Diurnal variations in enzymic activities in subcellular fractions of cactus phylloclades

Abstract

The activities of phosphopyruvate carboxylase (E.C. 4.1.1.31), malic enzyme (malate dehydrogenase (decarboxylating) E.C. 1.1.1.40), selected enzymes of TCA cycle and electron transport chain were determined in mitochondrial and supernatant fractions isolated from phylloclades of the cactus Nopalea dejecta, collected at noon and midnight. There were significant and consistent increases in the activities of mitochondrial and supernatant phosphopyruvate carboxylase, mitochondrial aconitate hydratase (E.C. 4.2.1.3) and soluble malate dehydrogenase (E.C. 1.1.1.37) and decreases in mitochondrial malate dehydrogenase and fumarate hydratase (E.C. 4.2.1.2) and in both mitochondrial and soluble malic enzyme in the midnight samples. Of the respiratory chain enzyme complexes tested, cytochrome c oxidase (E.C. 1.9.3.1) and succinate cytochrome c reductase had higher activities at noon, while reduced NAD-cytochrome c reductase and reduced NAD dehydrogenase (E.C. 1.6.99.3) activities were higher at midnight. The rate of succinate oxidation and coupled phosphorylation also varied, with a maximum at noon. The diurnal pattern of alteration in enzymic activities has a bearing on crassulacean acid fluctuation.