Arsenic binding proteins of mammalian systems: I. Isolation of three arsenite-binding proteins of rabbit liver.

It is well known that arsenite/arsenate (As3+/As5+) administered to rabbits is bound initially to cellular proteins of the liver before methylated arsenic metabolites appear in urine. This protein binding may decrease the in situ toxicity of inorganic arsenic by decreasing its metabolic availability until it is methylated enzymatically. We have investigated the binding of As3+ and As5+ to the cytosolic proteins of rabbit liver. The results indicate that when cytosolic proteins are incubated with inorganic arsenic, the amount of As3+ bound is 13 times greater than that for As5+. Arsenite-specific binding sites on cytosolic proteins were determined to be 67% of the total (specific and non-specific) number of possible binding sites. Ammonium sulfate fractionation, non-denaturing PAGE and gel filtration chromatography indicate that three liver proteins with molecular weights of 100 kDa, 450 kDa and > 2000 kDa strongly bind arsenite. The radioactive profiles after gel filtration chromatography of liver cytosolic proteins are very similar whether As3+ binding occurs in vitro or in vivo. Thus, the in vitro model appears to be valid for further study of these arsenite-binding proteins.