Biochemical Characterization of the Human Arsenite-stimulated ATPase (hASNA-I)

Arsenic is a potent toxin and carcinogen. In prokaryotes, arsenic detoxification is accomplished by chromosomal and plasmid-borne operon-encoded efflux systems. We have previously reported the cloning of hASNA-I, a human homologue of arsA encoding the ATPase component of the Escherichia coli arsenite transporter. Purified glutathione S-transferase (GST)-hASNA-I fusion protein was biochemically characterized, and its properties were compared with those of ArsA. The GST-hASNA-I exhibited a basal level of ATPase activity of 18.5 +/- 8 nmol/min/mg in the absence of arsenite. Arsenite produced a 1.6 +/- 0.1-fold stimulation of activity (p = 0. 0044), which was related to an increase in Vmax; antimonite did not stimulate activity. Two lines of evidence suggest that an oligomer is the most likely native form of hASNA-I. First, lysates of human embryo kidney 293 cells overproducing recombinant hASNA-I produced a single monomeric 37-kDa band on SDS-polyacrylamide gel electrophoresis (PAGE) and two distinct species when analyzed using nondenaturing PAGE. Second, chemical cross-linking of the 63-kDa GST-hASNA-I resulted in the formation of dimeric and tetrameric protein forms. The results indicate that hASNA-I is a distinct human arsenite-stimulated ATPase belonging to the same superfamily of ATPases represented by the E. coli ArsA protein.