<prompt> Which among the following are the substrates needed for purine salvage pathway?Hypoxanthine and XanthineHypoxanthine and AdenineAdenine and GuanineXanthine and Guanine  <response>: 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, Harpers Illustrated Biochemistry, 29e. 