Figure 1.

The metabolic scheme depicts the pathways involved in the catabolism and salvage of purine nucleotides. Inosine 5′-monophosphate (IMP) represents the branching point from which both guanylate and adenylate pools are generated. IMP dehydrogenase (4) converts IMP into xanthosine monophosphate (XMP), and adenylosuccinate lyase (3) converts succinyl-adenosine 5-monophosphate (S-AMP) into AMP. Purine nucleotides are converted to the corresponding nucleosides by 5′-nucleotidases (cytosolic 5′-nucleotidase I, 5, and cytosolic 5′-nucleotidase II, 6). Guanosine (Guo) and Inosine (Ino) are phosphorolytically cleaved into the bases guanine (Gua) and hypoxanthine (Hyp), through purine nucleoside phosphorylase (11). Adenosine (Ado) is deaminated to Ino by adenosine deaminase (9). Gua, Hyp, and Adenine (Ade) can be salvaged to the corresponding nucleotides by phosphoribosyltransferases (hypoxanthine-guanine phosphoribosyltransferase, 2, and adenine phosphoribosyltransferase, 12). Phosphoribosylpyrophospate (PRPP), necessary for the salvage of purine bases, is generated by phosphoribosylpyrophosphate synthetase (1), starting from ATP and ribose-5-phosphate (Rib-5-P). Ado can be generated from S-adenosylmethionine (SAM), through methyltransferases (13) and S-adenosylhomocysteine (SAH) hydrolase (10). Ado can be converted into AMP by adenosine kinase (8). Ade can be generated from SAM, through the polyamine synthesis pathway (14) and methylthioAdo phosphorylase (15). AMP is deaminated to IMP by AMP deaminase (7) and Gua is converted to xanthine (Xan) by guanase (16). Eventually, purine bases are converted to uric acid (UA) through xanthine oxidoreductase (17).