With the help of mass spectrometry, scientists have discovered new substrates and reaction pathways for a biomedically important protease enzyme (Nat. Chem. Biol., DOI: 10.1038/nchembio.126). The method could be used to understand the biology of proteases that don't yet have defined roles. Roughly 2% of the human genome codes for proteases, which hydrolyze peptide bonds, but many of their roles aren't understood because it's tough to identify their substrates. A team led by Alan Saghatelian of Harvard University has found new details about dipeptidyl peptidase 4 (DPP4), an enzyme that regulates a peptide hormone that lowers blood glucose levels and is the target of several diabetes drugs. The researchers used mass spectrometry and enzyme assays to compare peptide levels in the kidneys of normal mice, mice lacking DPP4, and normal mice treated with a DPP4 inhibitor. In addition to turning up new DPP4 substrates, their results suggest that DPP4 works together with another class of enzymes, the aminopeptidases. Although none of the new DPP4 substrates has a known biological role yet, they could serve as biomarkers for monitoring DPP4 activity.