Membranes containing wildtype seHAS (▴), or the 226,262 (○), 226,281 (▾), 226,367 (▿), 262,281 (▪), 262,367 (□) or 281,367 (•) double Cys-to-Ala mutants of seHAS were treated with 5 mM NEM at 4°C and the HAS activity of samples was determined at the indicated times as described in Methods. Panel B is a blowup of the early incubation times shown in Panel A. Results are the mean ± SEM for three separate experiments (n=3).