NAD(P)H-UBIQUINONE OXIDOREDUCTASES IN PLANT-MITOCHONDRIA

Plant (and fugal) mitochondria contain multiple NAD(P)H dehydrogenases in the inner membrane all of which are connected to the respiratory c hain via ubiquinone. On the outer surface, facing the intermembrane sp ace and the cytoplasm, NADH and NADPH are oxidized by what is probably a single low-molecular-weight, nonproton-pumping, unspecific rotenone -insensitive NAD(P)H dehydrogenase. Exogenous NADH oxidation is comple tely dependent on the presence of free Ca2+ with a K0.5 of about 1 muM . On the inner surface facing the matrix there are two dehydrogenases: (1) the proton-pumping rotenone-sensitive multisubunit Complex I with properties similar to those of Complex I in mammalian and fungal mito chondria. (2) a rotenone-insensitive NAD(P)H dehydrogenase with equal activity with NADH and NADPH and no proton-pumping activity. The NADPH -oxidizing activity of this enzyme is completely dependent on Ca2+ wit h a K0.5 of 3 muM. The enzyme consists of a single subunit of 26 kDa a nd has a native size of 76 kDa, which means that it may form a trimer.