Effect of photosynthesis on dark mitochondrial respiration in green cells

Green plant cells can generate ATP in both chloroplasts and mitochondria. Hence the effect of photosynthesis on dark mitochondrial respiration can be considered at a variety of levels. Turnover of ceitric acid cycle dehydrogenases, which is essential for supply of carbon skeletons for amino acid synthesis, seems to be largely unaffected during photosynthesis. The source of carbon for the anaplerotic function of the citric acid cycle in light is however, not known with certainty. NADH generated in these reactions is probably not oxidised via the mitochondrial electron transfer chain coupled to ATP synthesis. However, it may be oxidised by the alternative cyanide-insensitive pathway, exported to the cytosol via the oxaloacetate-malate dicarboxylate shuttle or directly utilised for cytosolic nitrate reduction. Oxidation of succinate via cytochrome oxidase may also be similarly inhibited in light. Whether increase in the cytosolic ATP/ADP ratio in light is responsible for the inhibition of mitochondrial electron transfer to O₂ is not clearly established, because the ATP/ADP ratio is reported to be already quite high in the dark. Effective collaboration between photophosphorylation and oxidative phosphorylation in order to maintain the cytosolic energy charge at a present high level is discussed.     

Regulation of NADH supply for nitrate reduction in green plants via photosynthesis and mitochondrial respiration.

The oxidation of NADH by mitochondrial respiration is inhibited by increased adenylate charge produced by photophosphorylation and under these conditions the reducing equivalents are used for nitrate reduction in the cytoplasm. Thus nitrate appears to function as an alternative electron acceptor to O₂ for NADH generated by the citric acid cycle dehydrogenases. This mechanism ensures that nitrate assimilation occurs only in light and thus the accumulation of toxic levels of nitrite in the dark is avoided.     

Inhibitory effects of Bcl-2 on mitochondrial respiration

In contrast to the well-established anti-apoptotic effect of Bcl-2 protein, we have recently demonstrated that Bcl-2 overexpression by vaccinia virus causes apoptosis in BSC-40 cells, while it prevents apoptosis in HeLa G cells. Given the key role of mitochondria in the process of apoptosis, we focused on effects of Bcl-2 expression on mitochondrial energetics of these two cell lines. In this study we present data indicating that BSC-40 cells derive their ATP mainly from oxidative phosphorylation whereas HeLa G cells from glycolysis. More importantly, we show that in both cell lines, Bcl-2 inhibits mitochondrial respiration and causes a decrease of the ATP/ADP ratio. However, it appears that BSC-40 cells cannot sustain this decrease and die, while HeLa G cells survive, being adapted to the low ratio of ATP/ADP maintained by glycolysis. Based on this observation, we propose that the outcome of Bcl-2 expression is determined by the type of cellular ATP synthesis, namely that Bcl-2 causes apoptosis in cells relying on oxidative phosphorylation.     

Inhibitor effects on photosynthesis,respiration and active ion transport inHydrodictyon africanum

Summary The effects of various inhibitors on photosynthesis, respiration, and active influx of K and Cl in light and dark inHydrodictyon africanum is reported. The inhibitors used were arsenate (uncouples electron-transport phosphorylations), dicyclohexylcarbodiimide (energy-transfer inhibitor in electron-transport phosphorylation), quinacrine (uncouples photophosphorylation and inhibits oxidative phosphorylation), and ethionine (traps adenylates as S-adenosyl ethionine). The action of these inhibitors, and of those previously used onHydrodictyon africanum, suggests that K influx requires ATP, while Cl influx requires some earlier manifestation of the ATP synthesizing process. Possible reasons for the greater sensitivity of K influx than of CO₂ fixation to treatments which interfere with photophosphorylation are discussed.     

The effects of drought stress on respiration of isolated corn mitochondria

Mitochondria were isolated from etiolated corn shoots (Zea mays L.) that were stressed to a measured water potential. The rates of mitochondrial respiration in state III, state IV, and without phosphate or ADP on a milligram protein basis decreased as water stress increased with succinate, malatepyruvate, or reduced nicotinamide adenine dinucleotide as substrates. Coupling (as determined by respiratory control and ADP/O ratios) did not decrease with increasing water stress. At water potentials greater than −35 bars all respiration had ceased.     

The impairing effects of chaetochromin D on mitochondrial respiration and structure

Chaetochromin D, a toxic secondary metabolite ofChaetomlum graclle, was examined for impairing effects on mitochondrial respiration and structure (swelling-induction) using isolated rat liver mitochondria to gain Insight into the molecular mechanism for Its cytotoxicity. Chaetochromin D exerted similar mode of effects to those of chaetochromin A, cephalochromin, and ustilaginoldin A on mitochondrial reactions, causing uncoupling of oxidative phosphorylation, depression of state 3 respiration, and induction of drastic swelling In mitochondria. Chaetochromin D induced the same style of swelling as that induced by chaetochromin A, being characterized by a very high rate and small amplitude of swelling. The swelling terminated In the middle and the amplitude was about half of the full swelling. Once the quick swelling ceased in the middle, subsequent swelling could not be elicited by the second addition of chaetochromin D at any of the concentrations tested.     

Nitric oxide and mitochondrial respiration.

Nitric oxide (NO) and its derivative peroxynitrite (ONOO-) inhibit mitochondrial respiration by distinct mechanisms. Low (nanomolar) concentrations of NO specifically inhibit cytochrome oxidase in competition with oxygen, and this inhibition is fully reversible when NO is removed. Higher concentrations of NO can inhibit the other respiratory chain complexes, probably by nitrosylating or oxidising protein thiols and removing iron from the iron-sulphur centres. Peroxynitrite causes irreversible inhibition of mitochondrial respiration and damage to a variety of mitochondrial components via oxidising reactions. Thus peroxynitrite inhibits or damages mitochondrial complexes I, II, IV and V, aconitase, creatine kinase, the mitochondrial membrane, mitochondrial DNA, superoxide dismutase, and induces mitochondrial swelling, depolarisation, calcium release and permeability transition. The NO inhibition of cytochrome oxidase may be involved in the physiological regulation of respiration rate, as indicated by the finding that isolated cells producing NO can regulate cellular respiration by this means, and the finding that inhibition of NO synthase in vivo causes a stimulation of tissue and whole body oxygen consumption. The recent finding that mitochondria may contain a NO synthase and can produce significant amounts of NO to regulate their own respiration also suggests this regulation may be important for physiological regulation of energy metabolism. However, definitive evidence that NO regulation of mitochondrial respiration occurs in vivo is still missing, and interpretation is complicated by the fact that NO appears to affect tissue respiration by cGMP-dependent mechanisms. The NO inhibition of cytochrome oxidase may also be involved in the cytotoxicity of NO, and may cause increased oxygen radical production by mitochondria, which may in turn lead to the generation of peroxynitrite. Mitochondrial damage by peroxynitrite may mediate the cytotoxicity of NO, and may be involved in a variety of pathologies.     

Na+ effects on mitochondrial respiration and oxidative phosphorylation in diabetic hearts

Intracellular Na+ is approximately two times higher in diabetic cardiomyocytes than in control. We hypothesized that the increase in Na+i activates the mitochondrial membrane Na+/Ca2+ exchanger, which leads to loss of intramitochondrial Ca2+, with a subsequent alteration (generally depression) in bioenergetic function. To further evaluate this hypothesis, mitochondria were isolated from hearts of control and streptozotocin-induced (4 weeks) diabetic rats. Respiratory function and ATP synthesis were studied using routine polarography and 31P-NMR methods, respectively. While addition of Na+ (1-10 mM) decreased State 3 respiration and rate of oxidative phosphorylation in both diabetic and control mitochondria, the decreases were significantly greater for diabetic than for control. The Na+ effect was reversed by providing different levels of extramitochondrial Ca2+ (larger Ca2+ levels were needed to reverse the Na+ depressant effect in diabetes mellitus than in control) and by inhibiting the Na+/Ca2+ exchanger function with diltiazem (a specific blocker of Na+/Ca2+ exchange that prevents Ca2+ from leaving the mitochondrial matrix). On the other hand, the Na+ depressant effect was enhanced by Ruthenium Red (RR, a blocker of mitochondrial Ca2+ uptake, which decreases intramitochondrial Ca2+). The RR effect on Na+ depression of mitochondrial bioenergetic function was larger in diabetic than control. These findings suggest that intramitochondrial Ca2+ levels could be lower in diabetic than control and that the Na+ depressant effect has some relation to lowered intramitochondrial Ca2+. Conjoint experiments with 31P-NMR in isolated superfused mitochondria embedded in agarose beads showed that Na+ (3-30 mM) led to significantly decreased ATP levels in diabetic rats, but produced smaller changes in control. These data support our hypothesis that in diabetic cardiomyocytes, increased Na+ leads to abnormalities of oxidative processes and subsequent decrease in ATP levels, and that these changes are related to Na+ induced depletion of intramitochondrial Ca2+.     

Measurements of photosynthesis and respiration in plants

Methods for measuring the rates of photosynthesis and respiration in plants are reviewed. Closed systems that involve manometric techniques, ¹⁴CO₂ fixation, O₂ electrodes and other methods for measuring dissolved and gas phase O₂ are described. These methods typically provide time-integrated rate measurements, and limitations to their use are discussed. Open gas exchange systems that use infra-red CO₂ gas analysers and differential O₂ analysers for measuring instantaneous rates of CO₂ and O₂ exchange are described. Important features of the analysers, design features of gas exchange systems, and sources of potential error are considered. The analysis of chlorophyll fluorescence parameters for estimating the quantum yield for O₂ evolution and CO₂ fixation is described in relation to new fluorescence imaging systems for large scale screening of photosynthetic phenotypes, and the microimaging of individual chloroplasts.     

Combined effects of cadmium and ozone on photosynthesis of Lycopersicon esculentum

Tomato (Lycopersicon esculentum Mill. cv. Pearson) plants were grown in growth chambers for 25 days with cadmium (Cd) and then exposed briefly to ozone (O₃). Gas exchange, chlorophyll a fluorescence, and pigment composition were analysed in leaves at the end of the treatment to assess the effects of a single pollutant and their combination on photosynthesis. The CO₂ assimilation rate was dramatically reduced in plants subjected to the combined treatment, while the single effect of Cd appeared less severe than that of O₃. The decline of CO₂ photoassimilation found in all O₃-exposed plants was attributed to both stomatal and nonstomatal limitations. Tomato plants seemed to detoxify Cd to a great extent, but this resulted in growth suppression. In response to O₃ exposure, the plants protected their photosystems by heat dissipation of excess energy via the xanthophyll cycle. Cd combined with O₃ affected adversely this cycle resulting in an increase in photosynthetic performance under the same experimental light conditions.     

The effects of some inhibitors on the rates of photosynthesis and respiration by Chlorella

The effects of several inhibitors on the rates of photosynthesis and of respiration by Chlorella pyrenoidosa have been studied. The inhibitors used in this study include potassium cyanide, hydroxylamine hydrogen chloride, dinitrophenol,and sodium azide. Each inhibitor seems to have its own characteristic action in photosynthesis and in respiration. With the exception of hydroxylamine, all the inhibitors show a stimulation of respiration at low concentrations of the inhibitors. Only dinitrophenol inhibits respiration to a marked extent. Potassium cyanide, hydroxylamine, and dinitrophenol are about equally effective in inhibiting photosynthesis, but sodium azide is nearly ten times as effective.     

Exercise and ovarian steroid hormones: their effects on mitochondrial respiration

The effect of exercise on mitochondria respiration was studied in gastrocnemius muscle of ovariectomized rats, pseudopregnant rats, and estrous rats. The estrous cycles were followed by vaginal smears. Rats were made pseudopregnant (PSP) by 45 s cervical stimulation with a glass rod on the day of estrous. The treadmill protocol (21 m/min, 10 grade uphill) induced a significant decrease in state 3 oxygen consumption (oxidative phosphorylation) in estrous (0.26 +/- 0.02 vs. 0.49 +/- 0.05 microatoms O min(-1) mg protein(-1)) and ovariectomized rats (0.18 +/- 0.03 vs. 0.40 +/- 0.03 microatoms O min(-1) mg protein(-1)). In contrast, pseudopregnant and progesterone-treated ovariectomized rats did not decrease state 3 nor state 4 respiratory rates. These results show that the effect of exercise on mitochondria respiration does vary according to the hormonal status.     

Cigarette smoke: in vitro effects of condensate fractions on mitochondrial respiration

Various fractions and subfractions (I₁, I₂, I₃, II₁, II₂, and II₃) isolated by the counter current distributions of tobacco smoke condensates (TSC) inhibited the State 3 respiration of rat liver mitochondria. All the fractions except fraction I₁ which contained a large portion of the water soluble components of TSC, stimulated the State 4 respiration. Respiratory control and phosphorylative activity of mitochondria were also impaired upon treatment with TSC fractions. It is concluded that inhibitors and uncouplers of respiratory chain are present in various fractions of tobacco smoke condensate.     

Community photosynthesis and respiration in experimental streams

Changes in relative contribution to total stream photosynthetic and respiratory rates by various community components of an open channel stream were estimated. Rates of photosynthetic production of plankton, benthos and macrophytes (with associated epiphytes) were followed through the growing season and compared with total estimates from a diurnal oxygen technique. Photosynthetic production by macrophytes was extremely high early in the growing season; but later declined and heterotrophic processes became predominant. In contrast, benthos production was initially low but became the primary source of photosynthesis later in the season. Plankton contributed little to stream photosynthesis and respiration.