Binding of dimethylarsinous acid to Cys-13α of rat hemoglobin is responsible for the retention of arsenic in rat blood

The metabolism, disposition, and carcinogenicity of arsenic differ dramatically between humans and rats. To understand the molecular basis of these differences, we have characterized arsenic species in rats that were treated with inorganic arsenate (iAs^V), monomethylarsonic acid (MMA ^V), or dimethylarsinic acid (DMA^V) for up to 15 weeks. Arsenic significantly accumulated in the red blood cells (RBCs) of rats in the form of hemoglobin (Hb) complexed with dimethylarsinous acid (DMA ^III), regardless of whether the rats were treated with iAs ^V, MMA^V, or DMA^V, suggesting rapid methylation of arsenic species followed by strong binding of DMA^III to rat Hb. The binding site for DMA^III was identified to be cysteine 13 in the α-chain of rat Hb with a stoichiometry of 1:1. Over 99% of the total arsenic (maximum 2.5-3.5 mM) in rat RBCs was bound to Hb for all rats examined (n = 138). In contrast, only 40-49% of the total arsenic (maximum ∼10 μM) in rat plasma was bound to proteins. The ratios of the total arsenic in RBCs to that in plasma ranged from 88-423 for rats that were fed iAs^V, 100-680 for rats that were fed MMA^V, and 185-1393 for rats that were fed DMA^V, when samples were obtained over the 15-week exposure duration. Previous studies have shown an increase in urothelial hyperplasia in rats fed DMA^V. This is the first article reporting that treatment with iAs^V in the drinking water also produces urothelial hyperplasia and at an even earlier time point than dietary DMA^V. Dietary MMA ^V produced only a slight urothelial response. A correlation between the Hb-DMA^III complex and urothelial lesion severity in rats was observed. The lack of cysteine 13α in human Hb may be responsible for the shorter retention of arsenic in human blood. These differences in the disposition of arsenicals may contribute to the observed differences between humans and rats in susceptibility to arsenic carcinogenicity.