The Amino Acids Required for the Complete Replacement of Endogenous Losses in the Adult Rat

Abstract

For the replacement of endogenous losses of nitrogen, the adult rat not need the following dietary amino acids : lysine, leucine, histidine, arginine, and phenylalanine. These amino acids are all required for growth in the rat. The adult rat requires both cystine and methionine for the maintenance of nitrogen equilibrium. The cystine requirement can be covered by dietary methionine, but methionine requirement cannot be satisfied by dietary cystine. The endogenous metabolism of the adult rat involves the destruction of tyrosine, or of tissue constituents derived from tyrosine, but not the destruction of phenylalanine. The losses thus incurred me be replaced by iether dietary tyrosine or by dietary phenylalanine. The differences between the dietary requirements of amino acids for growth and for maintenance of nitrogen equilibrium may be satisfactorily explained on the basis of differences between rates of supply and of demand with respect two functions. The adult rat may be maintained in nitrogen equilibrium on a nitrogen supply containing only the following amino acids : threonine, isoleucine, tryptophane, valine, methionine, tyrosine and norleucine. The precise function of norleucine in maintaining the nitrogenous integrity of the tissues needs further study. It may function in promoting the synthesis of lysine or of leucine, or of both.