INHIBITION OF A MAMMALIAN DEOXYRIBONUCLEIC ACID NUCLEOTIDYLTRANSFERASE BY ACTINOMYCIN D

Abstract

The inhibitory effect of Actinomycin D was studied in a partially purified preparation of deoxyribonucleic acid nucleotidyltransferase from Landschutz ascites-tumour cells. The inhibition exercised by the antibiotic appeared to obey competitive kinetics with respect to the DNA-primer. Relation of inhibition to the molar ratio of Actinomycin D to deoxyribonucleic acid-primer guanine residues revealed that 100% inhibition was achieved at a ratio of about unity. The nucleotidyltransferase was assayed also in the presence of only one triphosphate, thymidine 5[image]-triphosphate. The very limited incorporation observed in these circumstances probably took place at terminal positions in the primer, and was relatively resistant to the inhibitory action of Actinomycin D. It was concluded that low concentrations of Actinomycin D eliminated nucleotidyltransferase activity of the replicative type, but that much higher concentrations were required to eliminate the remaining and relatively resistant incorporation which was probably of the terminal type.