Figure 3.

Example of protein turnover calculation. A, Top row, the isotopic envelope represented as isotopic ratios for the peptide ATLAQLGYEKLDDIIGR from Fd-GOGAT (MLOC_70866.1), at 24, 48, 72, and 96 h, with all isotopes normalized to the monoisotopic value. Bottom row, as determined by nonnegative least squares (NNLS), the NA form of the peptide, containing zero labeled ¹⁵N atoms, and the H form of the peptide, containing one or more ¹⁵N atoms. The RIA can then be measured as NA/(NA + H). B, The inverse of the natural log (−LN) of RIA for all time points for the protein was used to determine a degradation rate for the protein. C, Average ¹⁵N enrichment of labeled peptide populations at different time points through the course of the experiment.