A.
Free purine and pyrimidine bases are constantly released in cells during the metabolic degradation of nucleotides. Free purines are in large part salvaged and reused to make nucleotides, in a pathway much simpler than the de novo synthesis of purine nucleotides described earlier.
B.
One of the primary salvage pathways consists of a single reaction catalyzed by adenosine phosphoribosyltransferase, in which free adenine reacts with PRPP to yield the corresponding adenine nucleotide.
C.
Free guanine and hypoxanthine (the deamination product of adenine ) are salvaged in the same way by hypoxanthine-guanine phosphoribosyltransferase.
D.
A similar salvage pathway exists for pyrimidine bases in microorganisms, and possibly in mammals.