Studies on the dual requirement for phenylalanine and tyrosine in tissue cultures.

Studies in several laboratories (l-4) have established that isolated tissues growing or surviving in vitro require both phenylalanine and tyrosine. This finding is in marked contrast to the well known sparing effect of tyrosine on the phenylalanine requirement of the whole animal (5-7). In the latter case, this sparing effect has been attributed (8) to the action of “phenylalanine hydroxylase,” an enzyme system consisting of two parts, Fraction I, found only in the liver, and Fraction II, found in most tissues studied (9). The dual requirement for phenylalanine and tyrosine by tissue cultures is in accord with the observations on phenylalanine hydroxylase distribution. It is also possible, however, that isolated tissue cells may resemble certain microorganisms (10) in which these two amino acids arise by separate metabolic pathways from a common precursor. The survival of chick embryonic heart tissues cultivated in vitro in completely synthetic media has proved a sensitive tool for the study of amino acid interrelationships (11, 12). Accordingly, a study of the phenylalanine-tyrosine requirement in this system was undertaken to clarify the relationship between these two amino acids. The present communication also reports data on the stereospecificity of the amino acid requirement and the effect of two antagonists, P-2-thienylalanine and pfluorophenylalanine. The results indicate that these two antagonists inhibit different reactions, and some of the characteristics which distinguish these reactions are discussed.