This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Amino Acid Metabolism, Subject name: Biochemistry
1st product of tryptophan catabolism is? 
Select and explain. 
A. Kynerunine, B. Bradykinin, C. PAF, D. Xantheurenate
A: Kynerunine
Answer: A: Kynerunine 
Explanation: ANSWER: (A) KynerunineREF: Harper's 27th ed ch: 29Tryptophanisdegradedto amphibolic intermediatesvia thekynurenine-anthranilate pathway. Tryptophan oxygenase (tryptophan pyrrolase) opens the indole ring, incorporates molecular oxygen, and forms N-formylkynurenine.Hydrolytic removal of the formyl group of AT-formylkynurenine, catalyzed by kynurenine formylase, 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.