ERK1/2 cysteines are oxidized to sulfenic acid in response to PDGF in WI-38 fibroblasts. Cells were serum-starved for 18 h, treated with PDGF, and lysed in the presence of DCP-Bio1 as described in Figures 1 and 2. Affinity-captured proteins were immunoblotted for total (A) or phospho-ERK1/2 (B). Lower panels summarize ERK band intensity from multiple experiments (n=4) normalized to AhpC, then to the sample with highest band intensity. *, p<0.05; **, p< 0.01; ****, p<5x10−10