Amino-acids in Nutrition.* Ii. the Nutritive Value of Lactalbumin: Cystine and Tyrosine as Growth-limiting Factors in That Protein. by Barnett Sure

Since 1911, when Osborne and Mendel published t,heir first work on “Feeding experiments with isolated food substances,” there has been made accessible to investigation the problem of the biological evaluat’ion of proteins as correlated with their chemical analyses. It is true that for a time t.he advance made in the solution of this problem was obscured by the lack of appreciation of the necessity of the presence of the water-soluble and fat-soluble vit’amines, but this was soon rectified as information in connection with t,hese vitamines was accumulat.ed by Hopkins (l), McCollum and Davis (a), and by Osborne and Mendel themselves (3). However, with the incorporation of the water-soluble vitamine in the diet in the form of the extracts in which it is known new complications were introduced, inasmuch as this also meant the introduction of a small amount of nitrogen in unknown forms. Some of t’he earlier work in this field was, therefore, open to much crit’icism, as unquestionably in many instances the amino-acid deficiency of a protein was covered by 6he amino-acid nitrogen contained in the vitamine preparat,ions used in excessive amounts. Even with the present technique of ration-compounding this difficulty has not been entirely circumvented, but it has at least been minimized by the use of vitamine preparations low in nitrogen and minimum in amount (4).