Arsenic binding to glutathione The metabolism ofArsenic in the cells involves the reduction of trivalent Arsenic to pentavalentarsenic.
This reaction consists of a redox cycle involving a bio-thiol(glutathione) with the the production of a tris-glutathionyl-Arsenitespecies. The multiple methylations of Arsenite by S-adenosyl-methionineto the generation of trimethy-arsine (haemolytic toxin) also involves glutathione.Glutathione presence in the intermediate conjugate forms of methylated Arsenicspecies helps these molecules to be removed from the cells by the multidrugresistance proteins (having ATP-binding cassette). Dimethylarsonic acid(carcinogenic end-metabolite) also reacts with glutathione having a highcytolethal effect on cells. Moreover various enzymes and regulatory elementscan contribute to the arsenic biotransformation by contributing individual ormultiple cysteine thiol groups in vicinity in proteins, for example thiolgroups required for catalytic activity. Arsenic binding to metallothioneinsMetallothioneinsare expressed by various organisms including bacteria, fungi, planta andvertebrates. They belong to a protein family of ubiquitous nature characterizedby low molecular weight, high metal and cysteine content. They are capabale ofbinding essential metal ions (zinc, copper)_and toxic heavy metals (Arsenic,cadmium).
Studies haverevealed that bioaccumulation of Arsenic in Sea weed species F. vesiculosus is achieved through the binding of Arseniteto the cysteine rich metallothioneins. Moreover arsenic is also known to bindto mammalian metallothioneins in rabbit and human species.It is presentabundantly in kidneys and liver of mammals. Further studies on humanmetallothioneins were consistent with the hypothesis that Arsenite has abinding preference for three vicinal thiol groups. ? and ? domain of human metallothionein contains 11and 9 cysteines respectively. All the 9 cysteine were involved in binding to threeArsenite molecules in ? domain while in the case of ? domain only 9 out of 11 cysteineresidues were involved in binding to three Arsenites.
This leave two cysteine residuesprotonated with no fourth Arsenite engaged in binding.