Ammonia metabolism

Sources of ammonia and toxicity mechanisms

Ammonia is constantly generated in all body systems. The basic intracellular source is amino acid catabolism in tissues. Additionally, it is released during the breakdown of biogenic amines (oxidative deamination mediated by monoamine oxidases) and the degradation of nucleotides (adenine and guanine).

An important external source is the intestine, where putrefaction of undigested proteins occurs under the influence of microflora, alongside the breakdown of urea by bacterial urease. From the digestive tract, ammonia is absorbed and channeled into the portal vein, where its concentration significantly exceeds that of the general systemic circulation.

Despite continuous production, plasma maintains a minimal level of ammonia. This is achieved through instant cellular binding. Free ammonia ($NH_3$) in the cytosol and blood captures a proton, transforming into the ammonium cation ($NH_4^+$). The accumulation of these ions causes pronounced neurotoxicity: they disrupt the transmembrane transport of monovalent cations ($Na^+$ and $K^+$), leading to fatal impairments in nerve impulse conduction.

Transport forms and cellular detoxification

For safe transport to excretory organs, ammonia must be converted into non-toxic compounds.

The primary detoxification reaction in many tissues (especially muscle) is the synthesis of glutamine. The enzyme glutamine synthetase has an extremely high affinity for ammonia and binds it to glutamate with ATP expenditure. Glutamine is a neutral amino acid that, unlike glutamate, easily leaves cells via facilitated diffusion, acting as the main "transport taxi" for nitrogen.

Brain tissue utilizes an enhanced two-step mechanism that disposes of two $NH_3$ molecules simultaneously:

1. Glutamate dehydrogenase catalyzes the reductive amination of $\alpha$-ketoglutarate using NADH to form glutamate.
2. Glutamine synthetase converts the resulting glutamate into glutamine.

This pathway is energetically favorable for neurons as it preserves the carbon skeleton in the form of an amino acid.

Another crucial transport form is alanine, which predominantly transports nitrogen from muscles and the intestine. In intestinal cells (enterocytes), the acquired glutamine is cleaved by glutaminase into glutamate and ammonia. Subsequently, alanine aminotransferase (ALT) transfers the amino group to pyruvate, forming alanine.

The role of the liver in urea synthesis

The liver is the central organ for global detoxification, where alanine, glutamine, and other amino acids deliver bound ammonia via the bloodstream.

In hepatocytes, transport forms release their nitrogen: glutamine is hydrolyzed to release $NH_3$, and glutamate undergoes oxidative deamination. Alanine transfers its amino group to $\alpha$-ketoglutarate, while its carbon residue—pyruvate—is directed toward glucose synthesis (gluconeogenesis). The resulting glucose returns to the tissues (a process known as the glucose-alanine cycle).

The liberated ammonia, along with aspartate, enters the ornithine urea cycle. Through the formation of carbamoyl phosphate, the liver binds nitrogen with carbon dioxide into non-toxic urea. Every day, the liver synthesizes about 25 grams of urea, which is then filtered and eliminated by the kidneys.

Kidney function: ammoniagenesis and acid-base balance

Renal tissue plays a critical role not only in excreting end products of nitrogen metabolism, but also in the fine regulation of water-electrolyte and acid-base balance (ABB).

The kidneys utilize glutamine for acid excretion. In renal tubular cells, a two-step removal of nitrogen takes place:

• Under the action of glutaminase, the first ammonia molecule is cleaved (forming glutamate).
• Glutamate dehydrogenase cleaves the second molecule, leaving $\alpha$-ketoglutarate, which is utilized in the Krebs cycle.

The released ammonia ($NH_3$) actively binds free protons ($H^+$) and acid anions (such as chlorides or sulfates), forming ammonium salts. Since the generated ammonium cation ($NH_4^+$) cannot be reabsorbed back into the blood, it is excreted in the urine.

This mechanism becomes especially significant during acidosis (blood acidification). Excess protons induce the activity of glutaminase, and ammonium salt excretion can rapidly surge from a normal 0.5 g/day to 10 g/day. This allows the body to effectively remove excess acids while simultaneously preventing the loss of vital $Na^+$ and $K^+$ ions.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Glucose-alanine cycle (Cori-Cahill cycle)
• Detailed chemical reactions of the ornithine cycle (Krebs-Henseleit cycle)
• Molecular mechanisms of glutaminase regulation during acidosis
• Specifics of glutamine metabolism in enterocytes

Other topics in the "Amino acid metabolism" section

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