This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'Which among the following are the substrates needed for purine salvage pathway? \nSelect and explain. \nA. Hypoxanthine and Xanthine, B. Hypoxanthine and Adenine, C. Adenine and Guanine, D. Xanthine and Guanine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Conversion of purines, their ribonucleosides, and their deoxyribonucleosides to mononucleotides involves "salvage reactions". The more impoant mechanism involves phosphoribosylation by PRPP of a free purine (Pu) to form a purine 5\'-mononucleotide (Pu-RP). Phosphoryl transfer from ATP, catalyzed by adenosine- and hypoxanthine-phosphoribosyl transferases, conves adenine, hypoxanthine, and guanine to their mononucleotides. A second salvage mechanism involves phosphoryl transfer from ATP to a purine ribonucleoside. Phosphorylation of the purine nucleotides, catalyzed by adenosine kinase, conves adenosine and deoxyadenosine to AMP and dAMP. Similarly, deoxycytidine kinase phosphorylates deoxycytidine and 2\'-deoxyguanosine, forming dCMP and dGMP. Ref: Rodwell V.W. (2011). Chapter 33. Metabolism of Purine & Pyrimidine Nucleotides. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper\'s Illustrated Biochemistry, 29e.', 'role': 'assistant'}]