¹⁵N metabolic labeling: evidence for a stable isotope effect on plasma protein levels and peptide chromatographic retention times.

Many quantitative proteomics methods rely on protein and peptide labeling with stable isotopes. We have recently found that the introduction of ¹⁵N into organisms via in vivo metabolic labeling affects protein expression levels as well as metabolic pathways and behavioral phenotypes. Here, we present further evidence for a stable isotope effect based on the plasma proteome analysis of ¹⁵N-labeled mice. We compared plasma proteomes of ¹⁵N-labeled and unlabeled (¹⁴N) mice by quantitative MS. We found a number of protein level differences, some of which were verified immunochemically. In addition, we observed divergent chromatographic retention time and peak full width at half maximum (FWHM) between ¹⁵N-labeled and ¹⁴N tryptic peptides. Our data point toward a systemic effect of the introduction of heavy isotopes in vivo.