peptide metabolism is a complex process involving many proteins working in concert . 
 mass spectrometry ( ms)-based global peptide profiling of mice lacking dipeptidyl peptidase 4 ( dpp4 ) identified endogenous dpp4 substrates and revealed an unrecognized pathway during proline peptide catabolism that interlinks aminopeptidase and dpp4 activities . 
 together , these studies elucidate specific aspects of dpp4-regulated metabolism and , more generally , highlight the utility of global peptide profiling for studying peptide metabolism in vivo .