This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
During vigorous strenuous exercise, which of the following amino acid is liberated from the skeletal muscles in maximum amount into circulation 
Select and explain. 
A. Glutamate, B. Glutamine, C. Branched chain amino acids, D. Alanine
D: Alanine
Answer: D: Alanine 
Explanation: Answer: d) AlanineALANINE* Alanine is a non-essential glucogenic amino acid.* Alanine can be formed by transamination of pyruvate.* The enzyme is alanine amino transferase (ALT).* Pyruvate + Glutamate - Alanine + alpha ketoglutarate* This reaction requires pyridoxal phosphate (PLP).* Under conditions of starvation, the glucose alanine cycle is of special metabolic significance.* Alanine is quantitatively the most important amino acid taken up by the liver from peripheral tissues, particularly from skeletal muscle.* It forms a major participant in inter-organ transport of nitrogen Glucose-Alanine Cycle (Cahill Cycle)* Alanine is transported to liver, transaminated to pyruvate and converted to glucose.* This glucose may again enter the glycolytic pathway to form pyruvate, which in turn, can be transaminated to alanine.* Glucose-alanine cycle is important in conditions of starvation.* Thus net transfer of amino acid (nitrogen) from muscle to liver and corresponding transfer of glucose (energy) from liver to muscle is effected.* Alanine cycle is intimately related with Cori's cycleAmino AcidArterial Concentration Rest240 min Exercise mmol/l Glycine188160Alanine192233Leucine126151!Valine242243Isoleucine6081!