Hens Produce Artificially Enriched super( 13)C Egg Proteins for Metabolic Tracer Studies

Clinicians and researchers studying protein metabolism in vivo, typically use isotopically-labeled free amino acids as metabolic tracers rather than isotopically-labeled proteins, because such proteins are commercially unavailable. However, the use of free amino acids in lieu of protein tracers violates the critical assumption that tracer molecules undergo the identical biochemical reactions as the tracee molecules of interest. To address this problem, the authors have synthesized super( 13)C-labeled proteins using egg laying hens and investigated the relationship between tracer dose and method of delivery on super( 13)C-protein production. The time required for the super( 13)C-enrichment in eggs to return to background levels at the end of the enrichment period was about twice that required to initially reach isotopic equilibrium with the diet, indicating significant biochemical discrimination of endogenous super( 13)C amino acids. Delivering small amounts of super( 13)C amino acid tracers in the drinking water of hens is the most effective way to produce super( 13)C-enriched proteins to for tracer studies that do not require delta super( 13)C-enrichment above 200%.