The distribution of succinate dehydrogenase and malate dehydrogenase among components of tobacco-leaf extracts

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Abstract
The activities of succinate dehydrogenase, malate dehydrogenase, and aconitate hydratase were measured on fractions of tobacco-leaf ex-tracts prepared by differential centrifuging. The dehydrogenases were also measured in fractions produced when the chloroplast and mitochondrial preparations were resolved by sucrose gradient centrifug-ing into chloroplasts, chloroplast fragments, and mitochondria. All the succinate dehydrogenase of the extracts behaved as if it were attached to mitochondria. It sedimented along with the succinate-oxidase system, and the succinate-dehydrogenase activity was sufficient to account for its expected role in this oxidase system. Most of the malate dehydrogenase was recovered in the supernatant fraction. The small proportions (5-19%) present in the mitochondrial and chloroplast preparations were reduced by washing. More enzyme appeared to be washed out of the chloroplasts and mitochondria as they sedimented in sucrose gradients. Most of the aconitate-hydratase activity of leaf extracts was recovered in the supernatant fraction. The rest occurred in the mitochondrial fraction and could be partially removed by washing. It is suggested that some of the malate dehydrogenase that was recovered in the supernatant fraction originated in chloroplasts and mitochondria, and that some of the aconitate hydratase originated in mitochondria.