The role of NADP in the mitochondrial matrix

Many diverse metabolic processes are coupled to the turnover of the coenzyme NADP in the matrix of plant mitochondria. NADPH can be produced via the NADP-specific isocitrate dehydrogenase as well as via enzymes like NAD-malic enzyme, NAD-malate dehydrogenase and Delta(1)-pyrroline-5-carboxylate dehydrogenase. Although not NADP-specific, the latter enzymes can all catalyse the reduction of NADP(+) at appreciable rates. The NADPH produced can be used in folate metabolism, by glutathione reductase for protection against oxidative damage, and by thioredoxin reductase in the (putative) regulation of metabolic pathways via thiol-group reduction. It can also be oxidized by the respiratory chain via a Ca2+-dependent NADPH dehydrogenase - this is a potential way of regulating the NADP reduction level in the matrix and thus, indirectly, the other processes. It is now possible to present an integrated picture of NADP turnover inside the mitochondrion.