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BIOENERGETICS AND PHOTOSYNTHESIS

Isolated Durum Wheat and Potato Cell Mitochondria Oxidize Externally Added NADH Mostly via the Malate/Oxaloacetate Shuttle with a Rate That Depends on the Carrier-Mediated Transport¹

Dipartimento di Scienze Animali, Vegetali e dell'Ambiente, Facoltà di Agraria, Università del Molise, Via De Sanctis, 86100 Campobasso, Italy (D.P., S.D.P., S.P.)

We investigated whether and how mitochondria from durum wheat (Triticum durum Desf.) and potato (Solanum tuberosum), isolated from etiolated shoots and a cell suspension culture, respectively, oxidize externally added NADH via the mitochondrial shuttles; in particular, we compared the shuttles and the external NADH dehydrogenase (NADH DH_Ext) with respect to their capacity to oxidize external NADH. We found that external NADH and NADPH can be oxidized via two separate DH_Ext, whereas under conditions in which the activities of NAD(P)H DH_Ext are largely prevented, NADH (but not NADPH) is oxidized in the presence of external malate (MAL) and MAL dehydrogenase, in a manner sensitive to several non-penetrant compounds according to the occurrence of the MAL/oxaloacetate (OAA) shuttle. In durum wheat mitochondria and potato cell mitochondria, the rate of NADH oxidation was limited by the rate of a novel carrier, the MAL/OAA antiporter, which is different from other carriers thought to transport OAA across the mitochondrial membrane. No NAD(P)H oxidation occurred arising from the MAL/Aspartate and the -glycerophosphate/dihydroxyacetonphosphate shuttles. We determined the kinetic parameters of the enzymes and the antiporter involved in NADH oxidation, and, on the basis of a kinetic analysis, we showed that, at low physiological NADH concentrations, oxidation via the MAL/OAA shuttle occurred with a higher efficiency than that due to the NADH DH_Ext (about 100- and 10-fold at 1 µM NADH in durum wheat mitochondria and in potato cell mitochondria, respectively). The NADH DH_Ext contribution to NADH oxidation increased with increasing NADH concentration.

² These authors contributed equally to the paper.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.028548.

^* Corresponding author; e-mail passarel{at}unimol.it; fax 39-0-874-404678.

Received June 13, 2003; returned for revision July 15, 2003; accepted August 27, 2003.

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