Current Medicinal Chemistry - Volume 11, Issue 9, 2004

Hydrogen peroxide (H2O2) can be detected in freshly-voided human urine from healthy subjects and has been proposed as a “biomarker” of oxidative stress. This paper summarizes our studies to examine the extent to which urinary H2O2 measurement fulfils the criteria for the “ideal biomarker”. Levels of H2O2, standardised for creatinine, varied widely between subjects. In most subjects, levels also varied considerably when measurements were made at different times and on different days. A reproducible increase in urinary H2O2 was detected in all subjects examined after drinking coffee, a beverage rich in H2O2. By contrast, green tea decreased urinary H2O2 levels. We conclude that the H2O2 in coffee is not excreted into urine. Instead, hydroxyhydroquinone from coffee is absorbed, excreted and oxidises in urine to produce H2O2. No other confounders of urinary H2O2 have been identified to date. Work is underway to compare H2O2 levels with variations in other biomarkers of oxidative damage, to test the possibility that there are daily or other periodic variations in oxidative damage rates.

Endothelial dysfunction, characterized by a loss in nitric oxide bioactivity, is an early event in the development of atherosclerosis and determines future vascular complications. Emerging evidence suggests a causal role for oxidative stress in this process. Reactive oxygen species can directly inactivate nitric oxide, modulate protein function and act as cellular signaling molecules. These events contribute to the initiation and progression of endothelial dysfunction. Considerable data also indicates that antioxidant compounds limit oxidative damage and restore endothelial function. The purpose of this review is to discuss these data and suggest novel approaches for lowering the oxidative stress in the vasculature.

Organisms are constantly exposed to many different forms of reactive oxygen species and reactive nitrogen species that damage proteins, nucleic acids, and lipids, leading to loss of biological function. The possibility that reactive oxygen / nitrogen-mediated protein damage contributes to the aging process is supported by results of many studies showing that aging is associated with the accumulation of such protein damage. Summarized here are results of studies , showing that the accumulation of ,protein damage is a complex function of a multiplicity of factors that govern the intracellular levels of reactive oxygen / nitrogen species, on the one hand, and a multiplicity of factors that govern the degradation and / or repair of damaged proteins, on the other. Basic mechanisms involved in the modification of proteins by various forms of reactive oxygen / nitrogen species are also discussed.

Molecules in biological systems often can perform more than one function. In particular, many molecules have the ability to chemically scavenge free radicals and thus act in the test tube as antioxidant, but their main biological function is by acting as hormones, ligands for transcription factors, modulators of enzymatic activities or as structural components. In fact, oxidation of these molecules may impair their biological function, and cellular defense systems exist which protect these molecules from oxidation. Vitamin E is present in plants in 8 different forms with more or less equal antioxidant potential (α-,β,γ,δ- tocopherol / tocotrienols); nevertheless, in higher organisms only α-tocopherol is preferentially retained suggesting a specific mechanism for the uptake for this analogue. In the last 20 years, the route of tocopherol from the diet into the body has been clarified and the proteins involved in the uptake and selective retention of α -tocopherol discovered. Precise cellular functions of α -tocopherol that are independent of its antioxidant / radical scavenging ability have been characterized in recent years. At the posttranslational level, α- tocopherol inhibits protein kinase C, 5-lipoxygenase and phospholipase A2 and activates protein phosphatase 2A and diacylglycerol kinase. Some genes (e. g. scavenger receptors, α-TTP, α-tropomyosin, matrix metalloproteinase-19 and collagenase) are modulated by α-tocopherol at the transcriptional level. α- Tocopherol also inhibits cell proliferation, platelet aggregation and monocyte adhesion. These effects are unrelated to the antioxidant activity of vitamin E, and possibly reflect specific interactions of α-tocopherol with enzymes, structural proteins, lipids and transcription factors. Recently, several novel tocopherol binding proteins have been cloned, that may mediate the non-antioxidant signaling and cellular functions of vitamin E and its correct intracellular distribution. In the present review, it is suggested that the non-antioxidant activities of tocopherols represent the main biological reason for the selective retention of α-tocopherol in the body, or vice versa, for the metabolic conversion and consequent elimination of the other tocopherols.

α-Lipoic acid (LA), a naturally occurring dithiol compound, has long been known as an essential cofactor for mitochondrial bioenergetic enzymes. Aside from its enzymatic role, in vitro and in vivo studies suggest that LA also acts as a powerful micronutrient with diverse pharmacologic and antioxidant properties. Pharmacolo-gically, LA improves glycemic control, polyneuropathies associated with diabetes mellitus, and effectively mitigates toxicities associated with heavy metal poisoning. As an antioxidant, LA directly terminates free radicals, chelates transition metal ions (e.g. iron and copper), increases cytosolic glutathione and vitamin C levels and prevents toxicities associated with their loss. These diverse actions suggest that LA acts by multiple mechanisms both physiologically and pharmacologically, many of which are only now being explored. Herein, we review the known biochemical properties of LA with particular reference to how LA may be an effective agent to ameliorate certain pathophysiologies of many chronic diseases.

Oxidative stress results from an oxidant / antioxidant imbalance, an excess of oxidants and / or a depletion of antioxidants. A considerable body of recent evidence suggests that oxidant stress plays a major role in several aspects of acute and chronic inflammation and is the subject of this review. Immunohistochemical and biochemical evidence demonstrate the significant role of reactive oxygen species (ROS) in acute and chronic inflammation. Initiation of lipid peroxidation, direct inhibition of mitochondrial respiratory chain enzymes, inactivation of glyceraldehyde-3-phosphate dehydrogenase, inhibition of membrane Na+ / K+ ATP-ase activity, inactivation of membrane sodium channels, and other oxidative protein modifications contribute to the cytotoxic effect of ROS. All these toxicities are likely to play a role in the pathophysiology of shock, inflammation and ischemia and reperfusion. (2) Treatment with either peroxynitrite decomposition catalysts, which selectively inhibit peroxynitrite, or with SODm's, which selectively mimic the catalytic activity of the human superoxide dismutase (SOD) enzymes, have been shown to prevent in vivo the delayed tissue injury and the cellular energetic failure associated with inflammation. ROS (e.g., superoxide, peroxynitrite, hydroxyl radical and hydrogen peroxide) are all potential reactants capable of initiating DNA single strand breakage, with subsequent activation of the nuclear enzyme poly (ADP ribose) synthetase (PARS), leading to eventual severe energy depletion of the cells, and necrotic-type cell death. Antioxidant treatment inhibits the activation of PARS, and prevents the organ injury associated with acute and chronic inflammation.

An increasing body of evidence from animal models, human specimens and cell lines points to reactive oxygen species as likely involved in the pathways, which convey both extracellular and intracellular signals to the nucleus, under a variety of pathophysiological conditions. Indeed, reactive oxygen species (ROS), in a concentration compatible with that detectable in human pathophysiology, appear able to modulate a number of kinases and phosphatases, redox sensitive transcription factors and genes. This type of cell signalling consistently implies the additional involvement of other bioactive molecules that stem from ROS reaction with cell membrane lipids. The present review aims to comprehensively report on the most recent knowledge about the potential role of ROS and oxidised lipids in signal transduction processes in the major events of cell and tissue pathophysiology. Among the lipid oxidation products of ROS-dependent reactivity, which appear as candidates for a signalling role, there are molecules generated by oxidation of cholesterol, polyunsaturated fatty acids and phospholipids, as well as lysophosphatidic acid and lysophospholipids, platelet activating factor-like lipids, isoprostanes, sphingolipids and ceramide.

The platelet aggregation is a crucial step in a pathophisiology of thromboses, leading to development of cardio-vascular diseases (myocardial infarction, transient ischemic attacks, strokes, etc.). The final step in the aggregation is the binding of fibrinogen to receptor - glycoprotein IIb / IIIa (GP IIb / IIIa) on the surface of activated platelets. In recent years the increasing attention is paid to the role of fibrinogen antagonists in the prevention of thrombosis. The search for these compounds is based on the molecular design of structures mimicking some fragment of RGD (Arg-Gly-Asp) sequence, responsible for the binding of fibrinogen to GP IIb / IIIa. Up to now, a large number of potent and selective GP IIb / IIIa antagonists, including non-peptide inhibitors are identified (derivatives of benzodiazepines, aminobenzamidinosuccinyles, isoxazolines, isoquinolines). The modification of natural peptide structures for obtaining of more active and selective fibrinogen receptor antagonists is realized in several ways: substitution of main pharmacophores of RGD sequence; cyclization of RGD-containing peptides; design of conformationally constrained peptidomimetics. For the treatment of chronic cardio-vascular diseases, the clinic needs high orally active RGD-peptidomimetics. This task is realized by obtaining of prodrugs on the base of the most potent RGDmimetics. In our laboratory the molecular design and synthesis of non-peptide fibrinogen receptor antagonists were carried out. The series of RGD-mimetics on the basis of 4-oxo-(piperazine-1-yl)butyric acid as Argmimetic and β-aryl-β-alanines as Asp-Phe-mimetics were synthesized. Obtained RGD-mimetics showed a high antiaggregatory activity in vitro experiments with IC50 values of 10-7 - 10-9 M.

Potassium channels play a crucial role in controlling the cell membrane potential. Among the different varieties of K+ channels, the ATP-sensitive potassium channels (KATP channels) have been characterized in numerous cell types, such as skeletal and smooth muscle cells, endocrine cells, cardiac cells and central neurons. Several molecules are known to activate KATP channels and have been named “potassium channel openers” (PCOs). Such compounds may have a wide therapeutic potential and a few drugs are currently used as antihypertensive agents. Different chemical series of PCOs have been explored. This heterogeneous group of organic compounds comprises the benzopyran series including potent vasorelaxant drugs, such as cromakalim. The latter compound, a typical example of potassium channel opener, exerts its biological effect by activating KATP channels. This review presents recent developments in the chemistry of cromakalim analoges and reports chemical aspects governing their potency and tissue selectivity.