Increase in chloroplastic thiol groups by SO2 and its effect on light modulation of NADP-dependent glyceraldehyde 3-phosphate dehydrogenase.

In broken spinach chloroplasts the total amount of thiol groups is about 3.7 μmol mg(-1) chlorophyll. Two thirds are represented by the masked form (which is only titratable after unfolding of the protein). Of the free groups, those reacting with NBD·Cl (1.2-2.0 μmol mg(-1) chlorophyll) seem to be undergoing oxidation more readily than those reacting with DTNB (1.0 μmol mg(-1) chlorophyll). SO2 application causes a maximal increase of 25% in free thiols, and doubles the amount of the masked thiols. The light triggered increase in SH, which starts at an elevated level, runs parallel to that of the controls. SO2 application of 1.8 mg m(-3) (=28 nmol l(-1)) for 1 h does not affect the dark level of NADP-GPD but enhances the light modulation by increasing the ratio of activation. This enhancement is explained by an increase in masked thiol groups during the preceding fumigation period.