Purine Phosphoribosyltransferases Convert Purines to Nucleotides

In mammals specific enzymes for converting purine bases to nucleotides are present in many organs, and in heart muscle this may be the main source of purine nucleotides. The most important of these enzymes is hypoxanthine-guanine phosphoribosyltransferase, which catalyzes the formation of IMP from hypoxanthine and GMP from guanine  

This enzyme returns the major products of purine nucleotide catabolism to nucleotide forms. 

The equilibria in these phosphoribosyltransferase reactions favor nucleotide synthesis, and since the inorganic pyrophosphate released is rapidly hydrolyzed by inorganic pyrophosphatase, the coupling of these reactions makes the synthesis of nucleotide irreversible. However, the efficiency of salvage is heavily dependent on the intracellular concentration of PRPP. 

The role of hypoxanthine-guanine phosphoribosyltransferase in purine salvage has been confirmed by the abnormally high excretion of purines (as uric acid) in humans who lack hypoxanthine-guanine phosphoribosyltransferase. Studies of purine metabolism in cultures of cells from patients with this hereditary disorder also support this conclusion. 

Besides this salvage role, hypoxanthine-guanine phosphoribosyltransferase is probably important also for the transfer of purines from liver to other tissues. Purine biosynthesis de novo is especially active in the liver, and extrahe-patic cells that have a low capacity for the synthesis of purines de novo, such as erythrocytes and bone marrow cells, depend on uptake of hypoxanthine and xanthine from the 

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