Ammonia is the chemical compound of hydrogen and nitrogen. The molecular formula of ammonia is NH3. In the body, the substance is formed when proteins are broken down.
What is ammonia?
Ammonia is a colorless gas composed of three hydrogen atoms and one nitrogen atom. The gas has an extremely pungent odor. To the human body, ammonia is toxic. It is usually present there as a water-soluble salt. In this form, it is also called ammonium (NH4+). Ammonia is involved in various metabolic processes. However, it is formed in particular during protein breakdown in the intestine. Ammonia is also produced during cell metabolism and the breakdown of amino acids. Ammonia can severely damage the cells of the body. Therefore, it is converted to urea in the liver and then excreted through the kidneys in the urine.
Function, effect and tasks
Ammonia plays an important role in metabolism in the formation and breakdown of amino acids. However, in these metabolic processes, the ammonia is in the form of the ammonium. Ammonium and α-ketoglutarate are converted into glutamate in a special chemical process called reductive amination. Glutamate, also called glutamic acid, is an α-amino acid. Since the body can produce glutamic acid itself with the help of ammonium, it belongs to the non-essential amino acids. Glutamic acid is also an important component of proteins as an amino acid. Through the process of transamination, other non-essential amino acids can be produced from glutamate. However, glutamate is not only involved in amino acid synthesis, it is also one of the most important excitatory neurotransmitters in the central nervous system (CNS). At the same time, the amino acid is also a precursor of γ-aminobutyric acid (GABA). This in turn is the most important inhibitory neurotransmitter in the central nervous system. Glutamate is also said to have a positive effect on muscle development and the immune system.
Formation, occurrence, properties, and optimal levels
Ammonia is largely formed during the breakdown of amino acids. The main site of formation of free ammonia is the intestine. Primarily in the large intestine, ammonia is formed from undigested protein by bacterial action. The amino acids are first broken down again to glutamate. This amino acid is then cleaved by the enzyme glutamate hydrogenase into the original substances α-ketoglutarate and ammonia. Not all of the ammonia thus produced can serve again for amino acid synthesis. In larger quantities, ammonia also has a cytotoxic effect, so the body must have a means of breaking down ammonia. Creatures that are native to water can often release ammonia directly into the surrounding water through their skin. Humans must convert toxic ammonia to a non-toxic form before excretion. In a healthy liver, there is rapid absorption of the ammonia. This usually reaches the liver via the portal vein. The liver then converts ammonia or ammonium into urea. Urea is a white, crystalline and non-toxic substance. It is excreted in the form of a solution in the urine via the kidneys. The plasma normal value for ammonia is considered to be 27 to 90 μg ammonia/dl. This corresponds to an amount of 16 to 53 μmol/l. Blood ammonia levels are usually determined as part of a liver function test.
Diseases and disorders
Decreased blood serum ammonia levels have no clinical relevance. Elevated ammonia levels usually occur with decreased liver function. The breakdown of ammonia is highly impaired in liver cirrhosis. Liver cirrhosis is the end stage of many liver diseases. The stage is irreversible and thus cirrhosis is not curable. Typically, cirrhosis develops over years to decades. In Europe, the most common cause of liver cirrhosis is alcohol abuse. Chronic viral hepatitis can also result in cirrhosis. In cirrhosis, there is a loss of liver tissue and a connective tissue remodeling of the liver function cells. On the one hand, this disturbs the blood supply to the liver. On the other hand, the liver cells can no longer perform their detoxification task. A severe increase in ammonia levels due to impaired liver function can lead to hepatic encephalopathy. This is a functional disorder of the brain due to the insufficient detoxification function of the liver. The cause of this damage is probably the similarity of ammonium and potassium.When potassium and ammonium are exchanged, the so-called NMDA receptor is disturbed. This in turn allows increased calcium to enter the nerve cell. Cell death occurs. Hepatic encephalopathy can be divided into four stages. The latent or minimal hepatic encephalopathy precedes the four stages. This is manifested by poor concentration, reduced drive, or difficulty remembering things. In the first stage, there is a noticeable reduction in consciousness, marked impulsivity, and a disturbance in fine motor skills. In the second stage, the affected person suffers from orientation disorders, memory disorders, slurred speech and severe sleepiness. The third stage is associated with high-grade disturbance of consciousness, loss of orientation, muscle stiffness, fecal and urinary incontinence, and gait unsteadiness. The most severe form of hepatic encephalopathy is hepatic coma (stage 4). Patients are unconscious and cannot be awakened even by painful stimuli. Muscle reflexes are completely extinguished. Because of the penetrating odor, poisoning by gaseous ammonia is rather rare. Ammonia in gaseous form is absorbed primarily through the lungs. Due to a reaction with moisture, it has a strongly corrosive effect on the mucous membranes of the respiratory tract. Above a certain concentration, there is a danger to life. The ammonia can cause laryngeal edema, laryngospasm, pulmonary edema, or pneumonia, resulting in respiratory failure.
