Sensitivity of trypanosoma equiperdum to the action of tumor-inhibitory antibiotics in vitro

Trypanosoma equiperdum , a member of the brucei-evansi group of African trypanosomes, is sensitive to the following tumor-inhibitory antibiotics: actinomycin D, mitomycin C, porfiromycin, phleomycin, and pactamycin. Only pactamycin inhibited trypanosomal carbohydrate metabolism in vitro , an action that might have contributed to the ability of this antibiotic to depress protein synthesis and various pathways of nucleic acid metabolism. The inhibitory effects of the other antibiotics were restricted to trypanosomal nucleic acid metabolism. Actinomycin D, 1.6 × 10 −8 M, inhibited the incorporation of 3 H-adenine and 3 H-uracil into RNA by approximately 50%, but had no effect at this concentration upon the incorporation of derivatives of these isotopes into DNA. Higher concentrations had no effect upon the incorporation of 3 H-adenine and 3 H-thymidine into DNA, but did inhibit the incorporation of derivatives of 3 H-uracil into DNA. The distribution of radioactivity in the nucleotide components of the acid-soluble fractions suggested that the predominant action of actinomycin D was to inhibit RNA polymerase. Mitomycin C and its methyl congener, porfiromycin, 2.2 × 10 −4 M and 5.7 × 10 −4 M, respectively, inhibited the incorporation of derivatives of 3 H-adenine and 3 H-uracil into RNA and DNA by approximately 50%. At 7.5 × 10 −6 M and 1.1 × 10 −5 M, mitomycin C and porfiromycin inhibited the incorporation of 3 H-thymidine into DNA by approximately 50%, and at 7.5 × 10 −5 M and 7.2 × 10 −5 M, the evolution of 14 CO 2 derived from orotic acid- 14 COOH. The ability of the mitosane antibiotics, at relatively low concentrations, to inhibit the incorporation of thymidine into DNA suggested that interference with DNA replication was a prominent feature of this group. The data also indicated that at higher concentrations, mitomycin C and porfiromycin inhibited RNA polymerase—whether by direct action or as a consequence of their effects upon DNA synthesis remains unsettled. Phleomycin, 1.3 × 10 −4 M, inhibited the incorporation of 3 H-adenine and 3 H-uracil into RNA by approximately 50%. At this concentration, phleomycin inhibited the incorporation of derivatives of 3 H-uracil into DNA to a greater extent than the derivatives of 3 H-adenine. At 3.2 × 10 −6 M, phleomycin inhibited the incorporation of 3 H-thymidine into DNA by approximately 50%. The action of phleomycin seemed to be qualitatively similar to that of the mitosane antibiotics, except for its inability to inhibit orotidylic acid decarboxylase.