The propachlor modification is clearly localized to C152 based on MS2 spectra (46 total spectral counts) (Figure ). C152 is in the NAD + binding site, and is necessary for catalytic activity . Across two biological duplicates, we find a 26±7% drop in the unmodified C152 intensity, implying that about a quarter of GAPDH is modified. No evidence for modification at C156 was observed.