chlorophyll-a and duroquinone

We examined the damage to mitochondrial electron transport caused by photosensitization of a pheophorbide a derivative, DH-I-180-3, shown recently to induce necrosis of lung carcinoma cells with low dark toxicity. Confocal microscopy showed that DH-I-180-3 co-localized with dihydrorhodamine-123 suggesting that it mainly accumulates in mitochondria. The photosensitizer alone in the dark did not affect mitochondrial electron transport. Illumination of isolated mitochondria in the presence of DH-I-180-3 resulted in inhibition of both NADH- and succinate-dependent respiration. Measurement of the activity of each component of the electron transport chain revealed that Complex I and III were very susceptible to the treatment whereas Complex IV was resistant. We conclude that the photosensitizer is localized in mitochondria and, upon illumination, produces reactive oxygen species that inactivate Complexes I and III.

Topics: Animals; Benzoquinones; Cattle; Chlorophyll; Electron Transport; Electron Transport Complex I; Electron Transport Complex II; Electron Transport Complex III; Electron Transport Complex IV; Light; Mitochondria; NAD; Radiation-Sensitizing Agents; Succinic Acid

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).

Topics: Benzoquinones; Chlorophyll; Chloroplasts; Darkness; Dibromothymoquinone; Energy Metabolism; Fluorescence; Intracellular Membranes; Kinetics; Light; Light-Harvesting Protein Complexes; Photochemistry; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Photosystem II Protein Complex; Plastoquinone; Quinones

Quinones caused quenching of Chl a fluorescence in native and model systems. Menadione quenched twofold the fluorescence of Chl a and BChl a in pea chloroplasts, chromatophores of purple bacteria, and liposomes at concentrations of 50-80 microM. To obtain twofold quenching in Triton X-100 micelles and in ethanol, the addition of 1.3 mM and 11 mM menadione was required, respectively. A proportional decrease in the lifetime and yield of Chl a fluorescence in chloroplasts, observed as the menadione concentration increased, is indicative of the efficient excitation energy transfer from bulk Chl to menadione. The decrease in the lifetime and yield of fluorescence was close to proportional in liposomes, but not in detergent micelles. The insensitivity of the menadione quenching effect to DCMU in chloroplasts, and similarity of its action in chloroplasts and liposomes indicate that menadione in chloroplasts interacts with antenna Chl, i.e., nonphotochemical quenching of fluorescence occurs.

Topics: Bacterial Chromatophores; Bacteriochlorophylls; Benzoquinones; Chlorophyll; Chlorophyll A; Chloroplasts; Diuron; Fluorescence; Liposomes; Micelles; Pisum sativum; Quinones; Rhodobacter sphaeroides; Rhodospirillum rubrum; Spectrometry, Fluorescence; Ubiquinone; Vitamin K

The regulation of the protein kinase activity responsible for the phosphorylation of the light-harvesting complex of photosystem II (LHCII) 27-kDa polypeptide involved in the State I-State II transitions in Acetabularia thylakoids was investigated. The LHCII kinase of isolated thylakoids retains its activity in absence of light-driven electron flow or reductants added in the dark. However, the kinase is reversibly inactivated by addition of oxidants in vitro or by far red (710 nm) light in vivo. Inhibitors of the quinol oxidase site of the cytochrome b6.f complex inactivate the LHCII kinase in the dark, and also in the light, or in presence of duroquinol when the plastoquinone pool is reduced. Inhibitors of the quinone reductase site of the b6.f complex have practically no effect in the dark and stimulate the kinase activity in the light. Based on these data and on our previous report, showing specific loss of LHCII kinase activity in a Lemna mutant lacking the cytochrome b6.f complex (Gal, A., Shahak, Y., Schuster, G., and Ohad, I. (1987) FEBS Lett. 221, 205-210), we propose that the activity of the LHCII kinase is regulated by the redox state of a cytochrome b6.f complex component(s) which responds to the balance of electron flow from photosystem II via the plastoquinone pool to photosystem I.

Topics: Acetabularia; Atrazine; Benzoquinones; Chlorophyll; Chlorophyta; Cytochromes; Cytochromes f; Diuron; Kinetics; Light-Harvesting Protein Complexes; Oxidation-Reduction; Phosphorylation; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Photosystem II Protein Complex; Plant Proteins; Plastoquinone; Protein Kinases; Quinones