1. 1. Cycles of oxidation followed by reduction at pH 7.2 have been induced in uncoupled anaerobic mung bean mitochondria treated with succinate and malonate by addition of oxygen-saturated medium. Under the conditions used, cytochromes b557, b553, c549 (corresponding to c1 in mammalian mitochondria) and ubiquinone are completely oxidized in the aerobic state, but become completely reduced in anaerobiosis.
2. 2. The time course of the transition from fully oxidized to fully reduced in anaerobiosis was measured for cytochromes c549, b557, and b553. The intramitochondrial redox potential (IMPh) was calculated as a function of time for each of the three cytochromes from the time course of the oxidized-to-reduced transition and the known midpoint potentials of the cytochromes at pH 7.2. The three curves so obtained are superimposable, showing that the three cytochromes are in redox equilibrium under these conditions during the oxidized-to-reduced transition.
3. 3. This result shows that the slow reduction of cytochrome b557 under these conditions, heretofore considered anomalous, is merely a consequence of its more negative midpoint potential of +42 mV at pH 7.2, compared to +75 mV for cytochrome b553 and +235 mV for cytochrome c549. Cytochrome b557 is placed on the low potential side of coupling site II and transfers electrons to cytochrome c549via the coupling site.
4. 4. The time course of the transition from fully oxidized to fully reduced was also measured for ubiquinone. Using the change in intramitochondrial potential IMPh with time obtained from the three cytochromes, the change in redox state of ubiquinone with IMPh was calculated. When replotted as IMPhversus the logarithm of the ratio (fraction oxidized)/(fraction reduced), two redox components with n = 2 were found. The major component is ubiquinone with a midpoint potential Em7.2 = + 70 mV. The minor component has a midpoint potential Em7.2 = − 12 mV; its nature is unknown.
Abbreviations: IMPh, intramitochondrial potential, referred to the normal hydrogen electrode; Em7.2, midpoint potential at pH 7.2 相似文献
The X-ray repair cross-complementing group 1 (XRCC1) protein plays a central role in DNA repair pathways. Epidemiological studies have revealed the association between XRCC1 Arg280His polymorphism and cancer risk, but the results were inconsistent. We conducted this meta-analysis to assess the effect of XRCC1 Arg280His polymorphism on cancer susceptibility with accumulated data. Up to January 2012, 53 case‐control studies with 21,349 cases and 23,649 controls were available for our study. Summary odds ratios (OR) and corresponding 95% confidence intervals (CIs) for XRCC1 Arg280His polymorphism and cancer were estimated using fixed‐ or random-effects models when appropriate. Our meta-analysis identified that elevated cancer risk was statistically associated with the minor variant His allele and Arg–His/His–His genotypes both in the overall population (allele comparison, His versus Arg: OR = 1.16; 95% CI: 1.08–1.25; dominant comparison, Arg–His/His–His versus Arg–Arg: OR = 1.17; 95% CI: 1.08–1.27) and in terms of subgroup analyses by ethnicity for both Caucasians and non‐Caucasians. However, no significant result was observed in the stratified analysis by cancer type. Moreover, significantly increased cancer risk was observed in smokers. These findings indicated that XRCC1 Arg280His polymorphism may play a role in cancer development. 相似文献
Chilling injury (CI) is associated with the degradation of membrane integrity which can be aligned to phenolic oxidation activated by polyphenol oxidase (PPO) and peroxidase (POD), enzymes responsible for tissue browning. Phenylalanine ammonia-lyase (PAL) is a further enzyme prominent in the phenolic metabolism that is involved in acclimation against chilling stress. It was hypothesized that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by increasing the synthesis of total phenolics and PAL by activating the key enzyme regulating the shikimic acid pathway whilst inhibiting the activity of POD and PPO. Lemon fruit were treated with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, waxed, stored at −0.5, 2 or 4.5 °C for up to 28 days plus 7 days at 23 °C. Membrane integrity was studied by investigating membrane permeability and the degree of membrane lipid peroxidation in lemon flavedo following cold storage. The 10 μM MJ plus 2 mM SA treatment was most effective in enhancing chilling tolerance of lemon fruit, significantly reducing chilling-induced membrane permeability and membrane lipid peroxidation of lemon flavedo tissue. This treatment also increased total phenolics and PAL activity in such tissue while inhibiting POD activity, the latter possibly contributing to the delay of CI manifestation. PPO activity was found to be a poor biochemical marker of CI. Treatment with 10 μM MJ plus 2 mM SA resulted in an alteration of the phenolic metabolism, enhancing chilling tolerance, possibly through increased production of total phenolics and the activation of PAL and inhibition of POD. 相似文献
Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets.This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed. 相似文献
Human cells deficient in rate of excision repair of DNA damage induced by UV-radiation, i.e., xeroderma pigmentosum (XP) cells, are much more sensitive to the mutagenic effect of UV than are cells from normal persons. The lower frequency of mutants in the latter cells has been attributed to the fact that, unlike XP cells, they excise most of the potentially mutagenic lesions before these can be converted into mutations. If semi-conservative DNA synthesis on a template still containing unexcised lesions is responsible for introducing mutations and if replication of the gene of interest, e.g., hypoxanthine (guanine)phosphoribosyltransferase (HPRT) for thioguanine resistance or the elongation factor 2 (EF-2) for diphtheria toxin resistance, occurs at a particular time during S-phase, it should be possible to shorten the time available for such repair by synchronizing cells and irradiating them just as the gene is to be replicated. The predicted result would be a much higher frequency of mutants at one part in the S-phase than at other times. To test this, cells were synchronized using the alpha-polymerase inhibitor aphidicolin, which blocks cells at the G1/S border. Autoradiography, cytofluorimetry, and incorporation of tritiated thymidine studies showed that DNA synthesis started immediately after release from aphidicolin and was completed in 8-10 h. Cells irradiated with 6 J/m2 at various times post-release were assayed for survival and mutations. The frequency of thioguanine- or diphtheria toxin-resistant cells in the population was highest in cells irradiated during the first fifth of the S-phase, i.e., 0-1.5 h post-release. It was significantly lower in cells irradiated at later times. In contrast, UV-induced cytotoxicity showed no significant time dependence during S-phase. These data suggest that the HPRT and EF-2 genes are replicated early in S-phase. 相似文献
Phosphorylated and non-phosphorylated forms of the F0F1-ATPase subunit c from rat liver mitochondria (RLM) were purified and their effect on the opening of the permeability transition pore (mPTP) was investigated. Addition of dephosphorylated subunit c to RLM induced mitochondrial swelling, decreased the membrane potential and reduced the Ca2+ uptake capacity, which was prevented by cyclosporin A. The same effect was observed in the presence of storage subunit c purified from livers of sheep affected with ceroid lipofuscinosis. In black-lipid bilayer membranes subunit c increased the conductance due to formation of single channels with fast and slow kinetics. The dephosphorylated subunit c formed channels with slow kinetics, i.e. the open state being of significantly longer duration than in the case of channels formed by the phosphorylated form that had short life spans and fast kinetics. The channels formed were cation-selective more so with the phosphorylated form. Subunit c of rat liver mitochondria was able to bind Ca2+. Collectively, the data allowed us to suppose that subunit c F0F1-ATPase might be a structural/regulatory component of mPTP exerting its role in dependence on phosphorylation status. 相似文献