Purine metabolism overview. Schematic overview of main enzymes and transporters involved in purine conversion and uptake. ENT1-2 indicates equilibrative nucleoside transporter 1-2; CNT1-3, concentrative nucleoside transporter 1-3; PNP, purine nucleoside phosphorylase; dCK, deoxycytidine kinase; dGK, deoxyguanine kinase; NT5C, cytosolic 5′ nucleotidase 1A; NT5C1A, cytosolic 5′ nucleotidase 1A; NT5C2, cytosolic 5′ nucleotidase; and RRM1 and RRM2, ribonucleotide reductase subunit 1 and subunit 2.