Probing the Role of Cysteine Thiyl Radicals in Biology: Eminently Dangerous, Difficult to Scavenge

An ultraviolet-driven rescue pathway for oxidative stress to eye lens protein human gamma-D crystallin

Human gamma-D crystallin (HGD) is a major constituent of the eye lens. Aggregation of HGD contributes to cataract formation, the leading cause of blindness worldwide. It is unique in its longevity, maintaining its folded and soluble state for 50-60 years. One outstanding question is the structural basis of this longevity despite oxidative aging and environmental stressors including ultraviolet radiation (UV). Here we present crystallographic structures evidencing a UV-induced crystallin redox switch mechanism. The room-temperature serial synchrotron crystallographic (SSX) structure of freshly prepared crystallin mutant (R36S) shows no post-translational modifications. After aging for nine months in the absence of light, a thiol-adduct (dithiothreitol) modifying surface cysteines is observed by low-dose SSX. This is shown to be UV-labile in an acutely light-exposed structure. This suggests a mechanism by which a major source of crystallin damage, UV, may also act as a rescuing factor in a finely balanced redox system.

Free Radical Lipid Peroxidation Induced by Reactive Halogen Species

The review is devoted to the mechanisms of free radical lipid peroxidation (LPO) initiated by reactive halogen species (RHS) produced in mammals, including humans, by heme peroxidase enzymes, primarily myeloperoxidase (MPO). It has been shown that RHS can participate in LPO both in the initiation and branching steps of the LPO chain reactions. The initiation step of RHS-induced LPO mainly involves formation of free radicals in the reactions of RHS with nitrite and/or with amino groups of phosphatidylethanolamine or Lys. The branching step of the oxidative chain is the reaction of RHS with lipid hydroperoxides, in which peroxyl and alkoxyl radicals are formed. The role of RHS-induced LPO in the development of human inflammatory diseases (cardiovascular and neurodegenerative diseases, cancer, diabetes, rheumatoid arthritis) is discussed in detail.

Nitric oxide and thiols: Chemical biology, signalling paradigms and vascular therapeutic potential

Nitric oxide (• NO) interactions with biological thiols play crucial, but incompletely determined, roles in vascular signalling and other biological processes. Here, we highlight two recently proposed signalling paradigms: (1) the formation of a vasodilating labile nitrosyl ferrous haem (NO-ferrohaem) facilitated by thiols via thiyl radical generation and (2) polysulfides/persulfides and their interaction with • NO. We also describe the specific (bio)chemical routes in which • NO and thiols react to form S-nitrosothiols, a broad class of small molecules, and protein post-translational modifications that can influence protein function through catalytic site or allosteric structural changes. S-Nitrosothiol formation depends upon cellular conditions, but critically, an appropriate oxidant for either the thiol (yielding a thiyl radical) or • NO (yielding a nitrosonium [NO+ ]-donating species) is required. We examine the roles of these collective • NO/thiol species in vascular signalling and their cardiovascular therapeutic potential.

Actions of Thiols, Persulfides, and Polysulfides as Free Radical Scavenging Antioxidants

Significance: The essential roles of thiol compounds as redox signaling mediators and protectors have been established. Recently, the roles of persulfides and polysulfides as mediators involved in numerous physiological processes have been revealed. Recent Advances: Recently, it became possible to detect and measure persulfides and polysulfides in human fluids and tissues and their physiological functions, including cellular signaling and protection against oxidative stress, have been reported, but the underlying mechanisms and dynamics remain elusive. Critical Issues: Physiological functions of thiol compounds have been studied, focusing primarily on two-electron redox reactions. In contrast, the contribution of one-electron redox mechanisms, that is, free radical-mediated oxidation and antioxidation, has received much less attention. Considering the important effects of free radical-mediated oxidation of biological molecules on pathophysiology, the antioxidant functions of thiol compounds as free radical scavengers are challenging issues. Future Directions: The antioxidant actions and dynamics of thiols, hydropersulfides, and hydropolysulfides as free radical scavenging antioxidants and their physiological significance remain to be established. Antioxid. Redox Signal. 39, 728-743.

Investigation into Propolis Components Responsible for Inducing Skin Allergy: Air Oxidation of Caffeic Acid and Its Esters Contribute to Hapten Formation

Propolis is a resin-like material produced by bees from the buds of poplar and cone-bearing trees and is used in beehive construction. Propolis is a common additive in various biocosmetics and health-related products, despite the fact that it is a well-known cause of contact allergy. Caffeic acid and its esters have been the primary suspects behind the sensitization potency of propolis-induced contact allergy. However, the chemical structures of the protein adducts formed between these haptens and skin proteins during the process of skin sensitization remain unknown. In this study, the reactivity of three main contact allergens found in propolis, namely, caffeic acid (CA), caffeic acid 1,1-dimethylallyl ester (CAAE), and caffeic acid phenethyl ester (CAPE), was investigated. These compounds were initially subjected to the kinetic direct peptide reactivity assay to categorize the sensitization potency of CA, CAAE, and CAPE, but the data obtained was deemed too unreliable to confidently classify their skin sensitization potential based on this assay alone. To further investigate the chemistry involved in generating possible skin allergy-inducing protein adducts, model peptide reactions with CA, CAAE, and CAPE were conducted and analyzed via liquid chromatography-high-resolution mass spectrometry. Reactions between CA, CAAE, and CAPE and a cysteine-containing peptide in the presence of oxygen, both in closed and open systems, were monitored at specific time points. These studies revealed the formation of two different adducts, one corresponding to thiol addition to the α,β-unsaturated carbonyl region of the caffeic structure and the second corresponding to thiol addition to the catechol, after air oxidation to o-quinone. Observation of these peptide adducts classifies these compounds as prehaptens. Interestingly, no adduct formation was observed when the same reactions were performed under oxygen-free conditions, highlighting the importance of air oxidation processes in CA, CAAE, and CAPE adduct formation. Additionally, through NMR analysis, we found that thiol addition occurs at the C-2 position in the aromatic ring of the CA derivatives. Our results emphasize the importance of air oxidation in the sensitization potency of propolis and shed light on the chemical structures of the resultant haptens which could trigger allergic reactions in vivo.

Visible Light Thiyl Radical-Mediated Desilylation of Arylsilanes

A straightforward, visible-light triggered desilylation of arylsilanes by thiyl radicals is presented. Silyl groups are often used to block a reactive position in multi-step organic synthesis, for which a mild cleavage at a late-stage will provide new possibilities and disconnection routes by C_Ar -Si cleavage/deprotection. In this work, commercially available and cheap disulfides are employed for the first time in this type of C(sp² )-Si bond cleavage reactions. Thus, upon irradiation with visible-light, homolytic cleavage of the disulfide give rise to the corresponding thiyl radical that allows for a radical chain mechanism. This methodology represents a mild, fast and simple approach suitable for a broad variety of simply substituted arylsilanes. Moreover, the procedure could be easily extended to natural products and therapeutic derivatives, showing its robustness and synthetic application potential.

Antioxidant action of persulfides and polysulfides against free radical-mediated lipid peroxidation

Hydrogen sulfide, hydropersulfides, and hydropolysulfides have been revealed to play important physiological roles such as cell signaling and protection against oxidative stress, but the underlying mechanisms and dynamics of action remain elusive. It is generally accepted that these species act by two-electron redox mechanisms, while the involvement of one-electron redox chemistry has received less attention. In this study, the radical-scavenging activity of hydrogen persulfide, hydrogen polysulfides (HS_nH n = 2-4), and diallyl- or dialkyl-sulfides (RS_nR, n = 1-4) was measured. Furthermore, their antioxidant effects against free radical-mediated human plasma lipid peroxidation were assessed by measuring lipid hydroperoxides. It was found that disodium disulfide, trisulfide, and tetrasulfide acted as potent peroxyl radical scavengers, the rate constant for scavenging peroxyl radical being 3.5 × 10⁵, 4.0 × 10⁵, and 6.0 × 10⁵ M^-1 s^-1 in PBS pH 7.4 at 37 °C respectively and that they inhibited plasma lipid peroxidation efficiently, the efficacy is increased with the catenation number. Disodium tetrasulfide was 1.5 times as reactive as Trolox toward peroxyl radical and inhibited plasma lipid peroxidation more efficiently than ascorbate and Trolox. On the other hand, diallyl- and dialkyl-sulfides did not exert significant radical-scavenging activity, nor did they inhibit lipid peroxidation efficiently, except for diallyl tetrasulfide, which suppressed plasma lipid peroxidation, despite less significantly than disodium tetrasulfide. Collectively, this study shows that hydrogen persulfide and hydrogen polysulfides act as potent radical-scavenging antioxidants and that, in addition to two-electron redox mechanisms, one electron redox reaction may also play important role in the in vivo defense against deleterious oxidative stress.

Reactive Sulfur Species (RSS) in Physiological and Pathological Conditions and in Therapy

Sulfur is a multivalent and nonmetallic chemical element with the symbol S and the atomic number 16 [...]