Ammonia, produced in animal cells as a result of catabolism, must be transported to the liver for its eventual conversion to urea. Figure 20.14 shows schematically how amino acids are employed to carry ammonia in the form of amino groups to the liver where it is eventually converted to urea. In most tissues, the toxic ammonia is converted to the nontoxic, and electrically neutral, glutamine. Glutamine is transport in the blood to the liver where it is cleaved hydrolytically by glutaminase in the following reaction:

Glutamine + H2O -> Glutamate + NH3

In the muscle, where glycolysis is active, the glucose-alanine cycle is used to remove toxic ammonia (Figure 20.14). Here glutamate dehydrogenase links ammonia to -ketoglutarate to form glutamate. Glutamate then donates its amine group to pyruvate (from glycolysis) to form alanine and -ketoglutarate. Alanine is transported to the liver, where it loses the amine group by a reversal of the previous processes, yielding ammonia for urea synthesis and pyruvate. Pyruvate, in turn, undergoes gluconeogenesis to form glucose, which is released to the blood and travels back to the muscles. Thus, muscle gets rid of ammonia and pyruvate and receives glucose (ultimately) in exchange.