The kinetics of hydroxyl-radical-induced strand breakage of hyaluronic acid. A pulse radiolysis study using conductometry and laser-light-scattering

On the Mechanism and Kinetics of Synthesizing Polymer Nanogels by Ionizing Radiation-Induced Intramolecular Crosslinking of Macromolecules

Nanogels-internally crosslinked macromolecules-have a growing palette of potential applications, including as drug, gene or radioisotope nanocarriers and as in vivo signaling molecules in modern diagnostics and therapy. This has triggered considerable interest in developing new methods for their synthesis. The procedure based on intramolecular crosslinking of polymer radicals generated by pulses of ionizing radiation has many advantages. The substrates needed are usually simple biocompatible polymers and water. This eliminates the use of monomers, chemical crosslinking agents, initiators, surfactants, etc., thus limiting potential problems with the biocompatibility of products. This review summarizes the basics of this method, providing background information on relevant aspects of polymer solution thermodynamics, radiolysis of aqueous solutions, generation and reactions of polymer radicals, and the non-trivial kinetics and mechanism of crosslinking, focusing on the main factors influencing the outcomes of the radiation synthesis of nanogels: molecular weight of the starting polymer, its concentration, irradiation mode, absorbed dose of ionizing radiation and temperature. The most important techniques used to perform the synthesis, to study the kinetics and mechanism of the involved reactions, and to assess the physicochemical properties of the formed nanogels are presented. Two select important cases, the synthesis of nanogels based on polyvinylpyrrolidone (PVP) and/or poly(acrylic acid) (PAA), are discussed in more detail. Examples of recent application studies on radiation-synthesized PVP and PAA nanogels in transporting drugs across the blood-brain barrier and as targeted radioisotope carriers in nanoradiotherapy are briefly described.

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.

In vivo cell wall loosening by hydroxyl radicals during cress seed germination and elongation growth

Loosening of cell walls is an important developmental process in key stages of the plant life cycle, including seed germination, elongation growth, and fruit ripening. Here, we report direct in vivo evidence for hydroxyl radical ((*)OH)-mediated cell wall loosening during plant seed germination and seedling growth. We used electron paramagnetic resonance spectroscopy to show that (*)OH is generated in the cell wall during radicle elongation and weakening of the endosperm of cress (Lepidium sativum; Brassicaceae) seeds. Endosperm weakening precedes radicle emergence, as demonstrated by direct biomechanical measurements. By (3)H fingerprinting, we showed that wall polysaccharides are oxidized in vivo by the developmentally regulated action of apoplastic (*)OH in radicles and endosperm caps: the production and action of (*)OH increased during endosperm weakening and radicle elongation and were inhibited by the germination-inhibiting hormone abscisic acid. Both effects were reversed by gibberellin. Distinct and tissue-specific target sites of (*)OH attack on polysaccharides were evident. In vivo (*)OH attack on cell wall polysaccharides were evident not only in germinating seeds but also in elongating maize (Zea mays; Poaceae) seedling coleoptiles. We conclude that plant cell wall loosening by (*)OH is a controlled action of this type of reactive oxygen species.

Oxidative damage to extracellular matrix and its role in human pathologies

The extracellular compartments of most biological tissues are significantly less well protected against oxidative damage than intracellular sites and there is considerable evidence for such compartments being subject to a greater oxidative stress and an altered redox balance. However, with some notable exceptions (e.g., plasma and lung lining fluid) oxidative damage within these compartments has been relatively neglected and is poorly understood. In particular information on the nature and consequences of damage to extracellular matrix is lacking despite the growing realization that changes in matrix structure can play a key role in the regulation of cellular adhesion, proliferation, migration, and cell signaling. Furthermore, the extracellular matrix is widely recognized as being a key site of cytokine and growth factor binding, and modification of matrix structure might be expected to alter such behavior. In this paper we review the potential sources of oxidative matrix damage, the changes that occur in matrix structure, and how this may affect cellular behavior. The role of such damage in the development and progression of inflammatory diseases is discussed.

Degradation of matrix glycosaminoglycans by peroxynitrite/peroxynitrous acid: evidence for a hydroxyl-radical-like mechanism

The oxidant peroxynitrite/peroxynitrous acid (ONOO-/ONOOH) is generated at sites of inflammation via reaction of O2.- with .NO. Previous studies have shown that these species can oxidize cellular targets, but few data are available on damage to extracellular matrix and its components, despite evidence for matrix modification in a number of pathologies. In the current study we show that reaction of ONOO-/ONOOH with glycosaminoglycans results in extensive polymer fragmentation. Bolus authentic ONOO-/ONOOH modifies hyaluronan, heparin, and chondroitin, dermatan, and heparan sulfates, in a concentration-dependent, but O2-independent, manner. The ONOO-/ONOOH generator 3-(4-morpholinyl)sydnoneimine produces similar time- and concentration-dependent damage. These reactions generate specific polymer fragments via cleavage at disaccharide intervals. Studies at different pH values, and in the presence of bicarbonate, are consistent with ONOOH, rather than the carbonate adduct, CO3.- or ONOO-, being the source of damage. EPR spin trapping experiments have provided evidence for the formation of carbon-centered radicals on glycosaminoglycans and related monosaccharides; the similarity of these spectra to those obtained with authentic HO. is consistent with fragmentation being induced by this oxidant. These data suggest that extracellular matrix fragmentation at sites of inflammation may be due, in part, to the formation and reactions of ONOOH.

Degradative action of reactive oxygen species on hyaluronan

Many human diseases are associated with harmful action of reactive oxygen species (ROS). These species are involved in the degradation of essential tissue or related components. One of such components is synovial fluid that contains a high-molecular-weight polymer--hyaluronan (HA). Uninhibited and/or inhibited hyaluronan degradation by the action of various ROS has been studied in many in vitro models. In these studies, the change of the molecular weight of HA or a related parameter, such as HA solution viscosity, has been used as a marker of inflicted damage. The aim of the presented review is to briefly summarize the available data. Their correct interpretation could contribute to the implementation of modern methods of evaluation of the antioxidative capacity of natural and synthetic substances and prospective drugs--potential inflammatory disease modifying agents. Another focus of this review is to evaluate briefly the impact of different available analytical techniques currently used to investigate the structure of native high-molecular-weight hyaluronan and/or of its fragments.

Chain scission of hyaluronan by carbonate and dichloride radical anions: potential reactive oxidative species in inflammation?

The reactions of the carbonate and dichloride radical anions, CO3- and Cl2-, with the extracellular matrix glycosaminoglycan hyaluronan (HA) have been studied using the kinetic technique of pulse radiolysis and also by steady-state irradiation combined with gel permeation chromatography/multiangle laser light scattering(gpc/MALLS) to measure the rates of reaction with HA and the yield of HA chain scission, respectively. For comparison, the same measurements were made for the reactions of the free radicals *OH, Br2*-, and N3*. The carbonate and dichloride radical anions were found to react relatively quickly with HA (7.0 x 10(5) and 6.9 x 10(6) dm3 mol(-1) s(-1), respectively) although they are much less reactive than the hydroxyl radical, *OH. Significant yields (20 and 38%, respectively) of chain scission of HA by these radical anions were also determined from the gpc/MALLS experiments, providing some support for their potential participation in the depolymerization of HA in vivo. These results are compared with data obtained for the other free radicals (hydroxyl, azide radicals, and dibromide radical anions) investigated in this study in order to gain an insight into their mechanism of reaction with HA. Earlier chain scission yields of HA by hydroxyl radicals determined by the authors have also been revised using the gpc/MALLS technique employed in the current study. The yields of 52% (absence of air) and 44% (in air) are much lower than the previous values. In the current study, the effect of oxygen on the yields of HA chain breaks is discussed in terms of the reactivity of HA peroxyl radicals in the presence of superoxide radical anions. The relevance of the results of this study to mechanisms of inflammation is discussed.

Chain scission of hyaluronan by peroxynitrite

The reaction of peroxynitrite with the biopolymer hyaluronan has been studied using stopped-flow techniques combined with detection of molecular weight changes using the combination of gel permeation chromatography and multiangle laser light scattering. From the effect of peroxynitrite on the yield of hyaluronan chain breaks, it was concluded that the chain breaks were caused by hydroxyl radicals which escape a cage containing the *OH NO*(2) radical pair. The yield of free hydroxyl radicals was determined as 5+/-1% (as a proportion of the total peroxynitrite concentration). At high peroxynitrite concentrations, it was observed that the yield of chain breaks leveled out, an effect largely attributable to the scavenging of hydroxyl radicals by nitrite ions present in the peroxynitrite preparation. These experiments also provided some support for a previous proposal that the adduct formed between ONOOH and ONOO(-) might itself produce hydroxyl radicals. The rate of this reaction would have to be of the order of 0.05 s(-1) to produce hydroxyl radical yields that would account quantitatively for chain break yields at high peroxynitrite concentrations. By carrying out experiments at higher hyaluronan concentrations, it was also concluded that an additional yield of chain breaks was produced by the bimolecular reaction of the polymer with ONOOH at a rate constant of about 10 dm(3)mol(-1)s(-1). At 5.3 x 10(-3)mol dm(-3) hyaluronan, this amounted to 3.5% chain breaks (per peroxynitrite concentration). These conclusions support the proposal that the yield of hydroxyl radicals arising from the isomerization of ONOOH to nitrate ions is relatively low.

Good genes, oxidative stress and condition-dependent sexual signals

The immune and the detoxication systems of animals are characterized by allelic polymorphisms, which underlie individual differences in ability to combat assaults from pathogens and toxic compounds. Previous studies have shown that females may improve offspring survival by selecting mates on the basis of sexual ornaments and signals that honestly reveal health. In many cases the expression of these ornaments appears to be particularly sensitive to oxidative stress. Activated immune and detoxication systems often generate oxidative stress by an extensive production of reactive metabolites and free radicals. Given that tolerance or resistance to toxic compounds and pathogens can be inherited, female choice should promote the evolution of male ornaments that reliably reveal the status of the bearers' level of oxidative stress. Hence, oxidative stress may be one important agent linking the expression of sexual ornaments to genetic variation in fitness-related traits, thus promoting the evolution of female mate choice and male sexual ornamentation, a controversial issue in evolutionary biology ever since Darwin.

Reactivity of cartilage and selected carbohydrates with hydroxyl radicals: an NMR study to detect degradation products

It was investigated to what extent isolated, monomeric and polymeric carbohydrates as well as cartilage specimens are affected by hydroxyl radicals generated by gamma-irradiation or Fenton reaction and what products can be detected by means of NMR spectroscopy. Resonances of all protons in glucose and other monosaccharides as well as carbon resonances in 13C-enriched glucose were continuously diminished upon gamma-irradiation. Formate and malondialdehyde were found as NMR detectable products in irradiated glucose solutions under physiologically relevant (aerated) conditions. In polysaccharide solutions (e.g. hyaluronic acid) gamma-irradiation and also treatment with the Fenton reagent caused first an enhancement of resonances according to mobile N-acetyl groups at 2.02 ppm. This indicates a breakdown of glycosidic bonds in polysaccharides. Using higher radiation doses or higher concentrations of the Fenton reagent formate was also detected. The same sequence of events was observed upon treatment of bovine nasal cartilage with the Fenton reagent. First, glycosidic linkages in cartilage polysaccharides were cleaved and subsequently formate was formed. In contrast, collagen of cartilage was affected only to a very low extent. Thus, HO-radicals caused the same action on cartilage as on isolated polymer solutions, inducing a fragmentation of polysaccharides and the formation of formate.

Radiation chemistry in the 1990s: pressing questions relating to the areas of radiation biology and environmental research

This preview discusses the possible future application of ionizing radiation in the study of the free-radical chemistry of some aqueous systems. With respect to the present state of knowledge in the area of chemistry related to radiobiology, ionizing-radiation damage to DNA will continue to be a major focus of radiobiological interest. It is noted that the purine-base, free-radical chemistry is still poorly understood, as is the chemistry of the direct effect of ionizing radiation on DNA. The role of transition metal ions, their interaction with hydrogen peroxide and the superoxide radical are areas of increasing interest, as are other reactions of the superoxide radical. The eventual chemical effects, on DNA, of the reactions of these species resemble those of the action of ionizing radiation and are therefore apt to be studied radiation-chemically. In contaminated water pollution control, procedures termed 'advanced oxidation processes' are gaining in importance. They are based on the action of the .OH radical. Radiation techniques will provide the tools for understanding the underlying reactions better.