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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.

Isoperoxidases were synthesized de novo in the embryonic axis of lentil. Nearly 50% decay of m RNA for peroxidase occurred in 3 hr. Detachment of the embryonic axis from the cotyledons caused a 2–3-fold increase in the production of peroxidase and a decrease in the lag period for enzyme synthesis. This increase in enzyme synthesis appeared to be due largely to a sharp rise in the intensity of 5–7 cathodic isoperoxidases as revealed by starch gel electophoresis. Two anodic isoperoxidases appeared with greater intensity in the attached axis.

Respiratory particles, presumably contaminated with chloroplast fragments, isolated from cactus phylloclades, gave tests for the enzymes of the tricarboxylic acid cycle and respiratory chain complexes, without special treatment for membrane disruption. However, detergent treatment resulted in a marked stimulation of many of the activities. The (untreated) particles were capable not only of oxidizing the tricarboxylic acid cycle acids, but also of satisfactorily coupling their oxidation with phosphorylation. Results obtained with inhibitors lent support to the occurrence of enzymes of the tricarboxylic acid cycle and the electron transfer chain. As the tissue aged, the respiratory particles showed a lowered ability to oxidize succinate and, more so, to effect esterification of phosphate; there was also a lowered activity of the enzyme complexes of the respiratory chain.