GAPDH activity in lysates was measured using a colorimetric assay for NAD + reduction in the presence of substrate. Treating HEK293T cells with 1 mM propachlor for 30 minutes decreased GAPDH activity by 25% (Figure ). This decrease is consistent with the amount of C152 adducts that we detect by mass spectrometry, but low considering the strong negative cooperativity between the two catalytic sites on the GAPDH tetramer 113 . Our proteomic characterization of propachlor-induced aggregation found an increase in GAPDH aggregation induced by propachlor treatment (FC in aggregates = 4.9, q-value = 0.008; Figure ). Similar results were obtained from Western Blot analysis assessing the levels of GAPDH in the pellet fraction after ultracentrifugation (Figure ), however there is no significant depletion of total GAPDH (Figure ). From this we can conclude that although GAPDH destabilization following propachlor treatment does lead to an increase in the aggregated fraction, the total burden of GAPDH aggregation on the cell remains small compared to the high levels of the soluble protein.