A lysis-derived PMSF and orthovanadate oxidant defines a distinct cysteine-engagement profile.

Reactive cysteines serve important functions in proteins, and characterizing their engagement by different electrophiles facilitates biological discovery and covalent drug development. Here, we show that the common lysis buffer components phenylmethylsulfonyl fluoride (PMSF) and orthovanadate generate a lysis-derived oxidant that engages cysteines during cell lysis. This oxidant sulfonylates N-acetyl-D-glucosamine kinase (NAGK) C217, producing a mobility shift on SDS-PAGE. C217 lies within the ATP-binding pocket, and a C217S mutant exhibits reduced ATP affinity and enzymatic activity. Competitive iodoacetamide-alkyne activity-based protein profiling (IAA-ABPP) chemoproteomics further showed that the PMSF/orthovanadate oxidant defines a cysteine-engagement profile that partially differs from that of pervanadate. These findings reveal an unrecognized source of chemical reactivity during protein extraction that expands the toolkit for cysteine-engagement profiling and underscores how sample preparation chemistry shapes chemoproteomic measurements.