Transport pathways for arsenic and selenium: a minireview

Abstract

Arsenic and selenium are metalloids found in the environment. Arsenic is considered to pose the most significant potential threat to human health based on frequency of occurrence, toxicity and human exposure. Selenium, on the other hand, ranks only 147th in toxicity but, in contrast to arsenic, is a required micronutrient. Whether a toxin or micronutrient, their metabolism requires that cells to accumulate these metalloids. In this review we discuss the membrane proteins that transport arsenic and selenium into cells, from bacteria to humans, as well as some of the efflux proteins involved in detoxification.

Fig. 1. Pathways of arsenic and selenium uptake and efflux in prokaryotes and eukaryotes.

Arsenate (As(V)) is taken up by phosphate transporters, while As(III) is taken up by aquaglyceroporins (GlpF in E. coli, Fps1p in yeast, and AQP7 and AQP9 in mammals), and hexose permeases (HXT1, HXT3, HXT4, HXT5, HXT7, or HXT9 in yeast, and GLUT1 and GLUT4 in mammals). In both E. coli and S. cerevisiae, arsenate is reduced to arsenite by the bacterial ArsC or yeast Acr2p enzymes. In both organisms, glutathione and glutaredoxin serve as the source of reducing potential. The proteins responsible for arsenate uptake and reduction in mammals have not yet been identified. In E. coli, arsenite is extruded from the cells by ArsB alone or by the ArsAB ATPase. In yeast Acr3p is a plasma membrane arsenite efflux protein, and Ycf1p, which is a member of the MRP family of the ABC superfamily of drug-resistance pumps, transports As(GS)₃ into the vacuole. In mammals, Mrp isoforms such as Mrp2, pump As(GS)₃ out of cells. Selenate is taken up by sulfate permeases, the CysAWTP ABC ATPase in bacteria, Sul1p in yeast and SLC26A11 in humans. By-and-large, the uptake pathways for selenite have not been identified.