Ammonia Metabolism and the Urea Cycle

Ammonia is a neurotoxic byproduct of amino acid and protein catabolism that the body must continuously dispose of. The liver does this chiefly through the urea cycle (the ornithine or Krebs-Henseleit cycle), which converts ammonia into urea for renal excretion. When hepatic urea synthesis fails or is bypassed, ammonia accumulates in the blood and can affect the brain.

Definition

The urea cycle is the hepatic pathway that converts ammonia, derived largely from amino acid catabolism and gut nitrogen, into urea for excretion, serving as the body's principal route of nitrogen disposal and ammonia detoxification.

Scope

The entry covers the sources of ammonia, the steps and compartmentalization of the urea cycle, the complementary role of glutamine synthesis in ammonia handling, and the consequences of impaired ureagenesis. It is a reference account of nitrogen disposal physiology; clinical syndromes of hyperammonemia are treated in the hepatic encephalopathy entry.

Core questions

• Where does circulating ammonia come from?
• What are the steps of the urea cycle and how is it compartmentalized in the hepatocyte?
• How do the liver and muscle use glutamine synthesis to buffer ammonia?
• What happens to ammonia when hepatic urea synthesis is impaired or bypassed?

Key concepts

• Ammonia as a nitrogen waste product
• Urea cycle (ornithine cycle)
• Mitochondrial and cytosolic cycle steps
• Carbamoyl phosphate synthetase I
• Glutamine synthesis and ammonia buffering
• Periportal versus perivenous hepatocyte zonation
• Portosystemic shunting
• Hyperammonemia

Mechanisms

Ammonia arises from the deamination of amino acids, from glutamine breakdown, and from gut bacterial metabolism of nitrogenous substrates absorbed into the portal blood (Braissant et al., 2013; Rui, 2014). Periportal hepatocytes convert ammonia and bicarbonate-derived carbamoyl phosphate into urea through the urea cycle, whose first steps occur in the mitochondrion and remaining steps in the cytosol, regenerating ornithine each turn (Krebs & Henseleit, 1932). Perivenous hepatocytes provide a high-affinity backup by incorporating residual ammonia into glutamine via glutamine synthetase, and skeletal muscle also takes up ammonia by forming glutamine. When functioning liver mass is lost or portal blood bypasses hepatocytes through portosystemic shunts, ammonia escapes detoxification and rises in the systemic circulation, where it can cross into the brain and disturb astrocyte and neuronal function.

Clinical relevance

The integrity of hepatic ureagenesis underlies the body's ability to keep blood ammonia low; its failure links liver disease and certain inherited enzyme defects to elevated ammonia and its neurological effects. This entry explains the physiology that connects nitrogen metabolism to clinical hyperammonemia and is not a basis for diagnosis or treatment of any individual.

Evidence & guidelines

The biochemistry of ammonia handling and its neurotoxicity is reviewed by Braissant and colleagues (2013), and the urea cycle's place in hepatic metabolism is described in standard physiology reviews (Rui, 2014). The cycle itself was first described by Krebs and Henseleit (1932).

History

Hans Krebs and Kurt Henseleit described the ornithine cycle of urea formation in 1932, one of the first metabolic cycles to be elucidated and a milestone in biochemistry. Later work mapped the enzymes, their compartmentalization within the hepatocyte, and the inherited urea cycle disorders that arise when individual steps fail.

Seminal works

• krebs-henseleit-1932
• braissant-2012

Frequently asked questions

Why is ammonia dangerous?

Ammonia is neurotoxic; when blood levels rise it can cross into the brain and disturb astrocyte function, which is why the body continuously converts it to the harmless, excretable molecule urea.

How can ammonia rise even when the urea cycle enzymes are intact?

In advanced liver disease, portal blood carrying gut-derived ammonia can bypass functioning hepatocytes through portosystemic shunts, so the ammonia is never presented to the urea cycle for detoxification.