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.