Free radical and enzymatic mechanisms for the generation of protein bound reducing moieties

Peroxynitrous acid induces structural and functional modifications to basement membranes and its key component, laminin

Basement membranes (BM) are specialized extracellular matrices underlying endothelial cells in the artery wall. Laminin, the most abundant BM glycoprotein, is a structural and biologically active component. Peroxynitrous acid (ONOOH), a potent oxidizing and nitrating agent, is formed in vivo at sites of inflammation from superoxide and nitric oxide radicals. Considerable data supports ONOOH formation in human atherosclerotic lesions, and an involvement of this oxidant in atherosclerosis development and lesion rupture. These effects may be mediated, at least in part, via extracellular matrix damage. In this study we demonstrate co-localization of 3-nitrotyrosine (a product of tyrosine damage by ONOOH) and laminin in human atherosclerotic lesions. ONOOH-induced damage to BM was characterized for isolated murine BM, and purified murine laminin-111. Exposure of laminin-111 to ONOOH resulted in dose-dependent loss of protein tyrosine and tryptophan residues, and formation of 3-nitrotyrosine, 6-nitrotryptophan and the cross-linked material di-tyrosine, as detected by amino acid analysis and Western blotting. These changes were accompanied by protein aggregation and fragmentation as detected by SDS-PAGE. Endothelial cell adhesion to isolated laminin-111 exposed to 10 μM or higher levels of ONOOH was significantly decreased (~25%) compared to untreated controls. These data indicate that laminin is oxidized by equimolar or greater concentrations of ONOOH, with this resulting in structural and functional changes. These modifications, and resulting compromised cell-matrix interactions, may contribute to endothelial cell dysfunction, a weakening of the structure of atherosclerotic lesions, and an increased propensity to rupture.

3,4-Dihydroxy-l-phenylalanine as a biomarker of oxidative damage in proteins: improved detection using cloud-point extraction and HPLC

Oxidized protein adducts are formed under conditions of oxidative stress and may represent a valuable biomarker for a variety of diseases which share this common aetiology. A suitable candidate biomarker for oxidized proteins is protein-bound 3,4-dihydroxyl-l-phenylalanine (l-DOPA), which is formed on 3'-hydroxylation of tyrosine residues by hydroxyl radicals. Existing methodologies to measure protein-bound l-DOPA employ lengthy acid hydrolysis steps (ca. 16h) which may cause artifactual protein oxidation, followed by HPLC with detection based on the intrinsic fluorescence of l-DOPA. We report a novel method for the measurement of protein-bound l-DOPA which involves rapid hydrolysis followed by pre-column concentration of 6-aminoquinolyl-derivatives using cloud-point extraction. The derivatized material is resolved by reversed-phase HPLC in less than 30min and has derivatization chemistry compatible with both UV and fluorescent detection, providing detection down to the femtomole level. The method provides identical results to those found with highly specific ELISA-based techniques and requires only basic instrumentation. The stability of the 6-aminoquinolyl-derivatives together with the fast and sensitive nature of the assay will be appealing to those who require large sample throughput.

Redox control of enzymatic functions: The electronics of life's circuitry

The field of redox biology has changed tremendously over the past 20 years. Formerly regarded as bi-products of the aerobic metabolism exclusively involved in tissue damage, reactive oxygen species (ROS) are now recognized as active participants of cell signaling events in health and in disease. In this sense, ROS and the more recently defined reactive nitrogen species (RNS) are, just like hormones and second messengers, acting as fundamental orchestrators of cell signaling pathways. The chemical modification of enzymes by ROS and RNS (that result in functional enzymatic alterations) accounts for a considerable fraction of the transient and persistent perturbations imposed by variations in oxidant levels. Upregulation of ROS and RNS in response to stress is a common cellular response that foments adaptation to a variety of physiologic alterations (hypoxia, hyperoxia, starvation, and cytokine production). Frequently, these are beneficial and increase the organisms' resistance against subsequent acute stress (preconditioning). Differently, the sustained ROS/RNS-dependent rerouting of signaling produces irreversible alterations in cellular functioning, often leading to pathogenic events. Thus, the duration and reversibility of protein oxidations define whether complex organisms remain "electronically" healthy. Among the 20 essential amino acids, four are particularly susceptible to oxidation: cysteine, methionine, tyrosine, and tryptophan. Here, we will critically review the mechanisms, implications, and repair systems involved in the redox modifications of these residues in proteins while analyzing well-characterized prototypic examples. Occasionally, we will discuss potential consequences of amino acid oxidation and speculate on the biologic necessity for such events in the context of adaptative redox signaling. © 2014 IUBMB Life, 66(3):167-181, 2014.

Free and Peptide-bound DOPA Can Inhibit Initiation of Low Density Lipoprotein Oxidation

Hydroxyl radicals have been shown to convert free tyrosine to 3,4-dihydroxyphenyl-alanine (DOPA) which has reducing properties. During protein or peptide oxidation such reducing species are also formed from tyrosine residues. Free DOPA or peptide-bound DOPA (PB-DOPA) is able to promote radical-generating events, facilitating the damage of biomolecules such as nucleic acids. Radical induced lipid oxidation in low density lipoprotein (LDL) transforms the lipoprotein into an atherogenic particle. As PB-DOPA has been found in atherosclerotic plaques, we tested the ability of free and PB-DOPA to influence LDL oxidation. Free DOPA, in contrast to tyrosine had strong inhibitory action on both, the copper-ion initiated and metal ion independent (AAPH-induced) lipid oxidation. Free DOPA also inhibited LDL oxidation induced by the copper transport protein ceruloplasmin. To test if PB-DOPA was also able to inhibit LDL oxidation, DOPA residues were generated enzymatically in the model peptides insulin and tyr-tyr-tyr, respectively. PB-DOPA formation substantially increased the ability of both molecules to inhibit LDL oxidation by copper or AAPH. We hypothesize that DOPA-peptides and -proteins may have the potential to act as efficacious antioxidants in the atherosclerotic plaque.

Oxidative activation of antioxidant defence

Living cells maintain a delicate balance between oxidizing and reducing species, and many disorders such as rheumatoid arthritis and Alzheimer's disease have been associated with a disturbed intracellular 'redox equilibrium'. The past few years have witnessed accelerated research into how natural redox responses and antioxidant defence systems are activated and how they restore a healthy redox balance. To function properly, many of these processes rely on a powerful sulfur redox chemistry, which is best exemplified by the complex, newly emerging cysteine-based redox regulation of the glutathione and thioredoxin pathways. Other redox systems based on oxidatively activated amino acid side chains in proteins are also becoming increasingly important, but are still barely understood or explored.

Nonprotein-bound iron and plasma protein oxidative stress at birth

We previously reported plasma nonprotein-bound iron (NPBI) as a reliable early indicator of intrauterine oxidative stress (OS) and brain injury. We tested the hypothesis that albumin, an NPBI serum carrier, is the major target of NPBI-induced OS. Twenty-four babies were randomly selected from 384 newborns constituting the final cohort of a prospective study undertaken to evaluate the predictive role of NPBI in cord blood for neurodevelopmental outcome. Twelve were selected in the group with lowest NPBI levels (0-1.16 microM) and good neurodevelopmental outcome and 12 in the group with highest NPBI levels (>or=15.2 microM) and poor neurodevelopmental outcome. Protein carbonyl groups were identified in cord blood samples by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and Western blotting with anti-2,4-dinitrophenyl (DNP) antibodies. Two series of immunoreactive spots, corresponding to serum albumin and alpha-fetoprotein, were found only in the group with highest NPBI levels. We found an association between NPBI and carbonylated proteins in babies with highest NPBI levels. Since NPBI may produce hydroxyl radicals through the Fenton reaction, the major target of OS induced by NPBI is its carrier: albumin. Oxidation of albumin can be expected to decrease plasma antioxidant defenses and increase the likelihood of tissue damage due to OS in the newborns.

Proteolytic-antiproteolytic balance and its regulation in carcinogenesis

Cancer development is essentially a tissue remodeling process in which normal tissue is substituted with cancer tissue. A crucial role in this process is attributed to proteolytic degradation of the extracellular matrix (ECM). Degradation of ECM is initiated by proteases, secreted by different cell types, participating in tumor cell invasion and increased expression or activity of every known class of proteases (metallo-, serine-, aspartyl-, and cysteine) has been linked to malignancy and invasion of tumor cells. Proteolytic enzymes can act directly by degrading ECM or indirectly by activating other proteases, which then degrade the ECM. They act in a determined order, resulting from the order of their activation. When proteases exert their action on other proteases, the end result is a cascade leading to proteolysis. Presumable order of events in this complicated cascade is that aspartyl protease (cathepsin D) activates cysteine proteases (e.g., cathepsin B) that can activate pro-uPA. Then active uPA can convert plasminogen into plasmin. Cathepsin B as well as plasmin are capable of degrading several components of tumor stroma and may activate zymogens of matrix metalloproteinases, the main family of ECM degrading proteases. The activities of these proteases are regulated by a complex array of activators, inhibitors and cellular receptors. In physiological conditions the balance exists between proteases and their inhibitors. Proteolytic-antiproteolytic balance may be of major significance in the cancer development. One of the reasons for such a situation is enhanced generation of free radicals observed in many pathological states. Free radicals react with main cellular components like proteins and lipids and in this way modify proteolytic-antiproteolytic balance and enable penetration damaging cellular membrane. All these lead to enhancement of proteolysis and destruction of ECM proteins and in consequence to invasion and metastasis.

Protein hydroperoxides are a major product of low density lipoprotein oxidation during copper, peroxyl radical and macrophage-mediated oxidation

Damage to apoB100 on low density lipoprotein (LDL) has usually been described in terms of lipid aldehyde derivatisation or fragmentation. Using a modified FOX assay, protein hydroperoxides were found to form at relatively high concentrations on apoB100 during copper, 2,2'-azobis(amidinopropane) dihydrochloride (AAPH) generated peroxyl radical and cell-mediated LDL oxidation. Protein hydroperoxide formation was tightly coupled to lipid oxidation during both copper and AAPH-mediated oxidation. The protein hydroperoxide formation was inhibited by lipid soluble alpha-tocopherol and the water soluble antioxidant, 7,8-dihydroneopterin. Kinetic analysis of the inhibition strongly suggests protein hydroperoxides are formed by a lipid-derived radical generated in the lipid phase of the LDL particle during both copper and AAPH mediated oxidation. Macrophage-like THP-1 cells were found to generate significant protein hydroperoxides during cell-mediated LDL oxidation, suggesting protein hydroperoxides may form in vivo within atherosclerotic plaques. In contrast to protein hydroperoxide formation, the oxidation of tyrosine to protein bound 3,4-dihydroxyphenylalanine (PB-DOPA) or dityrosine was found to be a relatively minor reaction. Dityrosine formation was only observed on LDL in the presence of both copper and hydrogen peroxide. The PB-DOPA formation appeared to be independent of lipid peroxidation during copper oxidation but tightly associated during AAPH-mediated LDL oxidation.

Perfluorocarbons attenuate oxidative lung damage

The aim of this study was to investigate the effect of tidal liquid ventilation (TLV) compared to conventional mechanical ventilation (CMV) on oxidative lung damage in the setting of acute respiratory distress syndrome (ARDS). After repeated lung lavages, 10 minipigs were treated with CMV or TLV for 4 hr before the animals were sacrificed. Samples for blood gas analysis and bronchial aspirate samples were withdrawn before the induction of lung injury, and at 10 min, 2 hr, and 4 hr after the beginning of ventilatory support. To assess lung oxidative damage, total hydroperoxide (TH) and advanced oxidation protein product (AOPP) concentrations were measured in bronchial aspirate samples. After 2 and 4 hr of ventilatory support, partial oxygen tension (PaO(2)) and base excess (BE) were significantly higher in the TLV group than in the CMV group, while PaCO(2) was slightly higher, but with no statistical significance. In the CMV group, the AOPP level was significantly higher at 4 hr than at baseline. TH and AOPP bronchial aspirate concentrations were higher in the CMV group than in the TLV group at 2 and 4 hr of ventilation. We conclude that animals treated with TLV showed lower oxidative lung damage compared to animals treated with CMV.

Protection of U937 cells from free radical damage by the macrophage synthesized antioxidant 7,8-dihydroneopterin

Interferon-gamma stimulation of human macrophages causes the synthesis and release of neopterin and its reduced form 7,8-dihydroneopterin (7,8-NP). The purpose of this cellular response is undetermined but in vitro experiments suggests 7,8-NP is an antioxidant. We have found 7,8-NP can protect monocyte-like U937 cells from oxidative damage. 7,8-NP inhibited ferrous ion and hypochlorite mediated loss of cell viability. Fe++ mediated lipid peroxidation was effectively inhibited by 7,8-NP, however, no correlation was found between peroxide concentration and cell viability. Hypochlorite was scavenged by 7,8-NP, preventing the loss of cell viability. 7,8-NP was less effective in inhibiting H2O2-mediated loss of cell viability with significant inhibition only occurring at high 7,8-NP concentrations. Analysis of cellular protein hydrolysates showed none of the oxidants caused the formation of any protein bound DOPA or dityrosine but did show 7,8-NP prevented the loss of cellular tyrosine by HOCl. Our data suggests macrophages may synthesize 7,8-NP for antioxidant protection during inflammatory events in vivo.

Generation and propagation of radical reactions on proteins

The oxidation of proteins by free radicals is thought to play a major role in many oxidative processes within cells and is implicated in a number of human diseases as well as ageing. This review summarises information on the formation of radicals on peptides and proteins and how radical damage may be propagated and transferred within protein structures. The emphasis of this article is primarily on the deleterious actions of radicals generated on proteins, and their mechanisms of action, rather than on enzymatic systems where radicals are deliberately formed as transient intermediates. The final section of this review examines the control of protein oxidation and how such damage might be limited by antioxidants.

Protein oxidation in aging and age-related diseases

Although different theories have been proposed to explain the aging process, it is generally agreed that there is a correlation between aging and the accumulation of oxidatively damaged proteins, lipids, and nucleic acids. Oxidatively modified proteins have been shown to increase as a function of age. Studies reveal an age-related increase in the level of protein carbonyl content, oxidized methionine, protein hydrophobicity, and cross-linked and glycated proteins as well as the accumulation of less active enzymes that are more susceptible to heat inactivation and proteolytic degredation. Factors that decelerate protein oxidation also increase the life span of animals and vice versa. Furthermore, a number of age-related diseases have been shown to be associated with elevated levels of oxidatively modified proteins. The chemistry of reactive oxygen species-mediated protein modification will be discussed. The accumulation of oxidatively modified proteins may reflect deficiencies in one or more parameters of a complex function that maintains a delicate balance between the presence of a multiplicity of prooxidants, antioxidants, and repair, replacement, or elimination of biologically damaged proteins.

Total hydroperoxide and advanced oxidation protein products in preterm hypoxic babies

Previous studies have shown that plasma lipoproteins are a common target of free radical-induced oxidative stress in hypoxic newborn infants. In contrast to lipids, the reaction of proteins with various oxidants during hypoxia has not been extensively studied. We tested the hypothesis that tissue hypoxia results in increased production of protein oxidation in cord blood of preterm newborns. Heparinized blood samples of 39 hypoxic and 16 control preterm newborns were obtained from the umbilical vein, after cord clamping immediately after delivery. Plasma levels of total hydroperoxide (TH), advanced oxidation protein products (AOPP), hypoxanthine (Hx), xanthine (Xa), and uric acid (UA) were measured. Higher Hx, Xa, UA, TH, and AOPP levels were found in hypoxic newborn infants than in controls. Statistically significant correlations were observed between: TH and Hx (r = 0.54, p = 0.003, n = 28), AOPP and Hx (r = 0.64, p = 0.0001, n = 27), and TH and AOPP plasma levels (r = 0.50, p = 0.02, n = 21). In summary, TH, AOPP, Hx, Xa, and UA production is increased in fetal blood during hypoxia. The more severe the hypoxia, the higher the lipid and protein damage by free radicals.

L-DOPA-quinone inactivates tryptophan hydroxylase and converts the enzyme to a redox-cycling quinoprotein

Tryptophan hydroxylase, the initial and rate limiting enzyme in the biosynthesis of serotonin (5-HT), is inactivated by the quinone of L-DOPA. L-DOPA itself has no effect on enzyme activity. The inactivation of tryptophan hydroxylase could be prevented by glutathione (GSH), dithiothreitol, cysteine, and ascorbic acid but not by scavengers of hydrogen peroxide (catalase), hydroxyl radical (DMSO), or superoxide (superoxide dismutase). All cysteinyl residues within tryptophan hydroxylase are modified after treatment with L-DOPA-quinone as revealed by loss of DTNB-reactivity and formation of cysteinyl-DOPA residues. L-DOPA-quinone also converts tryptophan hydroxylase to a redox-cycling quinoprotein. These results suggest a possible mechanism of 5-HT neuronal damage in Parkinson's Disease by a redox-cycling quinoprotein.

Tyrosine hydroxylase is inactivated by catechol-quinones and converted to a redox-cycling quinoprotein: possible relevance to Parkinson's disease

Quinone derivatives of DOPA, dopamine, and N-acetyldopamine inactivate tyrosine hydroxylase, the initial and rate-limiting enzyme in the biosynthesis of the catecholamine neurotransmitters. The parent catechols are inert in this capacity. The effects of the catecholquinones on tyrosine hydroxylase are prevented by antioxidants and reducing reagents but not by scavengers of hydrogen peroxide, hydroxyl radicals, or superoxide radicals. Quinone modification of tyrosine hydroxylase modifies enzyme sulfhydryl groups and results in the formation of cysteinyl-catechols within the enzyme. Catecholquinones convert tyrosine hydroxylase to a redox-cycling quinoprotein. Quinotyrosine hydroxylase causes the reduction of the transition metals iron and copper and may therefore contribute to Fenton-like reactions and oxidative stress in neurons. The discovery that a phenotypic marker for catecholamine neurons can be converted into a redox-active species is highly relevant for neurodegenerative conditions such as Parkinson's disease.

New paramagnetic species formed at the expense of the transient tyrosyl radical in mutant protein R2 F208Y of Escherichia coli ribonucleotide reductase

The highly conserved residue F208 in protein R2 of E. coli ribonucleotide reductase is close to the binuclear iron center, and found to be involved in stabilizing the tyrosyl radical Y122. in wild type R2. Upon the reconstitution reaction of the mutant R2 F208Y with ferrous iron and molecular oxygen, we observed a new EPR singlet signal (g = 2.003) formed concomitantly with decay of the transient tyrosyl radical Y122. (g = 2.005). This new paramagnetic species (denoted Z) was stable for weeks at 4 degrees C and visible by EPR only below 50 K. The EPR singlet could not be saturated by available microwave power, suggesting that Z may be a mainly metal centered species. The maximum amount of the compound Z in the protein purified from cells grown in rich medium was about 0.18 unpaired spin/R2. An identical EPR signal of Z was found also in the double mutant R2 F208Y/Y122F. In the presence of high concentration of sodium ascorbate, the amounts of both the transient Y122. and the new species Z increased considerably in the reconstitution reaction. The results suggest that Z is most likely an oxo-ferryl species possibly in equilibrium with a Y208 ligand radical.

The protein oxidation product 3,4-dihydroxyphenylalanine (DOPA) mediates oxidative DNA damage

A major product of hydroxy-radical addition to tyrosine is 3, 4-dihydroxyphenylalanine (DOPA) which has reducing properties. Protein-bound DOPA (PB-DOPA) has been shown to be a major component of the stable reducing species formed during protein oxidation under several conditions. The aim of the present work was to investigate whether DOPA, and especially PB-DOPA, can mediate oxidative damage to DNA. We chose to generate PB-DOPA using mushroom tyrosinase, which catalyses the hydroxylation of tyrosine residues in protein. This permitted us to study the reactions of PB-DOPA in the virtual absence of other protein-bound oxidation products. The formation of two oxidation products of DNA, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8oxodG) and 5-hydroxy-2'-deoxycytidine (5OHdC), were studied with a novel HPLC using gradient elution and an electrochemical detection method, which allowed the detection of both DNA modifications in a single experiment. We found that exposure of calf thymus DNA to DOPA or PB-DOPA resulted in the formation of 8oxodG and 5OHdC, with the former predominating. The formation of these DNA oxidation products by either DOPA or PB-DOPA depended on the presence of oxygen, and also on the presence and on the concentration of transition metal ions, with copper being more effective than iron. The yields of 8oxodG and 5OHdC increased with DOPA concentration in proteins. Thus PB-DOPA was able to promote further radical-generating events, which then transferred damage to other biomolecules such as DNA.

Decreased antioxidant defense mechanisms in rat liver after methanol intoxication

The primary metabolic fate of methanol is oxidation to formaldehyde and then to formate by enzymes of the liver. Cytochrome P-450 and a role for the hydroxyl radical have been implicated in this process. The aim of the paper was to study the liver antioxidant defense system in methanol intoxication, in doses of 1.5, 3.0 and 6.0 g/kg b.w., after methanol administration to rats. In liver homogenates, the activities of Cu,Zn-superoxide dismutase and catalase were significantly increased after 6 h following methanol ingestion in doses of 3.0 and 6.0 g/kg b.w. and persisted up to 2-5 days, accompanied by significant decrease of glutathione reductase and glutathione peroxidase activities. The content of GSH was significantly decreased during 6 hours to 5 days. The liver ascorbate level was significantly diminished, too, while MDA levels were considerably increased after 1.5, 3.0 and 6.0 g/kg b.w. methanol intoxication. Changes due to methanol ingestion may indicate impaired antioxidant defense mechanisms in the liver tissue.

Cysteine-dependent 5-S-cysteinyldopa formation and its regulation by glutathione in normal epidermal melanocytes

Recent evidence suggests that the melanogenesis intermediate 5-S-cysteinyldopa (5-S-CD) could display antioxidative activity. In the present study, the synthesis of 5-S-CD was examined in human epidermal melanocytes isolated from dark skin type VI (MT) and from white skin type III (GT). The MT melanocytes showed the higher melanin content and dopa oxidase activity. In addition, they produced eumelanin as shown by their ultrastructure, and the solubility and UV/visible absorption of the isolated pigment. Both MT and GT cells showed high levels of 5-S-CD (5.5-6.9 nmol/mg protein). 5-S-CD was also detected in culture supernatants from MT cells; the secretion rate was estimated to be 2.5 nmol/mg protein per 24 h. The role of cysteine and glutathione in 5-S-CD formation was investigated by exposing the melanocytes to the gamma-glutamylcysteine synthetase inhibitor L-buthionine sulfoximine (BSO). A strong reduction in glutathione levels (4-8% of the untreated controls) associated with an increase in cysteine levels (152-154%) was observed. In addition, BSO induced a moderate increase in the cellular levels of 5-S-CD (114-129%) and a decrease in dopa oxidase activity (75-83%). Our results indicate that the direct addition of cysteine to dopaquinone is the main source of 5-S-CD in human epidermal melanocytes. It is proposed that the synthesis of 5-S-CD is a mechanism regulating dopaquinone levels during pigment formation and/or a defence mechanism against oxidative stress.

The iron-selective chelator desferal can reduce chelated copper

It was shown that the iron-selective chelator desferal (desferrioxamine mesylate:DFO) can reduce Cu(II) as judged by measuring the formation of the complex between Cu(I) and a specific chelator for this species, neocuproine (NC), in phosphate buffer. It was found that under optimal conditions, 3 moles of Cu(II) could be reduced per mole of DFO. Studies of the kinetics of Cu(II) reduction by DFO revealed that the rate of Cu(II) reduction by DFO was considerably slower than that by ascorbate. In the case of both reductants, even in the absence of NC, Cu(I) complexes remained in aqueous solutions for at least 30 min. DFO could also reduce Cu complexed to histidine. The results presented highlight the interpretive dangers which can arise in studies involving multiple transition metals, especially in the presence of multiple chelators. Specifically, when desferal is used, it is important to be aware that any copper present may become reduced, and that any Cu(I) formed might participate in ongoing redox reactions.

Absence of increased susceptibility of LDL to oxidation in type 1 diabetics

Insulin-dependent diabetics have a greatly increased risk of developing premature coronary artery disease which is not entirely explained by known risk factors. A possible explanation may be enhanced oxidative modification of low density lipoprotein (LDL). The aim of this study was to determine firstly, whether or not LDL from moderately well controlled type 1 diabetics is more readily oxidisable than LDL from healthy non-diabetics and, secondly, to assess whether potential predictors of LDL oxidisability differ between type 1 diabetics and controls. Twenty type 1 diabetic men were carefully matched with healthy non-diabetic men on the basis of age and body mass index and each pair attended the department on the same morning for blood sampling. LDL oxidisability was assessed using both copper in PBS, 15 and 30 mM glucose, and with AAPH. There was no difference between type 1 diabetics and controls in the susceptibility of the LDL to either copper-dependent or non-transition metal-dependent oxidation. Furthermore, there was no difference between the groups for LDL vitamin E content, LDL fatty acid composition in cholesteryl esters, triglycerides or phospholipids, or LDL copper reductive capacity, but LDL glycation was elevated in the IDDM subjects. Given the absence of increased LDL oxidisability in these subjects, the recommendation of vitamin E supplementation in type 1 diabetics should be considered a secondary priority to achieving adequate glucose control.

Protein hydroperoxides can give rise to reactive free radicals

Proteins damaged by free-radical-generating systems in the presence of oxygen yield relatively long-lived protein hydroperoxides. These hydroperoxides have been shown by e.p.r. spectroscopy to be readily degraded to reactive free radicals on reaction with iron(II) complexes. Comparison of the observed spectra with those obtained with free amino acid hydroperoxides had allowed identification of some of the protein-derived radical species (including a number of carbon-centred radicals, alkoxyl radicals and a species believed to be the CO2 radical anion) and the elucidation of novel fragmentation and rearrangement processes involving amino acid side chains. In particular, degradation of hydroperoxide functions on the side chain of glutamic acid is shown to result in decarboxylation at the side-chain carboxy group via the formation of the CO2 radical anion; the generation of an identical radical from hydroperoxide groups on proteins suggests that a similar process occurs with these molecules. In a number of cases these fragmentation and rearrangement reactions give rise to further reactive free radicals (R., O2-./HO2., CO2-.) which may act as chain-carrying species in protein oxidations. These studies suggest that protein hydroperoxides are capable of initiating further radical chain reactions both intra- and inter-molecularly, and provide information on some of the fundamental mechanisms of protein alteration and side-chain fragmentation.

A newly discovered pathway of melanin formation in cultured retinal pigment epithelium of cattle

According to a recent hypothesis the melanin granules in the retinal pigment epithelium of mammals originate from photosensory membrane degradation. To test this hypothesis the retinal pigment epithelium of cattle was kept in tissue culture and exposed to gold-labelled rod outer segments. Gold granules were later detected inside phagosomes, melanosomes and mature melanin granules. Tyrosinase, the key enzyme in melanogenesis, was additionally localized inside phagosomes. These results indicate that in cultured retinal pigment epithelium the matrix of the melanosome can originate from phagosomes. Therefore, the melanosome is a specialized lysosome.

Reactive species and their accumulation on radical-damaged proteins

Hydroperoxides and catechols are described as novel reactive products of radical attack on proteins. These species, like other components of oxidized and otherwise damaged proteins, may accumulate in some biological systems. We propose that the reactive species may then attack other biomolecules, and constitute both a marker and a mechanism of age-related pathologies.

Does melanin turnover occur in the eyes of adult vertebrates?

This paper is a review of what is known about the turnover of melanin in iris, choroid, and retinal pigment epithelium (RPE) of the adult vertebrate eye. Differences in size and structure of choroideal and retinal pigment epithelial melanin granules are shown by electron micrographs. The classical stages of melanin synthesis, including the premelanosome, are shown in the RPE of adult hamsters that had been exposed to intense light. Degradation or synthesis of melanin also seem to occur in the melanocytes of the choroid in these animals. It is postulated that all three pigmented eye tissues (iris, RPE, and choroid) of adult vertebrates form melanin granules in vivo. However, nothing is known about the amount of this turnover.