Seventeen structurally related compounds were tested for their ability to substitute for phenylalanine or tyrosine in the nutrition of chick embryo heart fragments. DL-Alanyl-DL-phenylalanine replaced phenylalanine. All other compounds had negligible effects, and most were toxic at high concentrations. β-Phenylserine, a phenylalanine antagonist, actually prolonged the survival of chick heart cells but only if both phenylalanine and tyrosine were present. Similarly, optimal reversal of β-phenylserine toxicity was dependent on the presence of both amino acids. Although phenylalanine and tyrosine are not interconvertible in the present system, it has been shown that three phenylalanine antagonists, p-fluorophenylalanine, β-2-thienylalanine, and β-phenylserine, can be identified by their relationship to tyrosine, rather than to phenylalanine.
