This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urea Cycle and Disorders, Subject name: Biochemistry
Major source of ammonia in kidney - 
Select and explain. 
A. Glutamate, B. Glutamine, C. a-ketoglutarate, D. Alanine
B: Glutamine
Answer: B: Glutamine 
Explanation: Ans. is 'b' i.e., Glutamine Transport of ammoniao Ammonia is produced in most of the tissues. However, it should be transported to liver as urea cycle (formation of urea) occurs only in liver. Since free ammonia is highly toxic, it is never transported in free form in blood. Two mechanisms are available for transport of ammonia from the peripheral tissues to the liver.1) In the form of glutamine# In many tissues like liver, kidney and brain, ammonia combines with glutamate to yield glutamine, by the action of glutamine synthase. The brain is a rich source of glutamine synthase and it predominantly detoxifies ammonia by this routeQ.# Glutamine is a nontoxic major transport form of ammoniaQ. The glutamine is transported by blood to liver where deamination (removal of amino group) of glutamineQ takes place. Glutaminase cleaves glutamine to yield glutamate and free ammonia (ammonium ion). The ammonia is converted by liver to urea.# Formation and secretion ammonia by renal tubular cells maintain acid base balance. Ammonia is formed from glutamine by glutaminaseQ. Excretion of ammonia increases in metabolic acidosis and decreases in metabolic alkalosisQ.2) In the form of alanine# Alanine transports ammonia from muscles to liverQ through 'glucose-alanine cycle'. In muscle, glutamate is formed from ammonia and a-ketogiutarate by reversal of the glutamate dehydrogenase reaction. L-glutamate then transfers its a-amino group to pyruvate by transmination reaction to form alanine. Alanine is transported to liver. In liver alanine is converted to pyruvate and glutamate by transamination reaction. Glutamate undergoes oxidative deamination to release free ammonia, which is converted to urea. Pyruvate is converted to glucose by gluconeogenesis.