M. Tissut

Inhibitory effect of hydroxyflavones on the exogenous nadh dehydrogenase of plant mitochondrial inner membranes

Abstract The inhibitory properties of a series of 21 flavones were investigated on the uncoupled electron transfer of purified potato tuber mitochondria, with different substrates. Thirteen compounds were shown to be selective inhibitors of the external NADH dehydrogenase of the inner membrane. Among mono- and dihydroxyflavones, some good inhibitors were found, but their efficiency depended greatly on the position of hydroxylation (1 50 between 20 and 150 μM). For example, 8-hydroxyflavone was six-fold more effective than 5-hydroxyflavone. Except in the case of norwogonin, the aglycones tested were ineffective when they were substituted by more than two hydroxyls, without other substituents. This was the case for quercetin, for example. The best inhibitor of this series was platanetin (I 50 = 2 μM). This very lipophilic flavonol (-log P = 5.10) was characterized by the presence of a dimethylallyl chain at the 6-position. When this substituent was replaced by a methyl, (platanin), the effectiveness of the product remained high, but its selectivity was lowered. These results suggest that the value of the lipophilicity parameter (-log P), for the molecules able to attain this target must be between 3.7 and 5.1. However, the lipophilicity parameter on its own does not explain why some flavones act selectively on the external NADH dehydrogenase, probably at the level where the electrons are donated to the quinone pool.

Uncoupling activities of chalcones and dihydrochalcones on isolated mitochondria from potato tubers and mung bean hypocotyls

Abstract The uncoupling properties of 23 chalcones and dihydrochalcones were studied. Twelve compounds completely uncouple oxidative phosphorylation in mung bean hypocotyl and potato tuber mitochondria, four are weak uncouplers and seven are without effect. Usually, mung bean mitochondria are more sensitive to uncoupling agents than potato mitochondria. The uncoupling activity of chalcones and dihydrochalcones appears to be connected with the presence of hydrogen or hydroxyl groups in the 2′-position and hydrogen, hydroxyl or nitrate groups in the 4′-position. The α-β-unsaturated carbonyl system is not essential for activity. For the compounds which are not very lipophilic, substituents on the B-ring are without effect on the uncoupling properties. Phloretin appears to be an active uncoupler; its 6′-glucoside is without effect.

Physiological actions of dinoterb, a phenol derivative. 1. Physiological effects on the whole plant and on tissue fragments of pea.

Abstract The herbicide dinoterb is a tert. -butyl-2-dinitro-4,6-phenol. Its effects on the metabolism (water, dry matter, nitrogen, chlorophyll, and flavonol contents) were studied on pea plants ( Pisum sativum L.). Concurrently, photosynthesis and respiration intensities of treated plants or tissues were measured (O 2 emission or uptake measured on leaf fragments in the reaction vessel of a Clark-type electrode system). Dinoterb, which is an inhibitor of photosynthesis of isolated, physiologically active chloroplasts, also appeared to rapidly inhibit photosynthesis in the whole plant. This property was used for an indirect method of analysis of dinoterb movement in the leaf and in the plant. Dinoterb appears to have a complex mode of action: low concentrations of the herbicide, rapidly appearing in the whole treated leaf, inhibited photosynthesis, uncoupled oxidative phosphorylations, and began to inhibit respiratory oxygen consumption. High concentrations of dinoterb were responsible for important necrosis some days after treatment and we could show, by analysis of the flavonolic accumulation, that cells of the upper epidermis seemed to be first affected.

Effects of herbicidal carbamates on mitochondria and chloroplasts

Abstract At concentrations near 2 × 10 −4 M , barban, chlorpropham, and phenmedipham are inhibitors of the electron transfer in potato and mung bean mitochondria. The inhibition seems to be localized in the flavoprotein region. It affects preferentially the exogenous NADH dehydrogenation, in potato mitochondria (I 50 , 10 −4 M ). Succinate dehydrogenation is less inhibited. At noninhibiting concentrations, the studied carbamates cannot uncouple the oxidative phosphorylations. Photosynthesis is completely inhibited by 2.10 −7 M phenmedipham, 5 × 10 −5 M barban, and 2 × 10 −4 M chlorpropham. The inhibition takes place at the PS II level. Moreover, barban and chlorpropham are uncouplers of the photophosphorylations for concentrations between 5 × 10 −5 and 5 × 10 −4 M . The effects observed on mitochondrial respiration can also be found on respiration of Acer cultured cells. The effects on isolated chloroplast photosynthesis are also observed for slightly higher concentrations on cultured Chlorella and on pea and oat leaf fragments.

Physiological actions of dinoterb, a phenol derivative. 2. Effects on isolated plant mitochondria and chloroplasts.

Abstract Dinoterb, a contact herbicide, affects respiration and photosynthesis of mitochondria and chloroplasts. On mitochondria, at low concentrations, it acts as an uncoupler of oxidative phosphorylation; at higher concentrations, it inhibits the electron transport chains, probably before cytochrome c . On chloroplasts, dinoterb has a stimulatory effect on oxygen uptake in the reduced dichlorophenol-indophenol→methyl viologen couple; however, it is also an inhibitor of the Hill reaction and its site of inhibition is located before plastoquinone, near photosystem II.

Specific inhibition of mitosis in cell suspension cultures by a N-phenylcarbamate series

Abstract Acer cell suspension cultures were used as a tool for studying the possible specific inhibitory effect on mitosis of several N-phenylcarbamates. The existence of a specific mitosis inhibition was shown by several criteria: the increase in the number of cells in the culture was blocked, but the cells remained alive and their size was greatly enlarged with a concurrent increase in their dry weight and in their protein content. Among the studied carbamates, carbetamide and propham were true specific mitosis inhibitors in Acer cells: chlorpropham was also a specific inhibitor of mitosis at concentrations between 10 and 50 μM, but induced other phytotoxic effects at higher concentrations. Asulam had little effect on cell growth in Acer suspension cultures, and phenmedipham was highly toxic for these cells, in a way which is not yet understood, but which involves neither mitosis nor the electron transfer through PS II. These results reinforce those obtained when studying plant seedling growth in the presence of a great number of carbamates, which showed the structural features leading to a specific inhibitory effect on mitosis: for this, a N-phenyl-bound carbamic function is needed and the 4-position of the phenyl ring must be free. If the substituents in the 3- or 5-positions of the phenyl ring are too bulky or hydrophilic, the effect is decreased. The structure of the esterifying moiety seems to play a secondary role.

Mitochondrial changes during storage of untreated or CIPC-treated potatoes

Abstract During storage of potato tubers ( Solanum tuberosum L., var. Bintje), important changes appear which affect respiratory control, ADP O , intensity of O 2 consumption in the presence of different substrates, and NAD + dependence, In mitochondria extracted under strictly similar conditions, from patato tubers stored at 4°C, the respiratory control (RC) maintains a value near 4 for 4 to 5 months. It then declines progressively to low values. At 20°C, a stable RC of 4 can be observed for several months, which then decreases at the end of dormancy. Then, the RC increases sharply; at this stage, ADP O are abnormally low, and, some time later, NAD + dependence disappears. Mitochondria treated with 250 μ M chlorpropham show a 50% inhibition of the electron transfer with exogenous NADH as substrate. After tuber treatment with 1% chlorpropham, sprouting is inhibited for several months. The activities of mitochondria extracted from such tubers remain unaffected by the treatment. The use of this phenylcarbamate for potato tuber treatment permits obtaining functional mitochondria from tubers after a slightly longer period of storage.

Uncoupling activities of monensin in isolated mitochondria, chloroplasts and cells

Abstract At 1μM, the carboxylic ionophore monensin is an effective inhibitor of the light-dependent O 2 evolution in spinach class A chloroplasts. This effect is associated with an uncoupling activity which is obtained at the same concentration in isolated spinach thylakoids. This uncoupling effect is dependent on the presence of Na + (1 mM) or K + in the medium. The D 50 value is little affected when changing the concentration of thylakoid membranes from 9 to 109 μg chlorophyll ml −1 medium. In isolated potato mitochondria, in a medium containing Na + or K + , the uncoupling activity of monensin was very low, the full uncoupling effect needing 1 mM to occur. However, in cultured Acer cambium cells, 10 μM induced a full uncoupling effect after a 30 min incubation time. At 10 μM, monensin killed the cultured cells and induced a necrosis effect, after deposition on broad bean leaves. All these effects show that monensin cannot be considered as a selective inhibitor of transfer in Golgi vesicles. The biochemical mode of monensin uncoupling action appears to be complex, and seems not to need the presence of a free carboxyl, as its effects can be compared to those of valinomycin.