Facile sulfitolysis of the disulfide bonds in oxidized thioredoxin and glutaredoxin

Thioredoxins and glutaredoxins, in their oxidized form, possess a single disulfide bridge located on an edge of the small compact molecules. In contrast to most other disulfide‐containing proteins, this S‐S bridge is cleaved by millimolar concentrations of sulfite in the absence of protein denaturing agents at pH 7–8 and ambient temperature; however, the reaction is not quantitative. Sulfitolysis of Escherichia coli thioredoxin was found to be associated with an increase in fluorescence at 345 nm. A comparative study of sulfitolysis in 12 different thioredoxins and glutaredoxins of bacterial and plant origin has been made. Although they are all thought to be highly conserved in three‐dimensional structure, their reactivities towards sulfite and the effects of 6 M guanidinium chloride (not affecting, or enhancing sulfitolysis) very strongly in the series, with E. coli thioredoxin being less reactive and plant thioredoxins and E. coli glutaredoxin being more susceptible molecules. Contrary to expectation, reaction with sulfite is not generally correlated with the presence of negatively or positively charged amino acid residues near the disulfide loop but is determined by individual sequence and surface features in every single protein. These results confirm our hypothesis that thioredoxin sulfitolysis and inactivation [Würfel, M., Häberlein, I., Follmann, H. (1990) FEBS Lett. 268, 146–148] can occur in plant cells under physiological conditions and provide a biochemical rationale for the phytotoxicity of SO2.