Isotope Effects in Metabolism Of14n And15n From Unlabeled Dietary Proteins

The purpose of this study was to determine whether the excess of 15 N 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 15 N and its distribution are different in these two proteins. In both groups of rats, abundance of 15 N 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 15 N 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 15 N.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 15 N accumulated primarily in the nonessential group, and in leucine and valine. 15 N excess appeared to be greatest in amino acids which are extensively involved in nitrogen transfer. In threonine isolated from liver protein, abundance of 15 N was below that of dietary threonine, and this deficit increased with duration of feeding in both groups of rats.