Arsenic Biotransformations in Microbes and Humans, and Catalytic Properties of Human AS3MT Variants

Arsenic is the most pervasive environmental toxic substance. As a consequence of its ubiquity, nearly every organism has genes for resistance to inorganic arsenic. In one project I examined the role of glutaredoxin 2 (Grx2) in reduction of arsenate to arsenite. I demonstrated that Grx2 has both glutaredoxin thiol transfer activity and glutathione S-transferase (GST) activity. In a second project investigated arsenic resistance in a microbiome organism. I discovered that the human gut microflora B. vulgatus has eight continuous genes in its genome and these genes form an arsenical-inducible transcriptional unit. In two other projects I investigated the properties of two As(III) S-adenosylmethionine (SAM) methyltransferase (ArsM in microbes and AS3MT in animals). In this project we demonstrate that most fungal species have ArsM orthologs with only three conserved cysteine residues, and AfArsM from Aspergillus fumigatus methylates only MAs(III) and not As(III). For human, arsenic methylation process is thought to be protective from acute high-level arsenic exposure. However, with long term low-level exposure, hAS3MT is thought to produce intracellular methylarsenite (MAs(III)) and dimethylarsenite (DMAs(III)), which are considerably more toxic than inorganic As(III) and may contribute to arsenic-related diseases. Several single nucleotide polymorphisms (SNPs) in putative regulatory elements of the hAS3MT gene have been shown to be protective. In contrast, three previously identified exonic SNPs (R173W, M287T and T306I) may be deleterious. I identified five additional intragenic variants in hAS3MT (H51R, C61W, I136T, W203C and R251H). I purified the eight polymorphic hAS3MT proteins and characterized their enzymatic properties. Each enzyme had low methylation activity through decreased affinity for substrate, lower overall rates of catalysis and/or lower stability. I propose that amino acid substitutions in hAS3MT with decreased catalytic activity lead to detrimental responses to environmental arsenic and may increase the risk of arsenic-related diseases