Site-directed mutagenesis was utilized to produce the cys to ser mutant GRx1 enzymes, as described in Methods. Each enzyme (3–5 μM) was incubated with 1 mM sporidesmin for 20 min in 0.1 M K phosphate, pH 7.5, 10% ethanol at 30 °C. GRx1 activity was then measured using cys-SSG substrate. Each point represents the mean value, plus and minus the standard error, for at least three determinations. Where error bars are not evident they are within the size of the symbol.