Aminopterin inhibition in Aerobacter aerogenes: alanine and valine accumulation during the inhibition and their utilization on recovery

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Abstract
Throughout the period of inhibition of growth which follows the addition of the folic acid antagonist, aminopterin, to cultures of A. aerogenes in a simple chemically defined medium, alanine, valine and 5-amino-4-imidazolecarboxamide riboside accumulation in the medium. Inhibition of growth of A. aerogenes by aminopterin is accompanied by a decrease in deoxyribonucleic acid content, which markedly exceeds that of any other cell component. With the resumption of growth, which occurs after 15 to 18 hours through the inactivation of the aminopterin, the accumulated inter-mediates are reutilized rapidly, and the chemical composition of the cells becomes similar to that of normal cells grown in the absence of the antagonist. In the recovery cultures the accumulated alanine is utilized in the synthesis of a number of amino acids including leucine, for which it provides the x- and carboxyl-carbon atoms. The accumulated valine, however, is incorporated to a significant extent only into valine and leucine of the protein of A. aerogenes. The synthesis of leucine from valine involves the elimination of the carboxyl group of the latter amino acid. In normal cultures of A. aerogenes this occurs with the formation of a C1 unit other than CO2, which is utilized in the synthesis of the purine nucleus and the methyl group of thymine, and is probably formic acid. An analogy is drawn between the cleavage of 3-methyl-2-oxobutanoic acid with the production of formic acid, and the dissimilation of pyruvate to acetyl-coenzyme A and formic acid, and it is suggested that both of these reactions may be dependent upon folinic acid and susceptible to inhibition by aminopterin.