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Metabolism and Enzymology

Regulation of Nonphosphorylating Electron Transport Pathways in Soybean Cotyledon Mitochondria and Its Implications for Fat Metabolism ¹

Department of Botany Australian National University, Canberra ACT, 2601, Australia, Department of Biochemistry, University of Sussex, Falmer, Brighton BN1 9QG, U.K., Department of Botany, University of Adelaide, Adelaide SA, 5001, Australia

The respiration of mitochondria isolated from germinating soybean cotyledons was strongly resistant to antimycin and KCN. This oxygen uptake was not related to lipoxygenase which was not detectable in purified mitochondria. The antimycin-resistant rate of O₂ uptake was greatest with succinate as substrate and least with exogenous NADH. Succinate was the only single substrate whose oxidation was inhibited by salicyl hydroxamic acid alone, indicating engagement of the alternative oxidase. Concurrent oxidation of two or three substrates led to greater involvement of the alternative oxidase. Despite substantial rotenone-resistant O₂ uptake with NAD-linked substrates, respiratory control was observed in the presence of antimycin, indicating restriction of electron flow through complex I. Addition of succinate to mitochondria oxidizing NAD-linked substrates in state four stimulated O₂ uptake substantially, largely by engaging the alternative oxidase. We suggest that these properties of soybean cotyledon mitochondria would enable succinate received from the glyoxysome during lipid metabolism to be rapidly oxidized, even under a high cytosolic energy charge.

¹ Supported by grants from the Australian Research Grants Scheme (J. T. W. and D. A. D.) and the ANU Faculties Research Fund (D. A. D.).

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