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 methy late iAs. This work examined the metabolism of arsenite (iAs(III)), arsenat e (iAs(v)), methylarsine oxide ((MAsO)-O-III), methylarsonic acid (MAsv), d imethylarsinous acid (DMAsIII), and dimethylarsinic acid (DMAsV) in primary cultures of normal human hepatocytes. Primary rat hepatocytes were used as methylating controls, iAs(III) and (MAsO)-O-III were metabolized more exte nsively than iAs(v) and MAsv by either cell type. Neither human nor rat hep atocytes metabolized DMAsIII or DMAsV. Methylation of iAs(III) by human hep atocytes yielded methylarsenic (MAs) and dimethylarsenic (DMAs) species; (M AsO)-O-III was converted to DMAs. The total methylation yield (MAs and DMAs ) increased over the range of 0.1 to 4 mu M iAs(III). However, DMAs product ion was inhibited by iAs(III) in a concentration-dependent manner, and the DMAs/MAs ratio decreased. iAs(III) (10 and 20 mu M) inhibited both methylat ion reactions. Inhibition of DMAs synthesis resulted in accumulation of iAs and MAs in human hepatocytes, suggesting that dimethylation is required fo r iAs clearance from cells. Methylation capacities of human hepatocytes obt ained from four donors ranged from 3.1 to 35.7 pmol of iAs(III) per 10(6) c ells 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 maxim al methylation rates for either rat or human hepatocytes were attained betw een 0.4 and 4 mu M iAs(III). In summary, (i) human hepatocytes methylate iA s, (ii) the capacities for iAs methylation vary among individuals and are s aturable, and (iii) moderate concentrations of iAs inhibit DMAs synthesis, resulting in an accumulation of iAs and MAs in cells.