Diurnal variation in the activities of isocitrate and glucose-6-phosphate dehydrogenases in cactus phylloclades

Abstract

Remarkable diurnal variations in cactus phylloclades have been reported in the activities of various enzymes 1-5, protein and citric acid content 3. In view of the crassulacean type acid metabolism and succulent nature of the cactus phylloclades, rhythmic variation in enzymic activities could provide important correlations with the metabolism in this plant. Materials and methods. Details of sampling and homogenization procedures were those described earlier 3. The activities of isocitrate and glucose-6-phosphate dehydrogenases were estimated as described by SCOTT et al3 at limiting concentrations of enzyme. The reaction was run for 5 min at 30°C and terminated by the addition of ethanol.and sodium sulfate 8. A unit of enzyme caused the reduction of 1 ~tmole NADP per min under the assay conditions. For the best evaluation of diurnal variation, the activities were expressed on the basis of fresh weight of the tissue*.Results. Diurnal activities of isocitrate and glucose-6-phosphate dehydrogenases are described in the Table. The maximum activity of isocitrate dehydrogenase was at 16.00h and the minimum at 08.00 h, the former being 200% higher. In the case of glucose-6-phosphate dehydrogenase, maximum activity was, however, at 08.00 h and the minimum at midnight with a variation of 300%In order to test whether the spectrophotometric estimations of these dehydrogenases were affected by the presence of NADPH oxidizing system, the reduced coenzyme was incubated with cactus enzyme, No oxidizing system was found to be operative under the test conditions.Inspite of the fact that, in sampling, 15-24 phylloclades were taken at each operation, the biological variation and the statistical reproducibility was ascertained by making 5 separate estimations. In each case the pattern of variation was reproducible; there were little difference in the magnitude of variation. Statistical analysis of the results was found to be highly significant (p < 001)To evaluate the effect of some endogenous metabolic effectors, the activities were estimated on mixed homogenates. Enzyme preparations of maximum and minimum activity were mixed equally and simultaneous estimations on mixed and individual preparations were made. It was observed that the activities of both the enzymes were inhibited on mixing. The magnitude of inhibition was respectively 15 and 20% for isocitrate and glucose-6-phosphate dehydrogenases.Discussion. Like other enzymes of the Krebs cycle s, isocitrate dehydrogenase also showed maximum activity in the day time (16.00 h). Diurnal variation in organic acid content is known for malate and citrate pools in succulents 7. The diurnal variation in the activity of isocitrate dehydrogenase suggests its bearing on the acid metabolism at the citrate pool. The diurnal variation observed in glucose-6-phosphate dehydrogenase, a key enzyme of pentose phosphate pathway, is of considerable significance. RICHTER and PIRSON s found no rhythm in glucose-6-phosphate dehydrogenase activity in alternating light and darkness in Hydrodictyon. Accor...

Circadian Rhythmicity in the Activities of Phenylalanine Ammonia-Lyase from Lemna perpusilla and Spirodela polyrhiza

“…Among the more extensively investigated examples of oscillatory or rhythmic behavior at the metabolic level are the enzymic reactions of glycolysis in yeasts (24, 26). There are examples of soluble enzyme systems in plants which exhibit rhythmicity (17,19,25), and a recent review of biological rhythms and physiological timing in plants (16) suggests that rhythms are common in plant metabolism.In the Lemnaceae, rhythmic behavior of respiratory gas exchange has received considerable attention. Uptake of 02 by…”