Biological Value of Proteins in Relation to the Essential Amino Acids Which They Contain

In ten comparisons in human subjects of the proteins of whole egg, yeast, cottonseed meal, corn germ flour, beefsteak and haddock, with mixtures of the essential amino acids, compounded in the proportions one to another in which they occur in the proteins and supplying as much nitrogen as the protein, none of the mixtures possessed a biological value so high as that of the protein by from 10 to 40%. The mixtures contained arginine, histidine and lysine in the natural l (+) form and as the monohydrochloride salt. All the other acids isoleucine, leucine, methionine, phenylalanine, threonine, tryptophane and valine were available in sufficient amounts only as the dl synthetic products. In two experiments based on the hypothesis that if sufficient amounts of the natural isomers of the dl forms were supplied the unnatural isomers would be wasted, it was possible to duplicate closely the biological value of the proteins. A preliminary experiment was required to furnish data for calculation of the amount of nitrogen from unnatural isomers which would need to be excreted and subtracted from both sides of the balance sheet to produce the B.V. of the protein. Supplying these amounts in the dl isomers in a second experiment and deducting all the nitrogen of the unnatural isomers from both absorbed and urinary nitrogen of the amino acid period gave the predicted B.V. in one experiment and approached it in the other. Correction in this manner for nitrogen representing no nutritional synthetic value has been practised regarding the purine compounds in beverages and would be in order in the case of medicinal agents or urea administered in the course of an experiment as a test substance for kidney function. The unnatural isomers in these experiments insofar as they escape deamination, belong to this class of dispensable compounds; while insofar as they are deaminated and can be recognized as contributing extra nitrogen to the urea and ammonia fractions of the urine, they are in the same class as nonessential amino acids. The conclusion follows, that the use of dl synthetic amino acids, even those of the essential group, is nutritionally uneconomical.