Role of Intestinal Flora on Incorporation of 15N from Dietary, 15N-Urea, and 15N-Diammonium Citrate into Tissue Proteins in Pigs

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Abstract
To investigate the nutritional role of intestinal flora on the utilization of non-protein nitrogen (NPN) for protein synthesis, 15N-urea and 15N-diammonium citrate (DAC) were administered orally to germfree and specific pathogen free (SPF) piglets fed a 21.9% crude protein diet. The 15N from 15N-urea was incorporated into the protein of the liver, kidney, heart, muscles, and colon mucosa in the SPF piglet; however, none of the 15N from 15N-urea was incorporated into any tissue protein in the germfree piglets. On the other hand, the incorporation of 15N from 15N-DAC into tissue proteins was observed in both the germfree and SPF piglets. The 15N concentration in each tissue protein was higher when 15N-DAC was used in the SPF piglets, as compared to when 15N-urea was used. The urea N concentration in the supernatant of the ascending colon contents was higher in the germfree piglets given NPN-supplemented diets (+NPN) than in the SPF piglets (+NPN), while the concentrations of both ammonia N in the colon and of plasma ammonia N in the portal vein were higher in the SPF piglets (+NPN), as compared to the germfree piglets (+NPN). Our conclusion is that when nitrogen of dietary or endogenous urea enters the intestinal tract, it is a possible source of the nitrogen needed for protein synthesis only when the urea is converted into ammonia nitrogen by the action of intestinal-bacterial urease.