A 55-year-old man suffers from cirrhosis of liver. Toxins such as ammonia are not properly metabolized by the liver in this disease and can damage the organs such as brain. Which of the following amino acids covalently binds ammonia, transports and stores it in a non toxic form?
The correct answer is- D- Glutamate.
Glutamate occupies a central place in the amino acid metabolism. Basically it acts as a collector of amino group of the amino acids. Free ammonia is toxic to the body especially to brain cells; it is transported in the bound form to liver where it is finally detoxified forming urea.
The first line of defense from ammonia intoxication is conversion of glutamate to Glutamine. Excess of ammonia depletes glutamate and hence GABA level in brain, to compensate for glutamate, alpha keto glutarate is used, the decrease concentration of which subsequently depresses TCA and thus deprives brain cells of energy. Excess Glutamine is exchanged with Tryptophan, a precursor of Serotonin, resulting in hyper excitation. The symptoms of ammonia intoxication are all due to energy depletion and a state of hyper excitation.
As regards other options, Tryptophan has no role in the transport of ammonia. Alanine is a transporter of ammonia from muscle- Glucose Alanine cycle. Aspartate is required for transamination reactions and glutamine is produced from glutamate by coupling with ammonia.
Please read the central role of Glutamate for better understanding.
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