The purpose of this study was to determine whether the excess of 15N present in protein nitrogen of rat tissues is greater than that in nitrogen of the proteins consumed by the animals.Two groups of rats were fed diets in which the protein components were casein and soybean protein respectively. Both the natural excess of 15N and its distribution are different in these two proteins. In both groups of rats, abundance of 15N in nitrogen of liver and muscle protein exceeded that in dietary protein nitrogen after 2 weeks. When the same diets were continued for 8, 26, or 50 weeks, further accumulation of 15N in tissue proteins was slight and limited to a few amino acids. Protein nitrogen of nuclear, mitochondrial, microsomal, and soluble subcellular liver fractions contained essentially the same excess of 15N.Analyses of nitrogen from eight nonessential amino acids (proline, glutamic acid, alanine, aspartic acid, argmine, glycine, serine, tyrosine) and six essential ones (valine, leucine, lysine, histidine, phenylalanine, threonine) isolated from dietary and tissue proteins showed that 15N accumulated primarily in the nonessential group, and in leucine and valine. 15N excess appeared to be greatest in amino acids which are extensively involved in nitrogen transfer. In threonine isolated from liver protein, abundance of 15N was below that of dietary threonine, and this deficit increased with duration of feeding in both groups of rats.

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The purpose of this study was to determine whether the small increase in abundance of N15 normally found in nitrogenous compounds of biological origin is primarily due to mass discrimination in nitrogen metabolism, or to reproducible analytical errors.This problem was approached by repeatedly determining N15 in the same series of nine amino acids, either purchased in chromatographically pure form or isolated from proteins of rat liver, dog serum, and six plant sources. Proteins and amino acids included in the study were selected on the premise that concentration or redistribution of N15 might occur in such processes in urea formation, nitrogen transfer, or nitrogen fixation. Data for N15 excess in a series of alkaloids were also secured.Standard deviations obtained in series of analyses were too small, and values for N15 excess in the same amino acids isolated from different sources too variable, to permit interpreting the observed N15 excess as a reproducible error. Distribution of N15 in amino acids of the animal proteins studied resembled that observed when N15-labelled amino acids or ammonium compounds are given. Differences between results for amino acids of animal and plant origin also supported the idea that the small excess of N15 normally found is metabolically as well as statistically significant. Results for N15 in amide nitrogen likewise supported this view. The smallest excess of N15 occurred in amino acids from proteins of legumes, which fix nitrogen. In synthetic amino acids, the concentration of N15 was more often below than above normal abundance.

Prolonged administration of certain anterior pituitary extracts to animals results in at least two processes requiring nitrogen,--growth and lactation. It is obvious that in the production of gigantism in rats, by Evans and his coworkers (1, 2), and in the production of acromegaly in dogs by Putnam, Benedict, and Teel (3), large amounts of nitrogen must have been involved, although no observations on the nitrogen balance were made in the particular experiments. As a result of single injections of anterior pituitary extract, Teel and Watkins (4) observed a fall in the blood non-protein nitrogen of dogs. Teel and Cushing (5) also refer to experiments carried out in collaboration with Dr. Walter Bauer, in which it was found that anterior pituitary extracts administered to dogs which were consuming a constant diet caused a marked retention of nitrogen. The fall in blood nonprotein nitrogen observed by Teel and Watkins indicated that this retention was not secondary to renal impairment.It appeared of interest to the writer to study in some detail the effect of anterior pituitary extracts on the nitrogen economy of dogs. The term "growth" is at once recognized as a collective name for a considerable series of processes, and the term "growth hormone" suggests a single substance capable of initiating all of these. Should injections for a day or two result in a permanent gain in nitrogen, in absence of lactation, this result would be compatible with the idea that all of the processes required for laying down of permanent tissue had been stimulated and had gone to completion. On the other hand, a subsequent loss of the stored nitrogen would suggest that the factor supplied merely stimulated formation of reserve or "deposit" protein, 349on May 9, 2018 jem.rupress.org Downloaded from

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