Metabolism of Arsenic in Primary Cultures of Human and Rat Hepatocytes

The liver is considered a major site for methylation of inorganic arsenic (iAs). However, there is little data on the capacity of human liver to methylate iAs. This work examined the metabolism of arsenite (iAs^III), arsenate (iAs^V), methylarsine oxide (MAs^III O), methylarsonic acid (MAs^V), dimethylarsinous acid (DMAs^III), and dimethylarsinic acid (DMAs^) in primary cultures of normal human hepatocytes. Primary rat hepatocytes were used as methylating controls. iAsIII and MAs^III O were metabolized more extensively than iAs^V and MAs^V by either cell type. Neither human nor rat hepatocytes metabolized DMAs^III or DMAs^V. Methylation of iAsIII by human hepatocytes yielded methylarsenic (MAs) and dimethylarsenic (DMAs) species: MAs^III O was converted to DMAs. The total methylation yield (MAs and DMAs) increased over the range of 0.1 to 4 muM. iAs^III. However, DMAs production was inhibited by iAs^III in a concentration-dependent manner, and the DMAs/MAs ratio decreased. iAsIII (10 and 20 muM) inhibited both methylation reactions. Inhibition of DMAs synthesis resulted in accumulation of iAs and MAs in human hepatocytes, suggesting that dimethylation is required for iAs clearance from cells. Methylation capacities of human hepatocytes obtained from four donors ranged from 3.1 to 35.7 pmol of iAsIII per 10^6 cells per hour and were substantially lower than the methylation capacity of rat hepatocytes (387 pmol of iAs^III per 10^6 cells per hour). The maximal methylation rates for either rat or human hepatocytes were attained between 0.4 and 4 muM iAs^III. In summary, (i) human hepatocytes methylate iAs, (ii) the capacities for iAs methylation vary among individuals and are saturable, and (iii) moderate concentrations of iAs inhibit DMAs synthesis, resulting in an accumulation of iAs and MAs in cells.