Involvement of matrix NADP turnover in the oxidation of NAD-linked substrates by pea leaf mitochondria

The involvement of the internal rotenone-insensitive NADPH dehydrogenase on the inner surface of the inner mitochondrial membrane [ND_in(NADPH)] in the oxidation of strictly NAD⁺-linked substrates by pea ( Pisum sativum L.) leaf mitochondria was measured. As estimated by the inhibition caused by 5 μ M diphenyleneiodonium (DPI) in the presence of rotenone to inhibit complex I, the activity of ND_in(NADPH) during glycine oxidation (measured both as O₂ uptake and as CO₂ release) was 40–50 nmol mg⁻¹ protein min⁻¹. No significant activity of ND_in(NADPH) could be detected during the oxidation of 2-oxoglutarate, another strictly NAD⁺-linked substrate; this was possibly due to its relatively low oxidation rate. Control experiments showed that, even at 125 μ M , DPI had no effect on the activity of glycine decarboxylase complex (GDC) and lipoamide dehydrogenase. The relative activity of complex I, ND_in(NADPH), and ND_in(NADH) during glycine oxidation, estimated using rotenone and DPI, differed depending on the pyridine nucleotide supply in the mitochondrial matrix. This was shown by loading the mitochondria with NAD⁺ and NADP⁺, both of which were taken up by the organelle. We conclude that the involvement of NADP turnover during glycine oxidation is not due to the direct production of NADPH by GDC but is an indirect result of this process. It probably occurs via the interconversion of NADH to NADPH by the two non-energy-linked transhydrogenase activities recently identified in plant mitochondria.