Ammonia transport in the body

See also Urea Cycle Reactions, Urea, Uric Acid, The Nitrogen Cycle, Utilization of Ammonia, Metabolic Nitrogen Balance, Amino Acid Degradation, Ammonia Transport in the Body, Citric Acid Cycle, ATP as Free Energy Currency (from Chapter 12)... 

Ammonia is produced by almost all cells in the body however, only the liver has the enzymatic machinery to convert it to urea. Therefore, extra-hepatic ammonia must be transported to the liver. However, anunonia in the blood is toxic to cells, and therefore the nitrogen from amino acid catabolism is transported in blood either as glutamine or alanine. Glutamine is synthesized from Glu and ammonia in an ATP-requiring reaction that is catalyzed by glutamine synthetase. Alanine is formed from pyruvate in a transamination reaction catalyzed by alanine transaminase (ALT). 

The liver is the major site of amino acid metabolism in the body and the major site of urea synthesis The liver is also the major site of amino acid degradation. Hepatocytes partially oxidize most amino acids, converting the carbon skeleton to glucose, ketone bodies, or CO2. Because ammonia is toxic, the liver converts most of the nitrogen from amino acid degradation to urea, which is excreted in the urine. The nitrogen derived from amino acid catabolism in other tissues is transported to the liver as alanine or glutamine and converted to urea. 

Asteroids (and comets), however, can be mixed by coalescence with stony and icy bodies. If large enough, they may transport volatilizable material into earth s atmosphere (the Tunguska event is probably associated with an ammonia spike in the Greenland ice core record). The stony body can fall to the earth s surface but evaporates (in the Tunguska event there is speculation of cosmic carbon input on the other hand, nitrate increase as would be expected from NO formation by heating the atmosphere (see Chapters 2.6A.4 and 5.4.1) has not been found (Rasmussen et al. 1984, 1999, 2008). 

The large intestine extends from the ileocecal valve to the anus. It is wider than the small intestine except for the descending colon, which when empty may have the same diameter as the small intestine. Major functions of the colon are absorption of water, Na+, and other electrolytes, as well as temporary storage of excreta followed by their elimination. The colon harbors large numbers of mostly anaerobic bacteria that can cause disease if they invade tissues. These bacteria metabolize carbohydrates to lactate, short-chain fatty acids (acetate, propionate, and butyrate), and gases (CO2, CH4, and H2). Ammonia, a toxic waste product, is produced from urea and other nitrogenous compounds. Other toxic substances are also produced in the colon. Ammonia and amines (aromatic or aliphatic) are absorbed and transported to the liver via the portal blood, where the former is converted to urea (Chapter 17) and the latter is detoxified. The liver thus protects the rest of the body from toxic substances produced in the colon. Colonic bacteria can also be a source of certain vitamins (e.g., vitamin K, Chapter 36). 

In contrast to the case of lipids and carbohydrates, no special storage forms of either the nitrogen or the amino acid components of proteins exist. Dietary protein in excess of the requirement is catabolized to provide energy and ammonia, a toxic metabolite that is converted to urea in the liver and excreted by the kidneys. All body proteins serve a specific function (e.g., structural, catalytic, transport, regulatory) and are potential sources of carbon for energy production. 

Nitrogen is eliminated from the body as urea and ammonia. Urea synthesized in the liver is excreted by the kidneys. Urinary ammonia is produced in the kidney. The nitrogen in other tissues is transported to the liver and kidney in... 

The ammonia concentration measured in Hamilton Harbour, Ontario, Canada was typically 0.1-3 mg/L (100-3,000 ppb) in the early 1980s. This body of water is used for water transport, as a source for industrial cooling water, and as a receptor for waste water disposal (Snodgrass and Ng 1985). Measurements made a few years later (1987-1988), in contrast, showed much lower concentrations. Measured concentrations, however, were still greater than the International Joint Commission objective of 20 pg/L for more than half the year, and concentrations often exceeded the chronic toxicity threshold of 300 pg/L (Barica 1990). This work reported that ammonia loadings into Hamilton Harbor had decreased over the late 1970 s and 1980 s, and the measured concentrations may reflect that change. 

For example, fish excrete the ammonia produced by the decomposition of proteins directly into the watery environment in which they live. Birds, who consume less water by gram of weight than do most other animals, Urea is transported from the liver to the kidneys in the bloodstream. By the time it leaves the body in urine, its concentration is sixty to seventy times its concentration in the bloodstream. 1... 

Urea is the form in which amino groups derived from amino acids are disposed of from the body. One nitrogen atom of the urea molecule comes from free ammonia, the other from aspartate. All but two of the reactions of the cycle occur in the cytosol of liver hepatocytes the other two occur in the mitochondria. Urea is transported to the kidney for excretion into the urine. Urea is produced by the liver even during starvation, as skeletal muscle proteins are broken down to release amino acids to act as gluconeogenic precursors. The amino group is removed from these amino acids and converted into urea, which is then excreted in the urine. 

---