Pentavalent methylated arsenicals are substrates of human AQP9

Liver aquaglyceroporin AQP9 facilitates movement of trivalent inorganic arsenite (As^III) and organic monomethylarsonous acid (MAs^III). However, the transport pathway for the two major pentavalent arsenic cellular metabolites, MAs^V and DMAs^V, remains unknown in mammals. These products of arsenic metabolism, in particular DMAs^V, are the major arsenicals excreted in the urine of mammals. In this study, we examined the uptake of the two pentavalent organic arsenicals by human AQP9 in Xenopus laevis oocytes. Xenopus laevis oocytes microinjected with AQP9 cRNA exhibited uptake of both MAs^V and DMAs^V in a pH-dependent manner. The rate of transport was much higher at acidic pH (pH5.5) than at neutral pH. Hg(II), an aquaporin inhibitor, inhibited transport of As^III, MAs^III, MAs^V and DMAs^V via AQP9. However, phloretin, which inhibits water and glycerol permeation via AQP9, can only inhibit transport of pentavalent MAs^V and DMAs^V but not trivalent As^III and MAs^III, indicating the translocation mechanisms of these arsenic species are not exactly the same. Reagents such as FCCP, valinomycin and nigericin that dissipate transmembrane proton potential or change the transmemebrane pH gradient did not significantly inhibit all arsenic transport via AQP9, suggesting the transport of pentavalent arsenic is not proton coupled. The results suggest that in addition to the initial uptake of trivalent inorganic As^III inside cells, AQP9 plays a dual role in the detoxification of arsenic metabolites by facilitating efflux from cells.