Cysteines of chloroplast NADP-malate dehydrogenase form mixed disulfides

Chloroplast NADP-malate dehydrogenase (NADP-MDH) from pea and from spinach was N-terminally truncated by limited proteolysis with Staphylococcus aureus protease V8. The resulting monomeric enzymes lacking, respectively, the 37 and 38 N-terminal amino acids were inactive. Reduction and addition of low concentrations of guanidine-HCl (50-100 mM) resulted in a highly active enzyme of 850 units per mg protein. Equilibration of the truncated enzyme with various glutathione (GSH) redox buffers and assaying its activity in the presence of guanidine-HCl was used to establish the existence of protein-GSH mixed disulfides. This finding was further confirmed using incorporation of radioactively labelled thiol. The possible function of such cysteine modifications under oxidative stress and their regeneration by the thioredoxin system in the light is discussed.