Ammonia secretion from fish gill depends on a set of Rh glycoproteins

Abstract

Ammonia excretion from the gill in teleost fish is essential for nitrogen elimination. Although numerous physiological studies have measured ammonia excretion, the mechanism of ammonia movement through the membranes of gill epithelial cells is still unknown. Mammalian Rh glycoproteins are members of a family of proteins that mediate ammonia transport in bacteria, yeast, and plants. We identified the Rh glycoprotein homologs, fRhag, fRhbg, fRhcg1, and fRhcg2, of the pufferfish, Takijugu rubripes. North‐ern blot, in situ hybridization, and immunohistochem‐istry revealed that the pufferfish erythroid Rh glycop‐rotein homologue fRhag was present in red blood cells and the hematological organs (spleen and kidney) in fish. All four pufferfish Rh glycoproteins are specifically localized in the gill and line the pillar cells, pavement cells, and the mitochondrion‐rich cells. Het‐erologous expression in Xenopus oocytes showed that they mediate methylammonium (an analog of ammonium) transport. These results suggest that pufferfish Rh glycoproteins are involved in ammonia excretion from the gill. These findings challenge the classic view that ammonia excretion in the fish gill occurs by passive diffusion.—Nakada, T., Westhoff, C. M., Kato, A., Hirose, S. Ammonia secretion from fish gill depends on a set of Rh glycoproteins. FASEB J. 21, 1067–1074 (2007)