Methionine requirement for growth and utilization of its optical isomers.

Several years ago experiments were undertaken in this laboratory for the purpose of determining the minimum amount, of each indispensable amino acid which must be included in the food in order to induce maximum growth in the rat. This was accomplished by varying the proportion of one amino acid while maintaining the concentrations of all others at constant levels. Throughout the tests, liberal quantities of the non-essential amino acids were furnished except in t,he experiments involving methionine and phenylalanine. With these, the rations were devoid of cystine and tyrosine respectively, inasmuch as it had already become evident that the former can arise in viva from methionine, and the latter from phenylalanine. Thus, the values representing the minimum needs of the organism for methionine and phenylalanine really included the quantities of these two amino acids which were used in the manufacture of cystine and tyrosine. The results of these tests were summarized by one of us in 1937 (1). At the time, attention was called to the fact that the values were “to be regarded as strictly tentative.” Furthermore, it was pointed out that “factors such as the proportion of fat and carbohydrate in the ration, and the age, weight and sex of the subjects may play important r&es in determining the minimum level of a given component.” The experimental data upon which these tentative values were based were never published in full. After the tests had been completed, systematic investigations extending over several years revealed the necessity of altering our basal diet in order to provide a more appropriate (lower) fat content, a better source of inorganic salts, and a more adequate supply of vitamins (cf. Borman et al. (2)). All of these modifications permitted the subjects to gain at more rapid rates. It is well known that a ration which furnishes all the essential components is consumed by animals until their caloric requirements have been satisfied. It seemed probable, therefore,