One-electron oxidation and reduction of glycosaminoglycan chloramides: a kinetic study

Characterization of disulfide (cystine) oxidation by HOCl in a model peptide: Evidence for oxygen addition, disulfide bond cleavage and adduct formation with thiols

Disulfide bonds play a key role in stabilizing proteins by cross-linking secondary structures. Whilst many disulfides are effectively unreactive, it is increasingly clear that some disulfides are redox active, participate in enzymatic reactions and/or regulate protein function by allosteric mechanisms. Previously (Karimi et al., Sci. Rep. 2016, 6, 38752) we have shown that some disulfides react rapidly with biological oxidants due to favourable interactions with available lone-pairs of electrons. Here we present data from kinetic, mechanistic and product studies for HOCl-mediated oxidation of a protected nine-amino acid model peptide containing a N- to C-terminal disulfide bond. This peptide reacts with HOCl with k₂ 1.8 × 10⁶ M^-1 s^-1, similar to other highly-reactive disulfide-containing compounds. With low oxidant excesses, oxidation yields multiple oxidation products from the disulfide, with reaction predominating at the N-terminal Cys to give sulfenic, sulfinic and sulfonic acids, and disulfide bond cleavage. Limited oxidation occurs, with higher oxidant excesses, at Trp and His residues to give mono- and di- (for Trp) oxygenated products. Site-specific backbone cleavage also occurs between Arg and Trp, probably via initial side-chain modification. Treatment of the previously-oxidised peptide with thiols (GSH, N-Ac-Cys), results in adduction of the thiol to the oxidised peptide, with this occurring at the original disulfide bond. This gives an open-chain peptide, and a new mixed disulfide containing GSH or N-Ac-Cys as determined by mass spectrometry. Disulfide bond oxidation may therefore markedly alter the structure, activity and function of disulfide-containing proteins, and provides a potential mechanism for protein glutathionylation.

Free radical studies of components of the extracellular matrix: contributions to protection of biomolecules and biomaterials from sterilising doses of ionising radiation

The purpose of the current review is show how the principles and techniques of radiation chemistry have enabled the direct reactions of free radicals with biomolecules and biomaterials to be investigated at the molecular level. In particular, the review focusses on the free radical-induced fragmentation of glycosaminoglycans. Glycosaminoglycans are large linear polysaccharides consisting of repeating disaccharide units and are important components of the extracellular matrix (ECM) either in free form (hyaluronan) or as a component of proteoglycans. Oxidative damage of the extracellular matrix components by either enzymatic or non-enzymatic pathways may have implications for the initiation and progression of a range of human diseases. These include arthritis, kidney disease, cardiovascular disease, lung disease, periodontal disease and chronic inflammation. Oxidative damage to hyaluronan by reactive oxidative species and thus the potential mechanism of damage to the ECM and its role in human pathologies is reviewed with particular focus on damage initiated by potential in vivo free radicals such as superoxide, carbonate and hydroxyl radicals. Such knowledge has also allowed radiation protecting systems to be developed so that sterilising doses of radiation can be delivered to sensitive biomolecules such as proteins and glycosaminoglycans, and also to sensitive biomaterials such as tissue allografts.

Oxidation of glycosaminoglycans by free radicals and reactive oxidative species: A review of investigative methods

Glycosaminoglycans, in particular hyaluronan (HA), and proteoglycans are components of the extracellular matrix (ECM). The ECM plays a key role in the regulation of cellular behaviour and alterations to it can modulate both the development of human diseases as well as controlling normal biochemical processes such as cell signalling and pro-inflammatory responses. For these reasons, in vitro fragmentation studies of glycosaminoglycans by free radicals and oxidative species are seen to be relevant to the understanding of in vivo studies of damage to the ECM. A wide range of investigative techniques have therefore been applied to gain insights into the relative fragmentation effects of several reactive oxidative species with the ultimate goal of determining mechanisms of fragmentation at the molecular level. These methods are reviewed here.

The kinetics and mechanism of the oxidation of pyruvate ion by hypochlorous acid

Kinetic experiments and DFT calculations confirm a concerted oxygen atom transfer mechanism for the oxidation of pyruvic acid by HOCl.

Glycosaminoglycan degradation by selected reactive oxygen species

SIGNIFICANCE

Inflammatory diseases (such as arthritis) of the extracellular matrix (ECM) are of considerable socioeconomic significance. There is clear evidence that reactive oxygen species (ROS) and nitrogen species released by, for instance, neutrophils contribute to the degradation of the ECM. Here we will focus on the ROS-induced degradation of the glycosaminoglycans, one important component of the ECM.

RECENT ADVANCES

The recently developed "anti-TNF-α" therapy is primarily directed against neutrophilic granulocytes that are powerful sources of ROS. Therefore, a more detailed look into the mechanisms of the reactions of these ROS is reasonable.

CRITICAL ISSUES

Since both enzymes and ROS contribute to the pathogenesis of inflammatory diseases, it is very difficult to estimate the contributions of the individual species in a complex biological environment. This particularly applies as many products are not stable but only transient products that decompose in a time-dependent manner. Thus, the development of suitable analytical methods as well as the establishment of useful biomarkers is a challenging aspect.

FUTURE DIRECTIONS

If the mechanisms of ECM destruction are understood in more detail, then the development of suitable drugs to treat inflammatory diseases will be hopefully much more successful.

The mechanisms and physiological relevance of glycocalyx degradation in hepatic ischemia/reperfusion injury

SIGNIFICANCE

Hepatic ischemia/reperfusion (I/R) injury is an inevitable side effect of major liver surgery that can culminate in liver failure. The bulk of I/R-induced liver injury results from an overproduction of reactive oxygen and nitrogen species (ROS/RNS), which inflict both parenchymal and microcirculatory damage. A structure that is particularly prone to oxidative attack and modification is the glycocalyx (GCX), a meshwork of proteoglycans and glycosaminoglycans (GAGs) that covers the lumenal endothelial surface and safeguards microvascular homeostasis. ROS/RNS-mediated degradation of the GCX may exacerbate I/R injury by, for example, inducing vasoconstriction, facilitating leukocyte adherence, and directly activating innate immune cells.

RECENT ADVANCES

Preliminary experiments revealed that hepatic sinusoids contain a functional GCX that is damaged during murine hepatic I/R and major liver surgery in patients. There are three ROS that mediate GCX degradation: hydroxyl radicals, carbonate radical anions, and hypochlorous acid (HOCl). HOCl converts GAGs in the GCX to GAG chloramides that become site-specific targets for oxidizing and reducing species and are more efficiently fragmented than the parent molecules. In addition to ROS/RNS, the GAG-degrading enzyme heparanase acts at the endothelial surface to shed the GCX.

CRITICAL ISSUES

The GCX seems to be degraded during major liver surgery, but the underlying cause remains ill-defined.

FUTURE DIRECTIONS

The relative contribution of the different ROS and RNS intermediates to GCX degradation in vivo, the immunogenic potential of the shed GCX fragments, and the role of heparanase in liver I/R injury all warrant further investigation.

Efficiencies of fragmentation of glycosaminoglycan chloramides of the extracellular matrix by oxidizing and reducing radicals: potential site-specific targets in inflammation?

Hypochlorous acid and its conjugate base, hypochlorite ions, produced under inflammatory conditions, may produce chloramides of glycosaminoglycans, these being significant components of the extracellular matrix (ECM). This may occur through the binding of myeloperoxidase directly to the glycosaminoglycans. The N-Cl group in the chloramides is a potential selective target for both reducing and oxidizing radicals, leading possibly to more efficient and damaging fragmentation of these biopolymers relative to the parent glycosaminoglycans. To investigate the effect of the N-Cl group, we used ionizing radiation to produce quantifiable concentrations of the reducing radicals, hydrated electron and superoxide radical, and also of the oxidizing radicals, hydroxyl, carbonate, and nitrogen dioxide, all of which were reacted with hyaluronan and heparin and their chloramides in this study. PAGE gels calibrated for molecular weight allowed the consequent fragmentation efficiencies of these radicals to be calculated. Hydrated electrons were shown to produce fragmentation efficiencies of 100 and 25% for hyaluronan chloramide (HACl) and heparin chloramide (HepCl), respectively. The role of the sulfate group in heparin in the reduction of fragmentation can be rationalized using mechanisms proposed by M.D. Rees et al. (J. Am. Chem. Soc.125:13719-13733; 2003), in which the initial formation of an amidyl radical leads rapidly to a C-2 radical on the glucosamine moiety. This is 100% efficient at causing glycosidic bond breakage in HACl but only 25% efficient in HepCl, the role of the sulfate group being to favor the nonfragmentary routes for the C-2 radical. The weaker reducing agent, the superoxide radical, did not cause fragmentation of either HACl or HepCl although kinetic reactivity had been demonstrated in earlier studies. Experiments using the oxidizing radicals, hydroxyl and carbonate, both potential in vivo species, showed significant increases in fragmentation efficiencies for both HACl and HepCl, relative to the parent molecules. The carbonate radical was shown to be involved in site-specific reactions at the N-Cl groups, reacting via abstraction of Cl, to produce the same amidyl radical produced by one-electron reductants such as the hydrated electron. As for the hydrated electrons, the data support fragmentation efficiencies of 100 and 29% for reaction of carbonate radicals at N-Cl for HACl and HepCl, respectively. For the weaker oxidant, nitrogen dioxide, no fragmentation was observed, probably because of a low kinetic reactivity and low reduction potential. It seems likely therefore that the N-Cl group can direct damage to extracellular matrix glycosaminoglycan chloramides, which may be produced under inflammatory conditions. The in vivo species, the carbonate radical, is also much more likely to be site-specific in its reactions with such components of the ECM than the hydroxyl radical.