Ammonia is a nitrogen-containing compound that plays a critical role in normal metabolism but becomes neurotoxic at elevated concentrations.
Ammonia is produced primarily from dietary protein breakdown, bacterial metabolism in the colon, and deamination of glutamine in the small intestine.
In healthy individuals, ammonia is efficiently detoxified by the liver through the urea cycle and conversion to glutamine via glutamine synthetase.
Produced by colonic bacteria and by deamination of dietary glutamine.
Ammonia Metabolism
Ammonia produced in the gut is absorbed into the portal circulation, where portal vein concentrations are approximately ten-fold higher than arterial blood.
The liver extracts most of this ammonia through two main mechanisms: periportal hepatocytes convert it to urea via the urea cycle, while perivenous hepatocytes convert it to glutamine through glutamine synthetase.
The kidneys and skeletal muscle also participate in ammonia homeostasis, with muscle transforming ammonia into glutamine and kidneys generating ammonia from glutamine deamination depending on acid-base status.
Hyperammonemia and Clinical Significance
After intestinal absorption, NH3 is metabolized by the urea cycle in periportal hepatocytes and excreted via the kidneys and colon.
In cirrhosis portosystemic shunting and decline in hepatic urea synthesis causes an accumulation of NH3 in the blood, and increases levels in the CNS due to blood brain barrier disruption (Butterworth RF).
Associated with urea cycle disorders.
The testing for ammonia is delicate and subject to spuriously elevations.
Levels rise with standing blood because of in vitro deaminization of amino acids.
Elevations s of ALt and GGT enzymes are independently known to produce false elevations in ammonia levels.
Elevated blood ammonia (hyperammonemia) occurs when ammonia production exceeds clearance capacity or when detoxification mechanisms fail.
Hyperammonemia most common causes include liver cirrhosis (due to reduced hepatic function and portosystemic shunting), urea cycle disorders (genetic enzymatic deficiencies), organic acidurias, and fatty acid oxidation defects.
Hyperammonemia is particularly associated with hepatic encephalopathy, where ammonia crosses the blood-brain barrier and causes astrocytic dysfunction, cerebral edema, and altered neurotransmission.
The severity of neurological symptoms correlates with both the magnitude and duration of hyperammonemia, with acute elevations potentially causing life-threatening cerebral edema, particularly in fulminant hepatic failure.
Treatment strategies focus on reducing ammonia production and absorption from the gastrointestinal tract (using lactulose and rifaximin), providing alternative nitrogen excretion pathways (ammonia scavengers like sodium benzoate), and in severe cases, dialysis.