Mitochondrial-driven bicarbonate transport supports photosynthesis in a marine microalga

The CO(2)-concentrating mechanism (CCM) of the marine eustigmatophycean microalga Nannochloropsis gaditana consists of an active HCO(3)(-) transport system and an internal carbonic anhydrase to facilitate accumulation and conversion of HCO(3)(-) to CO(2) for photosynthetic fixation. Aqueous inlet mass spectrometry revealed that a portion of the CO(2) generated within the cells leaked to the medium, resulting in a significant rise in the extracellular CO(2) concentration to a level above its chemical equilibrium that was diagnostic for active HCO(3)(-) transport. The transient rise in extracellular CO(2) occurred in the light and the dark and was resolved from concurrent respiratory CO(2) efflux using H(13)CO(3)(-) stable isotope techniques. H(13)CO(3)(-) pump-(13)CO(2) leak activity of the CCM was unaffected by 10 microM 3(3,4-dichlorophenyl)-1,1-dimethylurea, an inhibitor of chloroplast linear electron transport, although photosynthetic O(2) evolution was reduced by 90%. However, low concentrations of cyanide, azide, and rotenone along with anoxia significantly reduced or abolished (13)CO(2) efflux in the dark and light. These results indicate that H(13)CO(3)(-) transport was supported by mitochondrial energy production in contrast to other algae and cyanobacteria in which it is supported by photosynthetic electron transport. This is the first report of a direct role for mitochondria in the energization and functioning of the CCM in a photosynthetic organism.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

[Effect of sodium alpha-ketoglutarate injected after x-ray treatment on the respiration and oxidative phosphorylation of hepatic mitochondria]

It have been found that total X-ray treatment (everyday expose to 1 Roentgen up to achievement of total doses 10, 20 30 Roentgen) inhibits the rate of ADP-stimulated respiration of rat liver mitochondria, decreases its efficiency and makes phosphorylation less couple to respiration. All this effects are present from the first period after treatment till the end of treatment. Intraperitoneal alpha-ketoglutarate injections during treatment decreases the inhibition of respiration and oxidative phosphorylation, increases the efficiency of respiration during the period from 1-st injected till the end of experiment.

UV-A irradiation induces a decrease in the mitochondrial respiratory activity of human NCTC 2544 keratinocytes

UV-A irradiation caused a dose-dependent decrease in cellular oxygen consumption (56%) and ATP content (65%) in human NCTC 2544 keratinocytes, one hour after treatment. This effect was partially reversed by maintaining the irradiated cells in normal culture conditions for 24 h. Using malate/glutamate or succinate as substrates for mitochondrial electron transport, the oxygen uptake of digitonin-permeabilised cells was greatly inhibited following UV-A exposure. These results strongly suggest that UV-A irradiation affects the state 3 respiration of the mitochondria. However, under identical conditions, UV-A exposure did not reduce the mitochondrial transmembrane potential. The antioxidant, vitamin E inhibited UV-A-induced lipid peroxidation, but did not significantly prevent the UV-A-mediated changes in cellular respiration nor the decrease in ATP content, suggesting that these effects were not the result of UV-A dependent lipid peroxidation. UV-A irradiation also led to an increase in MnSOD gene expression 24 hours after treatment, indicating that the mitochondrial protection system was enhanced in response to UV-A treatment. These findings provide evidence that impairment of mitochondrial respiratory activity is one of the early results of UV-A irradiation for light doses much lower than the minimal erythemal dose.

The effects of singlet oxygen produced by photodynamic action on the mitochondrial permeability transition differ in accordance with the localization of the sensitizer

We have examined whether the effects of singlet oxygen (1O2) produced by photodynamic action on the mitochondrial permeability transition (PT) can be modulated by the localization of photosensitizers in irradiated mitochondria. We have previously shown that oxidation due to 1O2 photogenerated in hematoporphyrin (HP)-loaded mitochondria can prevent opening of the PT pores, likely after degradation of some critical histidines (Salet et al, 1997, J. Biol. Chem. 272, 21938-21943). Equally, in the present study we have irradiated mitochondria in the presence of a structurally different photosensitizer producing 1O2, namely 4,5',8-trimethylpsoralen (TMP). Fluorescence studies show that TMP binds to protein sites which differ from those of HP. In sharp contrast with HP, TMP-driven photodynamic action triggers per se pore opening. Interestingly, this inducing effect is inhibited when TMP-treated mitochondria are irradiated after addition of mersalyl, a specific reagent protecting thiol groups of the inner mitochondrial membrane that are oriented toward the external hydrophilic phase. This fact suggests that 1O2-mediated thiol oxidation is responsible for TMP-photoinduced pore opening. Taken together, these findings suggest that 1O2 can activate or inactivate a cellular function such as mitochondrial PT depending on the site where it is produced in the mitochondrial membrane.

Photodynamic therapy-induced apoptosis in lymphoma cells: translocation of cytochrome c causes inhibition of respiration as well as caspase activation

L5178Y-R mouse lymphoma (LY-R) cells undergo rapid apoptosis when treated with photodynamic therapy (PDT) sensitized with the silicon phthalocyanine Pc 4. In this study we show that cytochrome c is released into the cytosol within 10 min of an LD99.9 dose of PDT. Cellular respiration is inhibited by 42% at 15 min, and 60% at 30 min after PDT treatment, and caspase 3-like protease activity is elevated by 15 min post-PDT. In digitonin-permeabilized cells addition of cytochrome c to the respiration buffer reverses PDT-induced inhibition of state 3 respiration via Complex I by 40-60%, and via Complex III by 50-90%. In contrast, extramitochondrial cytochrome c does not stimulate respiration in permeabilized control cells, and catalyzes only a low rate of oxygen consumption via electron transfer to cytochrome b5 on the outer mitochondrial membrane. These results demonstrate that PDT-induced inhibition of respiration is primarily due to leakage of cytochrome c into the cytosol rather than to damage to the major enzyme complexes of the electron transport chain. Whether or not inhibition of respiration influences the time course or extent of Pc 4-PDT-induced apoptosis in LY-R cells is not clear at the present time.

Effects of photodynamic action on respiration in nonphosphorylating mitochondria

We have studied the effects of singlet oxygen produced by photodynamic action on respiration in nonphosphorylating mitochondria (state 4). Isolated rat liver mitochondria were incubated with 3 microM hematoporphyrin and irradiated at 365 nm with a fluence rate of 25 W/m2. After short durations of irradiation, state 4 respiration with beta-hydroxybutyrate as substrate increases while respiration with succinate is negligibly affected. When mitochondria have been uncoupled with carbonylcyanide-p-trifluoromethoxyphenyl hydrazone before irradiation, no change occurs in beta-hydroxybutyrate-driven respiration, while succinate-driven respiration strongly decreases. Stimulation of state 4 NADH respiration cannot be explained by slippage of the NADH ubiquinone oxidoreductase because the stoichiometry of the redox pump was found insensitive to photodynamic action. In the light of the metabolite theory for linear enzymatic chains applied to state 4 respiration (Brand et al., Biochem. J. 255, 535-539, 1988), these results suggest that stimulation of NADH respiration is simply due to an increase of membrane leaks which occurs after irradiation. In the case of succinate-driven respiration, a strong inhibition of succinate dehydrogenase activity has been demonstrated after irradiation. It can be suggested that this inhibition introduces a negative control coefficient over state 4 respiration, counterbalancing the effects due to leakage.

Time-course of hypericin phototoxicity and effect on mitochondrial energies in EMT6 mouse mammary carcinoma cells

Photoactivated hypericin produces singlet oxygen and superoxide anion radical; however, the intracellular events contributing to toxicity are unknown. Clonogenic assays of oxygen-dependent hypericin phototoxicity to EMT6 cells have previously shown that 0.5 microM hypericin + 1.5 J cm(-2) fluorescent light is non-toxic and that 1.0 microM hypericin + 1.5 J cm(-2) fluorescent light produces LD40 toxicity. Intracellular events leading to toxicity were revealed at these doses. Lactate dehydrogenase leakage was elevated for both 0.5 microM and 1.0 microM hypericin + light immediately following irradiation. While values eventually returned to control levels for 0.5 microM hypericin + light, leakage increased over time for 1.0 microM hypericin indicating reversible and irreversible toxicity, respectively. Increases in lipid and protein oxidation were measured immediately following irradiation; however, these parameters return to control levels within 0.5 h for both doses. Both total cellular ATP levels and cellular respiration were depressed by approximately 50% of control values for 1.0 microM hypericin + light. These values were unchanged for 0.5 microM hypericin + light. Along with previously reported data demonstrating that light-activated hypericin can inhibit mitochondial succinoxidase in beef heart mitochondria in vitro, these data support oxidative stress-initiated mitochondrial damage as a key target in hypericin phototoxicity.