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
First product of tryptophan catabolism is: 
Select and explain. 
A. Kynerunine, B. Bradykinin, C. PAF, D. Xantheurenate
A: Kynerunine
Answer: A: Kynerunine 
Explanation: Ans. (a) KynerunineRef: Harper's Biochemistry, 30th ed. pg. 306-307* Tryptophan is degraded to amphibolic intermediates via the kynurenine-anthranilate pathway. Tryptophan oxygenase (tryptophan pyrrolase) opens the indole ring, incorporates molecular oxygen, and forms N-formyl kynurenine.* Hydrolytic removal of the formyl group of N-formyl kynurenine, catalyzed by kynurenine fortnylase, produces kynurenine.* Since kynureninase requires pyridoxal phosphate, excretion of xanthurenate in response to a tryptophan load is diagnostic of vitamin B6 deficiency.* Hartnup disease reflects impaired intestinal and renal transport of tryptophan and other neutral amino acids.