Scheibe, R, 1987, NADP*-malate dehydrogenase in Crplants: Regulation and roie of a light-activated enzmye. Plant, The thioredoxin-dependent light/dark modulation system of the chloroplast is described as a prerequisite enabling the flexible control of fluxes through the various parts of the CO2-fixation pathway. Both the rapid turnover of the reduced thiolcontaining form of the respective target enzyme, and the metabolite effect upon the reductive enzyme modolation, allow rapid adjustment of the amoimt of active species to the aaual requirements. The structural basis of the regulation of chloroplast NADP+-maiate dehydrogenase (EC 1,1,1,82) is described in more detail. The moduiable piastid enzyme is characterized by two sequence extensions not present in any other knowa NADP*-and/or NAD*-specific malate dehydrogenase. The NADP*malate dehydrogenase of Cj -plants is part of the "malate valve", which catalyzes the export of reducing equivalents in the form of malate from the chloroplast only when the NADPH to NADP* ratio is high, thus poising the NADPH to ATP ratio required for optimal carbon reduction in the light. The mode of regulation of other light/dark modulated enzymes is discussed.