Metabolism and toxicity of sodium arsenate in human erythrocytes

Abstract

Toxicity of arsenic species is dependent on chemical oxidation state. Inorganic arsenic in the trivalent state, arsenite or As(III), is more biologically active than pentavalent arsenic, arsenate or As(V), and is more toxic by most measures. As(V), however, is more stable and prevalent in the environment. One consequence of environmental exposure is peripheral vascular disease, which is primarily due to vascular changes, but toxicity to the erythrocyte has not been evaluated. To understand toxicity and the implications of arsenic oxidation state, human erythrocytes were utilized to model the uptake, biotransformation (metabolism) and toxicity of sodium arsenate, As(V). It was first established that biotransformation, both in vivo and in vitro, would not be limited by uptake of As(V) into the cell. Evidence suggested that reduction was accomplished by at least two separate pathways. All reductive metabolism was dependent on the presence of reduced thiols including both non-protein thiols (glutathione; GSH) and protein thiols (ProSH). These pathways are: (1) chemically mediated reduction by GSH and (2) protein mediated reduction. It was established that the protein-dependent pathway required a reduced protein thiol and also required the presence of GSH. This points to reduction through a redox coupling to form a protein mixed disulfide (ProSSG). Toxicity to the erythrocyte was evaluated by determining total cell death, morphologic changes and effects on the energy cofactor adenosine triphosphate (ATP). Based on these three parameters, the erythrocyte was more susceptible to As(V) and not As(III) as other tissues are. The morphologic effects on the cell were also consistent with ATP depletion. These changes were characterized by formation of morphologically altered cells that are unable to deform in circulation effectively and occlude the microcirculation. This could contribute to vascular tissue damage associated with arsenic-induced circulatory disorders. In summary, the erythrocyte is able to take in As(V) which is detrimental to the ability of the cell to perform its intended function. Biotransformation to As(III) would therefore be a detoxifying event, and understanding the factors involved in biotransformation will help to understand human susceptibility to arsenic-induced vascular disease.