pi cation radicals of ferrous and free base isobacteriochlorins: Models for siroheme and sirohydrochlorin

Theoretical and experimental optical, redox, and paramagnetic results are presented for models of siroheme, the iron isobacteriochlorin prosthetic group of nitrite and sulfite reductases, and of sirohydrochlorin, the metal-free siroheme that is an intermediate in the biosynthetic pathway to vitamin B₁₂. The facile oxidation of many isobacteriochlorins, which distinguishes them from porphyrins and chlorins, suggests that the siroheme macrocycle itself may undergo oxidation in the multi-electron enzymatic cycles that reduce nitrite to ammonia and sulfite to hydrogen sulfide. Extended Hückel MO calculations (i) help rationalize the redox properties of isobacteriochlorins compared with those of porphyrins and chlorins; (ii) indicate that Fe(II) pyridine carbonyl[(py) (CO)] complexes of isobacteriochlorins, unlike those of porphyrins and chlorins, should undergo oxidation from the macrocycle rather than the metal to yield π cation radicals; (iii) suggest that, in hexacoordinated Fe(II) isobacteriochlorin complexes, the site of oxidation—i.e., the metal or the macrocycle—will depend on the ligand field induced by the axial ligands; and (iv) predict similar unpaired spin density profiles for metal-free and (py) (CO)Fe(II) isobacteriochlorin radicals. Experimental data for three isomeric free-base and (py) (CO)Fe(II) complexes of dimethyloctaethylisobacteriochlorins support the theoretical calculations and establish the existence of Fe(II) isobacteriochlorin π cations in vitro.     

Energies and kinetics of radical pairs involving bacteriochlorophyll and bacteriopheophytin in bacterial reaction centers

Absorbance changes reflecting the formation of a transient radical-pair state, P^F, were measured in reaction centers from Rhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that P^F is an equilibrium mixture of two states, which appear to be mainly ¹[P^[unk]B^[unk]] and ¹[P^[unk]H^[unk]]. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm, and H is a bacteriopheophytin. In the presence of Q^[unk], the energy of ¹[P^[unk]B^[unk]] is about 0.025 eV above that of ¹[P^[unk]H^[unk]], ¹[P^[unk]H^[unk]] can decay to a triplet state, P^R, which also is an equilibrium mixture of two states, separated by about 0.03 eV. The lower of these appears to be mainly a locally excited triplet state of P, ³P; the upper state contains a major contribution from a triplet charge-transfer state, ³[P^[unk]B^[unk]]. The temperature dependence of delayed fluorescence from P^R indicates that ³P lies 0.40 eV below the excited singlet state, P^*, which is about 0.05 eV above ¹[P^[unk]H^[unk]]. The ^1,3[P^[unk]B^[unk]] charge-transfer states thus appear to interact with the locally excited states of P and B to give singlet and triplet states that are separated in energy by about 0.35 eV. This is 10⁶ times larger than the splitting between ¹[P^[unk]H^[unk]] and ³[P^[unk]H^[unk]] and implies strong orbital overlap between P^[unk] and B^[unk]. This is consistent with recent picosecond studies which suggest that electron transfer from P^* to B occurs within 1 ps and is followed in 4 to 10 ps by electron transfer from B^[unk] to H.     

Lipoproteins, free radicals and atherosclerosis

The Author reviews vascular, lipidic and oxidative factors in the genesis of atherosclerosis. He admits the possibility that an alteration in the arterial wall, an increase in circulating lipids or an oxidative stress may influence the precocity of atherosclerosis. The transport of lipoperoxides or of oxidized cholesterol by lipoproteins renders them toxic and susceptible to aggravate atherosclerosis.     

Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1: protein-cofactor (bacteriochlorophyll, bacteriopheophytin, and carotenoid) interactions.

The three-dimensional structures of the cofactors and protein subunits of the reaction center (RC) from the carotenoidless mutant strain of Rhodobacter sphaeroides R-26 and the wild-type strain 2.4.1 have been determined by x-ray diffraction to resolutions of 2.8 A and 3.0 A with R values of 24% and 26%, respectively. The bacteriochlorophyll dimer (D), bacteriochlorophyll monomers (B), and bacteriopheophytin monomers (phi) form two branches, A and B, that are approximately related by a twofold symmetry axis. The cofactors are located in hydrophobic environments formed by the L and M subunits. Differences in the cofactor-protein interactions between the A and B cofactors, as well as between the corresponding cofactors of Rb, sphaeroides and Rhodopseudomonas viridis [Michel, H., Epp, O. & Deisenhofer, J. (1986) EMBO J. 3, 2445-2451], are delineated. The roles of several structural features in the preferential electron transfer along the A branch are discussed. Two bound detergent molecules of beta-octyl glucoside have been located near BA and BB. The environment of the carotenoid, C, that is present in RCs from Rb. sphaeroides 2.4.1 consists largely of aromatic residues of the M subunit. A role of BB in the triplet energy transfer from D to C and the reason for the preferential ease of removal of BB from the RC is proposed.     

The evolution of free radicals and oxidative stress

The superoxide free radical has come to occupy an amazingly central role in a wide variety of diseases. Our metabolic focus on aerobic energy metabolism in all cell types, coupled with some chemical peculiarities of the oxygen molecule itself, contribute to the phenomenon. Superoxide is not, as we once thought, just a toxic but unavoidable byproduct of oxygen metabolism. Rather it appears to be a carefully regulated metabolite capable of signaling and communicating important information to the cell's genetic machinery. Redox regulation of gene expression by superoxide and other related oxidants and antioxidants is beginning to unfold as a vital mechanism in health and disease.     

Studies on free radicals,antioxidants, and co-factors

The interplay between free radicals, antioxidants, and co-factors is important in maintaining health, aging and age-related diseases. Free radicals induce oxidative stress, which is balanced by the body’s endogenous antioxidant systems with an input from co-factors, and by the ingestion of exogenous antioxidants. If the generation of free radicals exceeds the protective effects of antioxidants, and some co-factors, this can cause oxidative damage which accumulates during the life cycle, and has been implicated in aging, and age dependent diseases such as cardiovascular disease, cancer, neurodegenerative disorders, and other chronic conditions. The life expectancy of the world population is increasing, and it is estimated that by 2025, 29% of the world population will be aged ≥60 years, and this will lead to an increase in the number of older people acquiring age-related chronic diseases. This will place greater financial burden on health services and high social cost for individuals and society. In order to acheive healthy aging the older people should be encouraged to acquire healthy life styles which should include diets rich in antioxidants. The aim of this review is to highlight the main themes from studies on free radicals, antioxidants and co-factors, and to propose an evidence-based strategy for healthy aging.     

Ischemia, reperfusion and oxygen free radicals.

Alterations which occur during ischemia are reviewed. They modify the metabolic status in such a way they prepare the cell to an anomalous response to reoxygenation. The consequence of this disturbance is the generation of oxygen free radicals through several mechanisms, including the mitochondrial oxidative phosphorylation, the arachidonic acid cascade, the activation of xanthine oxidase, activation of phagocytes, iron mobilization, etc. Reduced glutathione is exhausted, proteins are inactivated. Lipid peroxidation induces membrane breakdown and cellular death.     

The link between free radicals and myocardial injury

In the first part of this study, oxygen derived radicals (O2-radical) were indirectly demonstrated as causative agents for ischemia/reperfusion injury by showing myocardial protection of radical scavengers. In 19 open chest dogs, the left anterior descending coronary artery was occluded for 90 min and subsequently reperfused for 60 min. Group A received SOD (15000 U/kg), catalase (5 mg/kg) and 20% mannitol (18 ml/kg) via the left atrium starting 15 min pre-occlusion and ending 15 min after reperfusion. Group B received infusion of saline and served as control. The severity of myocardial injury was evaluated by epicardial ECG, hemodynamics, echo cardiographical asynergy area and left ventricular wall thickness, histopathological findings and magnitude of necrotic area. Group A demonstrated less injury than group B in most of the parameters. In particular, ratios of necrotic to perfused areas determined by dual fluorescence methods were significantly limited in group A (34.1 +/- 12.0 vs 66.0 +/- 11.3%), indirectly verifying that the O2- radicals play an important role in the genesis of ischema/reperfusion injury. In the second part of the study, the direct measurement of radicals in freeze-clamped myocardium was reported using electron spin resonance (ESR) spectroscopy. With this method, it was not possible to demonstrate the generation of O2-radicals in ischemic/reperfused myocardium. However, the data suggested that coenzyme Q10 anion radical, which exists in the normal myocardium, might provide an index of tissue injury.     

一氧化氮等自由基与脑出血关系的研究

目的探讨一氧化氮等自由基与脑出血的关系。方法检测１３３例脑出血患者和１００例健康对照者血浆一氧化氮（Ｐ－ＮＯ）、维生素Ｃ（Ｐ－ＶＣ）、维生素Ｅ（Ｐ－ＶＥ）、β－胡萝卜素（Ｐ－β－ＣＡＲ）和过氧化脂质（Ｐ－ＬＰＯ）含量及红细胞超氧化物歧化酶（Ｅ－ＳＯＤ）、过氧化氢酶（Ｅ－ＣＡＴ）、谷胱甘肽过氧化物酶（Ｅ－ＧＳＨ－Ｐｘ）活性和过氧化脂质（Ｅ－ＬＰＯ）含量。结果与对照组比较,患者组Ｐ－ＮＯ、Ｐ－ＬＰＯ、Ｅ－ＬＰＯ均值显著升高（Ｐ＜０．００１）,Ｐ－ＶＣ、Ｐ－ＶＥ、Ｐ－β－ＣＡＲ、Ｅ－ＳＯＤ、Ｅ－ＣＡＴ、Ｅ－ＧＳＨ－Ｐｘ均值显著降低（Ｐ＜０．００１）;逐步回归发现,患者病情（ＮＤＳ）与Ｐ－ＮＯ、Ｐ－ＶＣ、Ｅ－ＬＰＯ值相关最为密切,颅内血肿量与Ｐ－ＮＯ、Ｐ－ＶＥ、Ｅ－ＬＰＯ值相关最为密切。结论脑出血患者体内自由基反应病理性加剧,氧化抗氧化平衡严重失     

The role of free radicals in leaf senescence

Decreased catalase activity is a consistent feature of leaf senescence. Although not as well documented, superoxide dismutase appears generally to decrease during leaf senescence. These changes suggest that free radical levels are likely to be higher in senescing tissues. The hydrogen peroxide-scavenging ability of chloroplasts due to the activity of the enzymes ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase appears to be established although there is no information on changes in levels of these enzymes in response to leaf senescence. In plants, unlike mammals, the direct reaction of glutathione with H2O2, catalysed by glutathione peroxidase, appears to be only a minor means of scavenging hydrogen peroxide. Senescence appears to be correlated with increases in lipid peroxidation and membrane permeability. The findings reviewed in this paper lend general support to the view that free radicals play a significant role in the multifactorial syndrome which constitutes leaf senescence.     

The role of oxidants and free radicals in reperfusion injury

While timely reperfusion of acute ischemic myocardium is essential for myocardial salvage, reperfusion results in a unique form of myocardial damage. Functional alterations occur, including depressed contractile function and decreased coronary flow as well as altered vascular reactivity. Both myocardial stunning and infarction are seen. Over the last two decades, it has become increasingly clear that oxidant and oxygen radical formation is greatly increased in the post-ischemic heart and serves as a critical central mechanism of post-ischemic injury. This oxidant formation is generated through a series of interacting pathways in cardiac myocytes and endothelial cells and triggers subsequent leukocyte chemotaxis and inflammation. Nitric oxide (NO) production and NO levels are also greatly increased in ischemic and post-ischemic myocardium, and this occurs through NO synthase (NOS)-dependent NO formation and NOS-independent nitrite reduction. Recently, it has been shown that the pathways of oxygen radical and NO generation interact and can modulate each other. Under conditions of oxidant stress, NOS can switch from NO to oxygen radical generation. Under ischemic conditions, xanthine oxidase can reduce nitrite to generate NO. NO and peroxynitrite can inhibit pathways of oxygen radical generation, and, in turn, oxidants can inhibit NO synthesis from NOS. Ischemic preconditioning markedly decreases NO and oxidant generation, and this appears to be an important mechanism contributing to preconditioning-induced myocardial protection.     

Experimental atherosclerosis and oxygen free radicals

Oxygen free radicals are known to produce cellular injury by peroxidation of phospholipids in the cell membrane. These free radicals might damage the endothelial cell and thus set the stage for atherosclerosis. The authors studied the effect of high-cholesterol diets on the genesis of atherosclerosis and lipid peroxidation products, malondialdehyde (MDA) in rabbits. The animals were divided into four groups each comprising 5 rabbits, on the basis of their diets. Group I, control diet; group II, cholesterol; group III, coconut oil; group IV, a mixture of cholesterol, coconut oil, and cholic acid. Rabbits were sacrificed five months after being on the respective diets. Blood samples were obtained for the measurements of total cholesterol, high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), very low density lipoprotein cholesterol (VLDL-C), triglycerides, and MDA at the end of the protocol. The aortas were removed from different animals for the identification of atherosclerotic plaques. Plaques were detected in all the animals in group II and group IV. The serum total cholesterol, LDL-C, and VLDL-C were significantly higher in animals of group II and IV than in those of group I. The values for serum total cholesterol, HDL-C, LDL-C, and VLDL-C in group III were not significantly different from those in group I. The blood MDA and serum triglycerides were also higher in animals of group II and IV than in those of group I. There were, however, no significant differences in these parameters in group III as compared with those in group I.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute hemorrhage and oxygen free radicals

Oxygen free radicals are known to produce cellular injury. There are various mechanisms during hemorrhagic shock that can lead to an increase in the oxygen free radicals, which would produce a loss of membrane integrity and a decrease in cardiac function and cardiac contractility. The authors studied the effect of acute hemorrhage and reperfusion on the hemodynamics, blood lactate, and oxygen free radicals. Dogs were divided into two groups, group I, hemorrhagic shock and reinfusion; group II, hemorrhagic shock and reinfusion with superoxide dismutase (SOD) and catalase treatment. The dogs were bled (50% of the estimated blood volume) over a period of five to twelve minutes. After an hour of bleeding, the shed blood was transfused over twenty to twenty-five minutes. Group II received SOD (0.7 mg/kg, IV) and catalase (0.7 mg/kg, IV) within ten minutes of bleeding. The hemodynamic measurements and collection of blood samples for measurement of oxygen free radicals and lactate were made before hemorrhage and at fifteen, thirty, and sixty minutes of hemorrhage and at fifteen, thirty, and sixty minutes after reperfusion of shed blood. There was a decrease in the cardiac function and index of myocardial contractility associated with an increase in the blood lactate after hemorrhage in group I. Similar but less marked changes in these parameters were observed in group II. Recovery of hemodynamic parameters with reinfusion was better in group II as compared with that in group I. There was an increase in blood oxygen free radicals (superoxide anions) in group I, especially after reinfusion. No such changes were observed in group II.(ABSTRACT TRUNCATED AT 250 WORDS)