Acids generated from the metabolism of excess dietary protein are excreted largely as urinary ammonium (NH4 +) produced by cells in the proximal tubule of the kidney. NH4 + is transported into the urine from collecting-duct cells through parallel movement of hydrogen ions (H+) and ammonia (NH4). Biver et al now report that not all ammonia moves by free diffusion – as thought previously – and that most of it crosses through the Rhcg protein, which functions as an ammonia channel. The
V-ATPase pump mediates H+ transport into the urine, where it recombines with NH3 to form NH4+, As for the initial NH4 + entry into these cells, an ion pump Na+ -K+ -ATPase, which can carry NH4, + place of potassium.
Rhcg is related to the rhesus (Rh) antigen protein of red blood cells.
Probably one third of ammonia crosses the membrane channel by diffusion and the other two thirds using the Rhcg mechanism.
Knepper 2008 Courier service for ammonia. Nature vol 456 p 336-7.
Biver et al 2008 A role for Rhesus factor Rhcg in renal ammonium excretion and male fertility Nature vol 456 pp 3339-343

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