Alteration of the Biochemical Effects of Ethidium Bromide by Exposure of Cells to Hypotonic Media

Abstract

ETHIDIUM bromide (3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide) has been shown to inhibit completely the incorporation of preformed purines (adenine, guanine and hypoxanthine) into nucleic acids in Ehrlich ascites tumour cells in vitro, but does not appreciably impair the incorporation into nucleic acids of purines synthesized de novo¹. This specific effect on preformed purine metabolism is not due to inhibition of their entry into these cells, conversion to nucleotides, or phosphorylation to nucleoside di- and tri-phosphates. Thus the purine nucleotides derived from exogenously supplied preformed purines are differentiated from those synthesized via the de novo pathway of purine nucleotide biosynthesis by this action of ethidium bromide. This suggests that these two types of purine nucleotides—chemically identical but derived from different sources—are separated within the cell. The nucleotides synthesized from exogenously supplied purines must mix with those made de novo at some point prior to their polymerization into nucleic acids, and it has been suggested that ethidium bromide interferes with this mixing process, which may be considered to be a movement of nucleotides from one compartment into another¹.