Interrelations of Phenylalanine and Tyrosine in the Chick

It has previously been established that the chick requires phenylalanine in its diet (1,2) as do the rat (3) and mouse (4), but for none of these animals is tyrosine an essential dietary constituent (2-6). Moss and Schoenheimer showed (‘i), by isotope tracer methods, that an appreciable amount of phenylalanine is converted to tyrosine in the rat even when a large amount of tyrosine is present in the diet. Bauer and Berg (4) considered that for the mouse a lack of tyrosine imposed an additional need for phenylalanine; Almquist and Grau (2) found that chicks fed diets containing 1 per cent nn-phenylalanine alone grew poorer than those fed diets which contained, in addition, 2 per cent n-tyrosine. A high level of nn-phenylalanine (3 per cent) obviated the need for tyrosine. Recently, Womack and Rose (8) demonstrated that in the rat tyrosine is capable of stimulating growth only when phenylalanine is furnished in suboptimum amounts. The present report is concerned with the results of feeding various levels of phenylalanine and tyrosine on the growth of chicks, and with determinations of the percentage retention for tissue formation of these components of the diet.