REGULATION OF CORN LEAF NADP-MALATE DEHYDROGENASE LIGHT-ACTIVATION BY THE PHOTOSYNTHETIC ELECTRON FLOW - EFFECT OF PHOTOINHIBITION STUDIED IN A RECONSTITUTED SYSTEM

The light-activation of the chloroplastic enzyme NADP-malate dehydrogenase is totally inhibited when chloroplasts are preilluminated with high light. The mechanism of this photoinhibition has been studied in a reconstituted chloroplast system composed of isolated thylakoids and the purified proteins of the ferredoxin-thioredoxin light activation system, by examinating the reduction state of the disulfide bridges located on the different proteins of the system and considered to be involved in the light activation process. The results indicate that, when the reduction of S-S groups on thioredoxin and on NADP-MDH is only partially decreased, NADP-MDH is totally inactive. A reduction vs. activity curve shows that the activity of this last enzyme strongly depends on its reduction state, more than 50% reduction being required for activity to appear. The physiological significance of these observations is discussed. © 1990.