STRUCTURAL BASIS FOR INHIBITION OF PROTEIN SYNTHESIS BY EMETINE AND CYCLOHEXIMIDE BASED ON AN ANALOGY BETWEEN IPECAC ALKALOIDS AND GLUTARIMIDE ANTIBIOTICS

The therapeutic properties of the ipecac alkaloids in the treatment of certain dysenteries and their effectiveness as emetics and expectorants was recognized as early as the 17th century.1 (-)-Emetine, an active principle of ipecac, was reported to be an amebicidal agent by Vedder2 and recently has proved beneficial in the therapy of nonspecific granulomas.3 The present communication will show that emetine is a potent inhibitor of protein synthesis in mammalian and other cells, a mode of action which may account for its therapeutic and toxic properties. The glutarimide antibiotics, of which (-)-cycloheximide is the best known member, are derived from various species of Streptomyces. These compounds also inhibit protein synthesis and exert amebicidal, antitumor,5' 6 and fungicidal7 actions. An analogy between the glutarimide antibiotics and the ipecac alkaloids will be demonstrated in this paper based on (a) similarities in their mode of action, (b) their configurational resemblances, and (c) their structure-activity relationships. The analogy provides a structural basis for the inhibition of protein synthesis by the ipecac and glutarimide compounds and suggests new pharmaceutical preparations of potential value. Methods and Materials.-Protein and polyphenylalanine synthesis by extracts of different species was measured by the procedure of Lamfrom,8 for rabbit reticulocytes; of Downey et al.,9 for Saccharomyces cerevesiae; of Nirenberg and Matthaei,10 as modified by Nathans et at.,11 for E. coli; and of Penman and Summers,"2 for HeLa cells. Components of the reaction mixtures are described in the legend to Table 1. Following the period of incubation, reactions were terminated by addition of 5% TCA (trichloroacetic acid) and the acid-insoluble material was washed by the procedure of Siekievitz.14 The precipitates were collected on Millipore membrane filters, and the radioactivity was determined in a Nuclear-Chicago low-background counter with an efficiency of 21% for C14. The transfer reaction was assayed in extracts of rat liver as described by Gasior and Moldave.13 (- )-Emetine and (-)-N-methylemetine were kindly provided by Dr. N. Whittaker of the Wellcome Laboratories, Kent, England; (-)-cephaeline and (+)-Omethylpsychotrine by Sandoz, Ltd., Basle; (-)-2,3-dehydroemetine and (+ )-1,2,3,4,5,11btrisdehydroemetine by Dr. A. Brossi of Hoffmann-La Roche, Nutley, N.J.; and (-+)-isoemetine by Dr. J. Elks of the Glaxo Laboratories, Middlesex, England. E. coli B sRNA was obtained from Nutritional Biochemicals, poly U (polyuridylic acid) from the Miles Laboratories, and radioisotopes from New England Nuclear. C14-phenylalanyl-sRNA and C14--leucyl-sRNA were prepared enzymatically by the procedure of von Ehrenstein and Lipmann.15 Coliphage MS2 RNA was a gift from Dr. D. Nathans. Inhibition of Protein Synthesis by Emetine.-Species specificity: The effect of emetine on protein synthesis in certain mammalian cells, yeast, and plants is shown in Table 1. Emetine can effect a 50 per cent inhibition of protein synthesis in HeLa cells and gametophytes of Anemia phylitidis at media concentrations of less than 10-611M. Under these in vivo conditions, emetine is concentrated 10- to 20fold by the cells. Protein synthesis in cell-free extracts prepared from HeLa cells and reticulocytes was inhibited by 50 per cent at concentrations of 8 X 10-5 M and 2 X 10-5 M, respectively.