Purine nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) from four sources (rat liver and brain, human erythrocytes, and calf spleen) has been shown to exhibit a low intrinsic activity towards adenine in the presence of ribose 1-phosphate. The Km for adenine determined with purine nucleoside phosphorylase from rat liver and human erythrocytes was 3.4 × 10−4 M and 4.1 × 10−4 M, respectively. Depending upon the source of the enzyme, the Vmax for ribosylation of hypoxanthine was 274–1630 times greater than the Vmax for ribosylation of adenine.Adenine is a competitive inhibitor of purine nucleoside phosphorylase from Sarcoma 180 ascites cells. An inhibition constant, Ki, of 5.5 × 10−4 M was determined for adenine with this enzyme and with selenoguanine as the variable substrate.
Purine Nucleoside Phosphorylase from Human Erythrocytes
