Purine nucleoside phosphorylase. Microheterogeneity and comparison of kinetic behavior of the enzyme from several tissues and species.

The purine nucleoside phosphorylases (PNPases) from human and rat erythrocytes and bovine spleen have been subjected to isoelectric focusing. The crystalline bovine spleen PNPase emerged as a single peak of pI = 5.4 whereas the rat erythrocytic PNPase was distributed into two variants of pI = 5.6 and 5.7 and the crystalline human erythrocytic enzyme produced six variants ranging from pI = 5.85 to 6.25. Treatment of human erythrocytic PNPase with dithiobisnitrobenzoate changed the enzyme to a more acidic form (pI = 5.05). The kinetic behaviors of these electrophoretic variants were studied and compared with the unresolved bovine erythrocytic PNPase. All six variants of the human erythrocytic PNPase and the two variants of rat erythrocytic PNPase displayed substrate activation at high concentrations of inosine and deoxyinosine. Bovine erythrocytic PNPase did not show activation with any of the nucleosides whereas with the bovine spleen enzyme activation occurred only with the deoxynucleosides, deoxyinosine and deoxyguanosine. The Km values for inosine, deoxyinosine, guanosine, deoxyguanosine, guanine, and hypoxanthine, where determined, ranged from 1.3 x 10-5 to 3.0 x 10-5 M for all the enzymes except the rat erythrocytic PNPase variants which have higher Km values for inosine (5.9 x 10-5 M, 8.3 x 10-5 M) and deoxyinosine (13 x 10-5 M, 20 x 10-5 M).