Structure-activity relationships for a class of inhibitors of purine nucleoside phosphorylase

Values of inhibition constants, K-i, for a family of structurally related, competitive inhibitors of calf spleen purine nucleoside phosphorylase (PNP) have been determined employing both inosine as substrate and a manual assa y and 2-amino-6-mercapto-7-methylpurine ribonucleoside (MESG) as substrate and a robot-based enzyme kinetics facility. Several of the values determine d robotically were confirmed employing the same substrate and a manual assa y. Surprisingly, for many of the inhibitors examined, values of K-i determi ned with MESG as substrate are smaller than those obtained employing inosin e as substrate by a factor that varies from less than 2 to 10. Values of co ncentrations required for 50% inhibition of PNP, IC50, have also been deter mined for the same family of inhibitors employing inosine as substrate. Val ues of IC50ino and those for K-i(ino) and K-i(mesg) for subsets of the inhi bitors have been employed as training sets to create quantitative structure -activity relationships (QSAR) which have substantial power to predict valu es of IC50 and K-i for inhibitors outside the training set. These QSAR mode ls should be useful in guiding future medicinal chemistry efforts designed to discover inhibitors of PNP having increased potency.