Crystallization and some properties of purine nucleoside phosphorylase from chicken liver

Purine nucleoside phosphorylase (purine nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) from chicken liver has been purified about 650 fold and crystallized. The crystalline enzyme was cube shaped and showed a specific activity of 46 units per mg of protein. The homogeneity of the crystalline enzyme was shown by polyacrylamide gel-disc electrophoresis. The sedimentation coefficient (s-degrees 2o,w) was 5.4 S. The crystalline enzyme was activated by the substrate inosine. The Hill coefficient was estimated to be 0.76, suggesting negative cooperativity with regard to the substrate inosine. The results of the kinetic analysis are consistent with the mechanism being a "rapid equilibrium random Bi-Bi reaction". The apparent equilibrium constant for phosphorolysis was 0.048.