The Interdependence among Amino Acids in their Utilization in the Endogenous Metabolism

Some 30 to 50% of the nitrogen lost in the endogenous catabolism may be replaced from a variety of incomplete dietary combinations of amino acids, even by mixtures containing none of those amino acids whose presence in the diet is essential for the attainment of nitrogen equilibrium. This much of the nitrogen requirement for the maintenance of the integrity of the tissues is thus apparently an undifferentiated one, that requires for its complete satisfaction no specific amino acids. The remainder of the requirement not only relates to apecific amino acids, but also is of such a nature that the utilization of the spicific amino acids required depends upon the simultaneous presence in the diet of certain combinations of the essential amino acids. Of this interdependence of the essential amino acids, it can be concluded, from the experiments reported in this paper, only that threonine and isoleucine, either individually or together, limit the utilization of the other essential amino acids and possibly occupy a key position in the anabolism consequent upon the endogenous disintegration of tissue constituents. Thus, the endogenous losses of nitrogen in the adult animal seem to result from the tessues of many types of nitrogenous constituents of relatively simple structure, rather than of complex amino-acid aggregates, such as the tissue proteins.