Protein and amino acid metabolism in laying hens receiving N-15 labeled casein. 7. Incorporation and distribution of labeled nitrogen into egg fractions

Abstract

Four colostomated hybrid laying hens were fed, for 6 days, a semisynthetic ration containing 15N labelled casein as sole protein source. Within this period, the hens laid 13 eggs which were fractionated into egg yolk, egg white and egg shell. The N content of these egg fractions was established. A TCE soluble and TCE precipitable fraction (TCE=trichloroacetic acid) were obtained from the fractions of egg yolk and egg white. Additionally, an analysis was made of the set of amino acids contained in the crude protein of the egg yolk, egg white and egg membrane (lysine, histidine and arginine only). The excess 15N atom% (15N') was established in the crude protein, in the nitrogen from the TCE soluble and TCE precipitable fractions and in the N from 17 amino acids of the egg yold and egg white. In the egg membrane fraction the 15N' atom% was determined as percentage of the total amount of N and of the amount of N contained in the basic amino acids. Essential differences between the 3 egg fractions were noticed with regard to the process of initial incorporation of labelled N and the intensity of this process after the onset of 15N application. 15N' was detected in the egg shell as early as 4 hrs after administration of the 15N' dose. Heavy nitrogen was found to be contained in the egg white of eggs that were laid on the 2nd day of experiment. The level of 15N' atom% in the egg white increased essentially during the following days of experiment; the 15N'quantities in the egg white were considerably higher than those in the egg yolk. Similar results were obtained for the process of 15N incorporation into the 17 amino acids. It is assumed that differences between the 3 fractions as observed in the level of labelling and in the pattern of 15N' incorporation are accounted for by the different sites and different times of protein synthesis for the different egg proteins.