Mechanism of S(H)2 reactions of disulfides: frontside vs backside, stepwise vs concerted.

Density functional theory calculations indicate that the S(H)2 reactions of disulfides with alkyl or aryl radicals take place via concerted backside displacement. The activation energies for reactions of Me* with RSSR (R = Me, Et, (i)Pr, (t)Bu) increase with the size of R, since larger R groups prevent the formation of an ideal geometry for SOMO-LUMO overlap. Frontside transition states can also be located, but these lie at least 11 kcal mol(-1) above the corresponding backside transition states.

Cholesterol secoaldehyde induces apoptosis in H9c2 cardiomyoblasts through reactive oxygen species involving mitochondrial and death receptor pathways.

Cholesterol secoaldehyde (ChSeco), a putative product of the reaction of ozone with cholesterol in aqueous environments, has been shown to induce apoptosis in H9c2 cardiomyoblasts. This study further investigated the involvement of apoptotic-related proteins and gene expression using RT-PCR, Western blot, and appropriate biochemical assays. The RT-PCR analysis revealed that ChSeco activates the expression of genes involved in the death receptor (extrinsic) pathway. The significance of this pathway was also evident from the increased activity of caspase-8. The overexpression of Apaf-1, loss of mitochondrial transmembrane potential, release of cytochrome c, and increased activity of caspase-9 provide further evidence for the involvement of a mitochondrial (intrinsic) pathway. Time-course analysis of ChSeco-exposed H9c2 cells showed an upstream increase in the generation of reactive oxygen species (ROS) and an associated decrease in the intracellular glutathione. N-acetyl-L-cysteine and Trolox significantly attenuated the ChSeco-induced ROS formation and cytotoxicity and also down-regulated the expression of the genes of all the players in either pathway. This study clearly shows that ChSeco induces apoptosis in H9c2 cells through ROS generation and the activation of both the intrinsic and the extrinsic pathway.

Free radical biology and medicine: it's a gas, man!

We review gases that can affect oxidative stress and that themselves may be radicals. We discuss O(2) toxicity, invoking superoxide, hydrogen peroxide, and the hydroxyl radical. We also discuss superoxide dismutase (SOD) and both ground-state, triplet oxygen ((3)O(2)), and the more energetic, reactive singlet oxygen ((1)O(2)). Nitric oxide ((*)NO) is a free radical with cell signaling functions. Besides its role as a vasorelaxant, (*)NO and related species have other functions. Other endogenously produced gases include carbon monoxide (CO), carbon dioxide (CO(2)), and hydrogen sulfide (H(2)S). Like (*)NO, these species impact free radical biochemistry. The coordinated regulation of these species suggests that they all are used in cell signaling. Nitric oxide, nitrogen dioxide, and the carbonate radical (CO(3)(*-)) react selectively at moderate rates with nonradicals, but react fast with a second radical. These reactions establish "cross talk" between reactive oxygen (ROS) and reactive nitrogen species (RNS). Some of these species can react to produce nitrated proteins and nitrolipids. It has been suggested that ozone is formed in vivo. However, the biomarkers that were used to probe for ozone reactions may be formed by non-ozone-dependent reactions. We discuss this fascinating problem in the section on ozone. Very low levels of ROS or RNS may be mitogenic, but very high levels cause an oxidative stress that can result in growth arrest (transient or permanent), apoptosis, or necrosis. Between these extremes, many of the gasses discussed in this review will induce transient adaptive responses in gene expression that enable cells and tissues to survive. Such adaptive mechanisms are thought to be of evolutionary importance.

The mechanism of the self-initiated thermal polymerization of styrene. Theoretical solution of a classic problem.

The Mayo and Flory mechanisms for the self-initiation of styrene polymerization were explored with B3LYP and BPW91 density functional calculations. The Diels-Alder dimer (AH) is the key intermediate, and the lowest energy pathway for AH formation is a stepwise mechanism via a gauche/sickle (*M2*Gs) or gauche/U-shaped (*M2*Gu) diradical. Ring closure of the 1,4-diradical to diphenylcyclobutane (DCB) is predicted to have a lower barrier than ring closure to AH. Dynamic effects are likely to play an important role in determining the rate of AH versus DCB formation. Hydrogen transfer from AH to styrene to generate two monoradical species is predicted to be a reasonable process that initiates monoradical polymerization.

Antioxidant-mediated augmentation of ozone-induced membrane oxidation.

The pulmonary epithelial lining fluid (ELF) contains substrates, e.g., ascorbic acid (AH2), uric acid (UA), glutathione (GSH), proteins, and unsaturated lipids, which undergo facile reaction with inhaled ozone (O3). Reactions near the ELF gas/liquid interface likely provide the driving force for O3 absorption ("reactive absorption") and constrain O3 diffusion to the underlying epithelium. To investigate the potential mechanisms wherein O3/ELF interactions may induce cellular damage, we utilized a red cell membrane (RCM) model intermittently covered by an aqueous film to mimic the lung surface compartmentation, and evaluated exposure-mediated loss of acetylcholinesterase activity (AChE) and TBARS accumulation. In the absence of aqueous reactants, O3 exposure induced no detectable changes in AChE or TBARS. AH2 and GSH preferentially induced oxidative damage in a dose-dependent fashion. AH2-mediated RCM oxidation was not inhibited by superoxide dismutase, catalase, mannitol, or Fe chelators. O3 reaction with UA, Trolox, or albumin produced no RCM oxidation but oxidation occurred when AH2 was combined with UA or albumin. Rat bronchoalveolar lavage fluid (BALF) also induced RCM oxidation. However, in vivo O3 exposure dampened the extent of BALF-mediated RCM oxidation. Although we cannot completely rule out O3 diffusion to the RCM, product(s) derived from O3 + AH2/GSH reactions (possibly O3*- or 1O2) likely initiated RCM oxidation and may suggest that in vivo, such secondary species account for O3 permeation through the ELF leading to cellular perturbations.

Re-evaluation of the relative potency of synthetic and natural alpha-tocopherol: experimental and clinical observations.

Nutritionists generally consider all-rac-alpha-tocopherol and RRR-alpha-tocopherol equivalent in vitamin E activity but disagree whether equivalency requires a dosage ratio of 1.36:1 or 2:1. In contrast, we hypothesize that all-rac- and RRR-alpha-tocopherols are not equivalent in any dosage ratio. Previous observations that all-rac- and RRR-alpha-tocopherols are distributed and eliminated via saturable and stereospecific pathways imply that their relative bioavailability varies with the saturation of these pathways and therefore varies with dosage. Indeed, previous studies observed that the relative bioavailability of all-rac- and RRR-alpha-tocopherols varies between tissues as well as with dose, time after dosing, and duration of dosing. This non-constant relative bioavailability predicts non-constant relative activity (i.e., non-parallel dose-concentration curves predict non-parallel dose-effect curves). Non-constant relative bioavailability suggests that a fixed dosage ratio of all-rac- and RRR-alpha-tocopherols cannot produce a fixed ratio of effects on all processes in all tissues at all times after all dosages. However, previous studies suggest that all-rac- and RRR-alpha-tocopherols have equivalent effects (parallel dose-effect curves) in vitamin E-deficient animals and non-vitamin E-deficient humans. We re-evaluate the data from these animal studies and find non-parallel dose-effect and concentration-effect curves. We discuss pharmacokinetic and pharmacodynamic reasons why previous studies in non-vitamin E-deficient humans did not find non-parallel dose-effect curves for all-rac- and RRR-alpha-tocopherols. We note that saturable elimination predicts that all-rac- and RRR-alpha-tocopherols might inhibit and/or induce elimination of other compounds (including 30-40% of prescription drugs) eliminated via the same saturable pathways, and stereospecific elimination predicts that all-rac- and RRR-alpha-tocopherol have non-parallel dose-effect curves for these interactions.