Purine nucleoside phosphorylase (PNP) from mouse leukemia cells L1210 was purified to homogeneity by a combination of ion exchange and affinity chromatography using AE-Sepharose 4B and 9-(p-succinylaminobenzyl)hypoxanthine as the matrix and the ligand, respectively. The native enzyme has a molecular weight of 104 000 and consists of three subunits of equal molecular weight of 34 000. The results of isoelectric focusing showed that the enzyme is considerably microheterogeneous over the pI-range 4.0-5.8 and most likely consists of eight isozymes. The temperature and pH-optimum of phosphorolysis, purine nucleoside synthesis and also of transribosylation is identical, namely 55 °C and pH 7.4. The transribosylation reaction proceeds in the presence of phosphate only. The following Km-values (μmol l-1) were determined for phosphorolysis: inosine 40, 2'-deoxyinosine 47, guanosine 27, 2'-deoxyguanosine 32. The Km-values (μmol l-1) of purine riboside and deoxyriboside synthesis are lower than the values for phosphorolysis (hypoxanthine 18 and 34, resp., guanine 8 and 11, resp.). An affinity lower by one order shows PNP for (-D-ribose-1-phosphate, (-D-2-deoxyribose-1-phosphate (Km = 200 μmol l-1 in both cases) and phosphate (Km = 805 μmol l-1). The substrate specificity of the enzyme was also studied: positions N(1), C(2) and C(8) are decisive for the binding of the substrate (purine nucleoside).
Substrate specificity and kinetic mechanism of purine nucleoside phosphorylase from Mycobacterium tuberculosis
