Amino Acid Esters as Inhibitors of Growth and of Aminoacyl Transfer Ribonucleic Acid Synthetases in Euglena and Astasia

Abstract

Abstract At moderate concentrations the methyl, ethyl, or butyl esters of 10 amino acids inhibit the growth of Astasia. At least 8 of these also inhibit the growth of Euglena, although Euglena is generally less sensitive to these esters than Astasia. None of the amino acids tested at comparable concentrations inhibit growth except cysteine. The concentrations of ester required for 50% inhibition of growth rate depend not only on the amino acid, but on the ester group, increasing inhibition being generally observed with increasing size of the alcohol moiety. Recovery takes place primarily by nonenzymatic hydrolysis of the ester. Inhibition is highly pH-sensitive, with little inhibition below pH 5 and maximal inhibition above pH 6. Experiments with 14C-leucine ethyl ester indicate that Astasia is much less permeable to this ester at low pH. These observations led to the hypothesis that the amino acid esters inhibited growth by interfering with the corresponding aminoacyl transfer ribonucleic acid (tRNA) synthetases. Consistent with this hypothesis were the observations that: (a) phenylalanine ethyl ester inhibited the synthesis of an induced acid phosphatase; (b) the methyl and ethyl esters of serine, which did not inhibit growth, did not inhibit a partially purified seryl-tRNA synthetase of Astasia; (c) the methyl and ethyl esters of leucine, which inhibited growth, competitively inhibited the partially purified leucyl-tRNA synthetase of Astasia. Other growth experiments demonstrated that, whereas leucine slightly protected against growth inhibition by leucine ethyl ester, phenylalanine potentiated growth inhibition due to phenylalanine ethyl ester. It was then found that partially purified phenylalanyl-tRNA synthetase is inhibited by excess substrate concentrations. It is suggested that some amino acid esters may be useful inhibitors of the first committed steps of protein synthesis.