A pilot study for the intrinsic labeling of egg proteins with 15N and 13C

The aim of this study was to produce intrinsically and uniformly doubly N-15-C-13-labeled proteins. These proteins can be used as intrinsic tracers of dietary amino acids, both alpha-amino groups and carbon skeletons, during postprandial metabolic utilization. Two (Rhodes) laying hens were fed for 16 days with a standard poultry diet supplemented with 0, 0.2% or 0.4% of a mixture of 20 doubly N-15-C-13-labeled AAs. A third hen was given a non-enriched diet, as the control. The eggs laid were collected over 24days, from 3 days before to 4 days after supplementation. The N-15 and C-13 enrichments in proteins from white and yolk were measured by EA-IRMS and GC-C-IRMS for enrichment in individual amino acids. After 10 days of supplementation, the N-15 enrichment reached an isotopic plateau at 1500 to 3000 parts per thousand, depending on the supplementation level, in both white and yolk while the C-13 enrichment was 220 to 650 parts per thousand in white and was 100 to 250 parts per thousand in yolk. The N-15 enrichment was similar among the amino acids, except for the aromatic ones in which the enrichment was lower. The delta C-13 values were variable among amino acids in both white and yolk, ranging from 77 parts per thousand for tyrosine to 555 parts per thousand for proline with the 0.2 % supplementation level. In conclusion, the incorporation of 0.2 % labeled amino acids in the hen diet allowed us to achieve sufficient enrichment for metabolic studies. However, due to the non-homogeneity of the C-13 labeling, adequate C-13 enrichment of individual amino acids must be considered depending on the investigated metabolic pathway. Copyright (C) 2011 John Wiley & Sons, Ltd.