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University of Toronto Scarborough
Biological Sciences
Jason Brown

SLIDE 2 Sources of Nitrogenous Wastes  95% of the waste nitrogen you’re producing is coming from protein catabolism. The other 5% comes from breakdown of purines and pyrimadines. SLIDE 3 Protein Catabolism  Animals can’t store surplus amino acids. If you want to store amino acids, you want to build them into functional proteins.  Breaking amino acids involves cutting the molecule and getting rid of the amine group. The leftover carbon skeleton is used to build other molecules or oxidized as a source of energy.  Amino acids catabolism yields: o Carbon skeleton o Ammonia (NH3/NH4+)  Amino acid catabolism yields ammonia which is toxic for a variety of reasons. Ammonia is toxic largely to the nervous system of animals because it inhibits metabolism in the nervous system so ATP levels fall in animals. At the same time, ammonia is good at displacing potassium from transporters. This disrupts electric signals.  Amino acid catabolism is not a simple process. Only 3 amino acids in our body can have their amino acids directly removed. One of them is glutamate.  The vast majority of amino acids can’t be deaminated directly. They need to have their amine group transferred to some other molecule to generate one of these amino acids that can be deaminated directly like glutamate.  For example an amino acid is transferred to an oxoglutarate to produce glutamate which can then be deaminated directly.  We liberate an ammonia molecule that we have to get rid of. The main way many cells dump ammonia is that they take it and build another kind of amino acid. So here we take a glutamate and use ATP to add an ammonia group to it to form Glutamine. This is the main transport form of ammonia in the animal body.  Glutamine isn’t toxic so we can transport it in the blood and transport it to the site where we can excrete it, pull the ammonia off and excrete it. SLIDE 4 “Phylogeny” of Nitrogen Metabolism  We can take cellular proteins and ingested proteins and hydrolyze them to liberate amino acids.  We can’t store excess amino acids so what do we do with them?  Some marine invertebrates such as crayfish can retain amino acids in their circulatory fluid and use them to adjust to the salinity of the environment in which they live in.  There are some marine invertebrates that retain excess amino acids and use it to maintain water balance.  In most animals, if there’s an excess, you either have to build them into new cellular proteins or get rid of them.  When you break down amino acids and are left with ammonia, one way to simply excrete it as it is. This is the route most aquatic animals take.  Many other animals take that ammonia and convert it to less toxic molecules such as uric acid and urea and then excrete those compounds. One exception is that some marine invertebrates like the squid retain certain amounts of ammonia. They use this ammonia in place of sodium because ammonia is lighter than sodium. This helps them maintain buoyancy. SLIDE 5 Ammonia is Toxic!  We have ammonotelic animals, and “non-ammonotelic” animals. Ammonotelic animals do the same thing that fish do, they simply take any ammonia they produce and get rid of it as fast as they make it as it is. o Excrete NH3 directly into the environment  Non-ammonotelic animals take the ammonia and convert it into less toxic form like urea and uric acid and can build up to a certain extent before they’re excreted.  An advantage of being ammonotelic is that it’s cheap; no energy needs to be invested. Ammonia is also easy to get rid of because it’s lipid soluble and membrane-permeable.  A disadvantage is that you need to get rid of it with a huge amount of water. So it’s not a good strategy if you don’t have a good source of water  The advantage of being non-ammonotelic is that you don’t have to excrete uric acid or urea with large amounts of water. You can allow them to accumulate with a small amount of water, and get rid of that small amount of water and a lot of urea and uric acid.  The disadvantage is that it takes energy to build urea or uric acid.  So ammonotelic animals are all aquatic. Things like aquatic invertebrates, fish, and amphibians.  Non-ammonotelic include terrestrial organisms like insects, arachnids, mammals, birds, lizards, and sharks.  Being ammonotelic or non-ammonotelic depends on how much water is in your environment. SLIDE 6 Ammonia Excretion requires a lot of water  Ammonia excretion requires a lot of water  Takes much less water if excreted as Urea  Even less water if excreted as uric acid SLIDE 7 Ammonotelism and ”non-ammonotelism” : Not always fixed stategies  A single individual can switch between ammonotelic and non-ammonotelic. An example of this is slender lungfish.  When it’s in water, it excretes nitrogen as ammonia.  Every once in a while, the pond dries up and the lung fish forms a caccoon. Now it becomes non- ammonotelic and converts ammonia into urea and stores it until the water returns. After the water returns it pees out all the urea that accumulated.  In the graphs if you look at the last two bars, the 40-day control means it’s living in water, or aestivated meaning it isn’t living in water.  If you look down and see all the different tissues, 40 days living in water shows little urea in the tissues, but 40 days living without water accumulates lots of urea in the tissues. SLIDE 8 Basic Chemistry  Ammonia is a base and can bind up protons (NH3). When ammonia picks up a proton it becomes ammonium (NH4).  The pK is the pH at which the equilibrium is such that it has equal amounts of the acid and base.  If you put ammonia in water, if the pH is brought to 9.5 you’ll have equal amounts of ammonium and ammonia.  If the pH drops below 9.5, ammonium becomes preferable. If the pH is above 9.5 the ammonia becomes preferable.  The pH in the body is about 7, so ammonium predominates in the body, so most of the ammonia is in the ammonium form in the cells and the circulatory system. SLIDE 9 Ammonotelic Animals: Mechanisms of ammonia excretion- Freshwater  What is the mechanism a fresh water fish uses to excrete ammonium?  Most fresh water fish excrete their ammonia across their gills. So in the blood we have ammonium and ammonia. Ammonium is the dominant form, but it’s not membrane permeable. Ammonia is membrane permeable. Ammonia is able to pass across the gills and into surrounding waters.  The problem here is that the water is divided into two spaces. The layer of water right up against the fish gill doesn’t get mixed well with the bulk water. There’s always a little layer of water around the gills that’s not exchanging materials with the rest of the water.  As this ammonia is diffusing into the boundary layer, it’s building up and not much of it is getting exchanged with the bulk water. This makes it hard to get rid of more ammonia and it doesn’t make a favorable diffusion gradient.  To solve this problem, the fish take CO2 from the blood and take it into the gill cells, the gill cells have enzymes called carbonic anhydrase which convert CO2 into 2 things, a bicarbonate ion and a proton.  The proton is pumped actively by ATPase into the boundary layer. Now the proton and ammonia come together to form ammonium. Now there’s a favorable diffusion gradient to allow more ammonia to diffuse from the gills.  The fish is acidifying water right next to the gill to convert ammonia into ammonium and keep the diffusion gradient favorable SLIDE 10 Ammonotelic Animals: Mechanisms of ammonia excretion- Marine  In marine fish a different mechanism is used. Sea water tends to be much more buffered than fresh water.  Secreting protons into a buffered solution won’t do much to change the pH of the solution.  Because seawater is well buffered, you can’t pump ions into it and expect it to change the pH and have the same reaction as the fresh water fish take place so you have to use a different mechanism. SLIDE 11 Ammonotelic Animals: Mechanisms of ammonia excretion- Marine  To a very small degree ammonia is diffused is lost across the gills. This accounts for about 5-10% of the total ammonia loss across the gills in marine fish.  Active excretion of ammonium ions accounts for most of the ammonia loss. This involves the Sodium/Potassium ATPase.  Na+/K+ ATPase in the gill cells actively pump sodium into the blood of the fish and pumping potassium in the gill cells. There’s very little sodium now in the gill cell and lots of it in the blood. This sets up a diffusion gradient for sodium to get back in the gill cell.  This involves the NKCC which stands for sodium potassium two chloride transporter. This transporter allows sodium to pass into the cell and couples that diffusion with the uptake of both potassium and chloride.  But you don’t see any potassium coming into the cell, you see ammonium coming in. Remember that one of the reasons ammonium is toxic because it can use any potassium transporter to act like potassium. This is how it replaces potassium and enters the gill cell.  Now we pump the sodium back out again, so in addition to the sodium coming into the gill cell from the blood, it also comes in from the water via an exchange known as the sodium ammonium exchanger.  This exchanger allows the sodium to come into the cell and exchanges that sodium with an ammonium. This is how marine fish excrete ammonia at their gills, in the form of ammonium.  All of this occurs because we can’t acidify sea water because it’s well buffered. SLIDE 12 “Non-ammonotelic” animals: Urea (ureotelic animals)- Adult amphibians, mammals, erthworms,
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