Utilization of nitrogen-15 urea for protein synthesis in domestic fowl

Experiments are described with eight broiler chickens weighing 0.7-1kg whose feed contained 50% fishmeal supplemented for three weeks with 2-4% urea of which part was then replaced by 15N-labelled urea for ten days (47mg of 15N daily). Three and seven days after the last peroral dose, 15N was incorporated to the extent of 0.047+-0.014 at.% excess and 0.005+-0.002 at.% excess in the TCA-precipitable plasma and muscle proteins respectively. A 15N excess of 0.302+-0.048 at.% was found in the amide-nitrogen of the plasma proteins and one of 0.045+-0.012 at.% in that of the muscle proteins. No differences in 15N enrichment were observed in relation to sampling time or urea supplementation. Despite the relatively high dose of 15N, very little became incorporated into the plasma proteins; about 50% was found in their amide groups. Only trace amounts of 15N were incorporated into the muscle proteins; however, their amide groups contained nine times the average concentration found in total nitrogen. The results show that, in chicks, urea nitrogen is utilized for protein synthesis to only a slight extent, about half of it becoming incorporated into the amide-nitrogen of proteins. The relative inability of such birds to utilize urea nitrogen for protein synthesis is due to the low activity of the main enzymes (urease in the alimentary tract and glutamate dehydrogenase in the liver) permitting the incorporation of urea nitrogen into amino acids. (author)