We all get tired of putting the rubbish out, but cells do it all the time. When proteins break down ammonia is produced, leaving animal cells with a big problem — ammonia is highly toxic.
Mammals detoxify ammonia into urea in the liver, but many marine organisms take the energetically cheaper route of ejecting it into their environment where it is diluted. Most achieve this by diffusion, but some actively pump the toxin out. However, new research by Dirk Weihrauch, working with an international team, has revealed that green shore crabs may be using a rather novel mechanism to dispose of their toxic ammonia(p. 2765).
Physiologists have had a long-standing collaboration with the shore crab when answering questions about ion transport. During Weihrauch's early research on the shore crab's nitrogen metabolism, he came across a paper showing that a cellular pump, the ubiquitous Na+/K+-ATPase, was capable of utilising ammonium ions(which are similar to potassium ions), to transport sodium ions across cell membranes. From here `it was only logical to see whether this protein was involved in ammonia transport' explains Weihrauch.
To find the proof Weihrauch and his team began their search with the shore crab, because they excrete ammonia through their gill's epithelial cells. To test the role of Na+/K+-ATPase the team `knocked out'the protein using a chemical inhibitor. Immediately, the gill's ability to dispose of its toxic ammonia was halved. Clearly,Na+/K+-ATPase played a role in ammonia excretion, but it wasn't the whole story. The team suspected a second `protein pump' may also be involved and all the evidence pointed to vacuolar-type H+-ATPase(V-ATPase). Without these two crucial proteins, the crab was unable to excrete its toxic ammonia.
The team believes that Na+/K+-ATPase pumps ammonia from the crab's blood into the epithelial cells of its gill, ready for excretion into the environment. In some freshwater fish, this excretion role is played by V-ATPase, which flushes ammonia into the surrounding water by establishing a proton gradient. Could it be that V-ATPase plays a similar role in the crab?
Weihrauch found that V-ATPase was barely present in the apical membrane of the crab's epithelial cells, but was found in the membrane surrounding tiny vesicles within each cell. This puzzled Weihrauch until he realised that V-ATPase traps ammonia inside these tiny vesicles. These vesicles are then transported to the outer surface of the cell where the ammonia is ejected into the crab's environment by exocytosis.
But why should the crab go to such lengths to dispose of its ammonia when diffusion might suffice? Weihrauch believes that the answer lies in the crab's ecology. Whilst scavenging on the ocean floor, crabs often sneak into the carcases of rotting fish where ammonia levels are high — this makes it impossible for the crab to dispose of its ammonia by diffusion.
Ammonia transport remains a mystery in most organisms. Weihrauch believes that we might even protect our brains from ammonia by packaging the toxin inside protective vesicles — perhaps we have more in common with crabs than meets the eye?