Improved Method to Determine Protein Turnover Rates with Heavy Water Labeling by Mass Isotopomer Ratio Selection

The synthesis and degradation rates of proteins form an essential component of gene expression control. Heavy water labeling has been used in conjunction with mass spectrometry to measure protein turnover rates, but the optimal analytical approaches to derive turnover rates from the mass isotopomer patterns of deuterium-labeled peptides continue to be a subject of research. Here, we describe a method that comprises (1) a nearest lookup of numerically approximated peptide isotope envelopes, coupled to (2) the selection of optimal mass isotopomer pairs based on peptide sequence rules, to calculate the molar fraction of new peptide synthesis in heavy water labeling mass spectrometry experiments. We validated our approach using an experimental calibration standard comprising mixtures of fully unlabeled and fully labeled proteomes. We then reanalyzed 17 proteome-wide turnover experiments from four mouse organs across multiple data sets and showed that the combined nearest-lookup and rule-based mass isotopomer ratio selection method increases the coverage of well-fitted peptides in protein turnover experiments by up to 58 ± 13%. The workflow is implemented in the Riana software tool for protein turnover analysis and may avail ongoing efforts to study the synthesis and degradation kinetics of proteins in animals on a proteome-wide scale.