Inhibition of terminal deoxynucleotidyltransferase by various 5-alkylated derivatives of 1-beta-arabinofuranosyluracil 5'-triphosphate: substituent effects on inhibitory action.

Inhibitory effects of 1-beta-D-arabinofuranosyluracil 5'-triphosphate (ara-UTP) and its 5-alkylated derivatives [1-beta-D-arabinofuranosylthymine 5'-triphosphate (ara-TTP): 1-beta-D-arabinofuranosyl-5-ethyluracil 5'-triphosphate (ara-EtUTP); 1-beta-D-arabinofuranosyl-5'-propyluracil 5'-triphosphate (ara-PrUTP); 1-beta-D-arabinofuranosyl-5-butyluracil 5'-triphosphate (ara-BuUTP)] on the activity of terminal deoxynucleotidyl-transferase (TdT) from calf thymus were examined. All these compounds inhibited TdT activity by competition with the natural substrate dTTP for the same substrate-binding site of the enzymes. The extent of inhibition by the inhibitor, however, decreased by introducing an alkyl group on the 5-position of the uracil nucleus, indicating the importance of inductive and steric effects of the 5-substituents on TdT. Of the four 5-alkylated ara-UTP's, ara-EtUTP was less inhibitory to TdT than the other compounds, and the potency of inhibition was restored by replacing 5-ethyl group with longer propyl or butyl group, suggesting that the hydrophobic effects of these 5-alkyl side chains are involved in the inhibitory action of the compounds. The results are compared and discussed with those of our previous report on the inhibition of DNA polymerase alpha and beta by these 5-alkylated ara-UTP's (Ono et al., 1981).