and GR from prolonged exposure of male New Zealand white rabbits to iAsV. Exposure of rabbits to iAsV in drinking water (5 mg/L) for 18 weeks caused a significant suppression of hepatic TR and GR activities, of approximately 30% and 20%, respectively, below controls. In vitro experiments suggested that trivalent inorganic arsenic (iAsIII) but not pentavalent arsenicals including iAsV, monomethylarsonic acid (MMAsV), and dimethylarsinic acid (DMAsV) affected the hepatic TR activity of rabbit. So it was suggested that in the present study iAsV ingested via drinking water was metabolized to reactive trivalent arsenicals, such as iAsIII, which may play an important role in the decreased TR and GR activities from prolonged exposure to iAsV observed in vivo.

A potential mechanism for the impairment of nitric oxide formation caused by prolonged oral exposure to arsenate in rabbits.

We have recently found evidence for impairment of nitric oxide (NO) formation and induction of oxidative stress in residents of an endemic area of chronic arsenic poisoning in Inner Mongolia, China. To investigate the underlying mechanisms responsible for these phenomena, a subchronic animal experiment was conducted using male New Zealand White rabbits. After 18 weeks of continuous exposure of rabbits to 5 mg/l of arsenate in drinking water, a significant decrease in systemic NO production occurred, as shown by significantly reduced plasma NO metabolites levels (76% of control) and a tendency towards decreased serum cGMP levels (81.4% of control). On the other hand, increased oxidative stress, as shown by significantly increased urinary hydrogen peroxide (H(2)O(2)) (120% of control), was observed in arsenate-exposed rabbits. In additional experiments measuring aortic tension, the addition of either the calcium ionophore A23187 or acethylcholine (ACh) induced a transient vasoconstriction of aortic rings prepared from arsenate-exposed rabbits, but not in those prepared from control animals. This calcium-dependent contractility action observed in aorta rings from arsenate-exposed rabbits was markedly attenuated by the superoxide (O2(.-)) scavenging enzyme Cu, Zn-SOD, as well as diphenyleneiodonium (DPI) or N(G)-nitro-L-arginine methyl ester (L-NAME), which are inhibitors for nitric oxide synthase (NOS). However, the cyclooxygenase inhibitor indomethacin or the xanthine oxidase blocker allopurinol had no effect on this vasoconstriction. These results suggest that arsenate-mediated reduction of systemic NO may be associated with the enzymatic uncoupling reaction of NOS with a subsequent enhancement of reactive oxygen species such as O2(.-), an endothelium-derived vasoconstricting factor. Furthermore, hepatic levels of (6R)-5,6,7,8-tetrahydro-L-biopterin (BH(4)), a cofactor for NOS, were markedly reduced in arsenate-exposed rabbits to 62% of control, while no significant change occurred in cardiac L-arginine levels. These results suggest that prolonged exposure of rabbits to oral arsenate may impair the bioavailability of BH(4) in endothelial cells and, as a consequence, disrupt the balance between NO and O2(.-) produced from endothelial NOS, such that enhanced free radicals are produced at the expense of NO.