The Ca(2+)-Regulation of the Mitochondrial External NADPH Dehydrogenase in Plants Is Controlled by Cytosolic pH

NADPH is a key reductant carrier that maintains internal redox and antioxidant status, and that links biosynthetic, catabolic and signalling pathways. Plants have a mitochondrial external NADPH oxidation pathway, which depends on Ca(2+) and pH in vitro, but concentrations of Ca(2+) needed are not known. We have determined the K(0.5)(Ca(2+)) of the external NADPH dehydrogenase from Solanum tuberosum mitochondria and membranes of E. coli expressing Arabidopsis thaliana NDB1 over the physiological pH range using O(2) and decylubiquinone as electron acceptors. The K(0.5)(Ca(2+)) of NADPH oxidation was generally higher than for NADH oxidation, and unlike the latter, it depended on pH. At pH 7.5, K(0.5)(Ca(2+)) for NADPH oxidation was high (≈100 μM), yet 20-fold lower K(0.5)(Ca(2+)) values were determined at pH 6.8. Lower K(0.5)(Ca(2+)) values were observed with decylubiquinone than with O(2) as terminal electron acceptor. NADPH oxidation responded to changes in Ca(2+) concentrations more rapidly than NADH oxidation did. Thus, cytosolic acidification is an important activator of external NADPH oxidation, by decreasing the Ca(2+)-requirements for NDB1. The results are discussed in relation to the present knowledge on how whole cell NADPH redox homeostasis is affected in plants modified for the NDB1 gene.