The action of rhein, 4,5 dihydroxyanthraquinone-2-carboxylic acid, on protein synthesis of neoplastic cells has been investigated. Rhein decreases amino acid incorporation in all cells tested. The inhibition of incorporation of labeled precursors into acid-insoluble material cannot be ascribed to an impairment of amino acid uptake, which is unaffected by the drug. Tests on cell-free system showed that rhein does not inhibit the TMV-mRNA directed in vitro protein synthesis, thus indicating that the protein machinery per se is not affected. The inhibition of protein brought about by the drug must be ascribed to an effect on the energy-yielding processes with a remarkable decrease in ATP content. The mechanism is similar to that of other metabolic inhibitors, but rhein, for its capability to inhibit both respiration and glycolysis, is effective at much lower concentrations.
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