Interaction of Trivalent Arsenicals with Metallothionein

Abstract

Arsenic is a human carcinogen, causing skin, bladder, and lung cancers. Although arsenic in drinking water affects millions of people worldwide, the mechanism(s) of action by which arsenic causes cancers is not known. Arsenic probably exerts some toxic effects by binding with proteins. However, few experimental data are available on arsenic-containing proteins in biological systems. This study reports on arsenic interaction with metallothionein and established binding stoichiometries between metallothionein and the recently discovered trivalent metabolites of arsenic metabolism. Size exclusion chromatography with inductively coupled plasma mass spectrometry analysis of reaction mixtures between trivalent arsenicals and metallothionein clearly demonstrated the formation of complexes of arsenic with metallothionein. Analysis of the complexes using electrospray quadrupole time-of-flight tandem mass spectrometry revealed the detailed binding stoichiometry between arsenic and the 20 Cys residues in the metallothionein molecule. Inorganic arsenite (As(III)) and its two trivalent methylation metabolites, monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)), readily bind with metallothionein. Each metallothionein molecule could bind with up to six As(III), 10 MMA(III), and 20 DMA(III) molecules, consistent with the coordination chemistry of these arsenicals. The findings on arsenic interaction with proteins are useful for a better understanding of arsenic health effects.

Combined ion-mobility and mass-spectrometry investigations of metallothionein complexes using a tandem mass spectrometer with a segmented second quadrupole

“…As-, MMA-, and DMA-MT2A Complexes Figure 5 shows the mass spectra of As 3ϩ , MMA, and DMA with MT2A. The maximum numbers of arsenic species bound by MT are: As 3ϩ , 6; MMA, 10; and DMA, 20; in accordance with previous observation [15]. The presence of the methyl groups affects binding with MT2A, which translates into changes in the crosssection.…”

“…Take the 4ϩ ions, the As 6 MT2A complex has a cross-section of 750 Å 2 ; the MMA 10 MT2A complex, 800 Å 2 ; and the DMA 20 MT2A complex, 960 and 1150 Å 2 . The increase in cross-section from the As 3ϩ to MMA to DMA complex can be interpreted as a decrease in "cross-linking" (As 3ϩ binds to three cysteinyl S Ϫ , MMA two, and DMA only one) [15] and/or an increase in the cross-section of the individual arsenic species due to increasing number of methyl groups. Similarly, for the 6ϩ ions, the As 6 MT2A complex gives a cross-section of 790 Å 2 ; the MMA 10 MT2A complex, 920 Å 2 ; and the DMA 20 MT2A complex, 1220 Å 2 .…”

“…Many mono-and divalent metals have been observed to complex with MTs, including Cu(I), Ag(I), Zn(II), Cd(II), and Hg(II) [11][12][13][14]. Recently, As(III), in the forms of arsenite, monomethylarsonous acid (MMA), and dimethylarsinous acid (DMA), has also been observed to bind with MTs [15]. Traditionally, methylation of arsenic is believed to be a detoxification process [16,17].…”

“…Similarly, CD apparently supported existence of the Cu 12 -MT complex. Recently, trivalent arsenicals were reported to bind with MTs [15]. Up to six arsenite, 10 MMA, and 20 DMA were observed to bind with MT2A using electrospray MS [15].…”

The nitric oxide prodrug, V-PYRRO/NO, mitigates arsenic-induced liver cell toxicity and apoptosis

“…Arsenic has been shown to induce MT expression both in vitro and in vivo [16][17][18]. Furthermore, Jiang et al reported that arsenic and its methylated metabolites interacted with MT in a stoichiometric fashion [16]. Recent evidence indicates that human populations poorly expressing MT may be more sensitive to chronic arsenic intoxication [19].…”