The regulatory isoform of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a light-activated enzyme constituted by subunits GapA and GapB. The NADPH-dependent activity of regulatory GAPDH from spinach chloroplasts was affected by the redox potential (E(m,7.9), -353 +/- 11 mV) through the action of thioredoxin f. The redox dependence of recombinant GapB (E(m,7.9), -347 +/- 9 mV) was similar to native GAPDH, whereas GapA was essentially redox-insensitive. GapB mutants having one or two C-terminal cysteines mutated into serines (C358S, C349S, C349S/C358S) were less redox-sensitive than GapB. Different mutants with other cysteines substituted by serines (C18S, C274S, C285S) still showed strong redox regulation. Fully active GapB was a tetramer of B-subunits, and, when incubated with NAD, it associated to a high molecular weight oligomer showing low NADPH-dependent activity. The C-terminal GapB mutants (C358S, C349S, C349S/C358S) were active tetramers unable to aggregate to higher oligomers in the presence of NAD, whereas other mutants (C18S, C274S, C285S) again behaved like GapB. We conclude that a regulatory disulfide, between Cys-349 and Cys-358 of the C-terminal extension of GapB, does form in the presence of oxidized thioredoxin. This covalent modification is required for the NAD-dependent association into higher oligomers and inhibition of the NADPH-activity. By leading to GAPDH autoinhibition, thioredoxin and NAD may thus concur to the dark inactivation of the enzyme in vivo.