Thiol groups in deoxyribonucleic acid nucleotidyltransferase

1. The inhibitory effects of iodoacetate, iodoacetamide, p-hydroxymercuribenzoate and sarkomycin were studied in a partially purified preparation of deoxyribonucleic acid nucleotidyltransferase from Landschutz ascites-tumour cells. All of these agents inhibited the activity of the enzyme, and it was shown that the inhibition exerted by the last three compounds obeyed non-competitive kinetics. 2. Inclusion of glutathione or 2-mercaptoethanol in the enzyme assays did not prevent the inhibition by iodoacetate or iodoacetamide, but did prevent inhibition by p-hydroxymercuribenzoate. Inhibition by sarkomycin could be partially prevented by glutathione or 2-mercaptoethanol. 3. The enzyme fraction also catalysed incorporation in the presence of only one triphosphate (thymidine 5′-triphosphate), and the limited incorporation observed in these circumstances was more resistant to the inhibitory action of iodoacetamide and p-hydroxy-mercuribenzoate than was the standard nucleotidyltransferase reaction (four triphosphates present). Levels of inhibition imposed on the standard reaction were achieved in the limited incorporation reaction with 2·5-fold higher concentrations of the two inhibitors. 4. The addition of certain bivalent cations to the standard system resulted in severe inhibition of the reaction: Zn²⁺ ions (10μm) gave 50% inhibition; ethylenediaminetetra-acetate (0·4mm) in the reaction mixture gave essentially complete protection against this inhibitory effect of Zn²⁺ ions. 5. Deoxyribonucleic acid-nucleotidyltransferase fractions prepared in the presence of a thiol and ethylenediaminetetra-acetate could be stored without loss of activity for 2 months at 0° or for 1 year at −70°.