Activity of Respiratory Pathways in Cultured Yam Cells under the Influence of Furostanol Glycosides

The role of furostanol glycosides (FGs) in the intensification of main metabolic processes in cultured yam (Dioscorea deltoidea Wall.) cells was shown. The effects of FGs were associated with changes in the complicated network of intracellular enzymatic reactions. This was evidenced by an increase in the activities of the enzymes from the glucose glycolytic oxidation pathway (hexokinase and fructose-1,6-difosphate aldolase) and the malate dehydrogenase complex (oxidoreductive—NADH/NAD-malate dehydrogenase and malic enzyme—decarboxylating NADP-malate dehydrogenase) as well as the stimulation of mitochondrial respiration accompanied by changes in activity of the alternative respiratory pathway. At the lag-phase of the growth cycle, the cell treatment with FGs was capable to guide glucose conversion through the glycolytic pathway. This was witnessed by a decrease in the activity of glucose-6-phosphate dehydrogenase and increase in that of hexokinase. Inhibition of NADPH oxidase afforded by diphenyliodonium chloride resulted in a decrease in activities of the key enzymes of glycolysis and the pentose phosphate cycle of glucose oxidation. Simultaneously, the aldolase activity increased and raised the content of its product glyceraldehyde 3-phosphate. The effect may explain the earlier-observed activation of the aldehyde-utilizing enzymes diminishing the initial POL level. The modulating action of FGs on activities of the enzymes of sugar glycolytic oxidation was shown. This was expressed as the activation of the tested enzymes at the lag- and exponential phases of the cellular growth and as the absence of the FG effect at the stationary phase. The short-term exposure of yam cells to FGs intensified the total mitochondrial respiration. An inhibitory analysis revealed the dominance of the cytochrome respiration pathway at the lag-phase, while its contribution became considerably lower at the later growth phases. FGs did not affect the rate of this respiration path but increased the potential activity of alternative oxidase (AO) during the lag- and exponential phases of the growth cycle. The FG-induced activation of AO, which may be intended to maintain a balance between carbohydrate metabolism and the rate of electron transport, is discussed.