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.

Protein oxidation and aging

Organisms are constantly exposed to various forms of reactive oxygen species (ROS) that lead to oxidation of proteins, nucleic acids, and lipids. Protein oxidation can involve cleavage of the polypeptide chain, modification of amino acid side chains, and conversion of the protein to derivatives that are highly sensitive to proteolytic degradation. Unlike other types of modification (except cysteine oxidation), oxidation of methionine residues to methionine sulfoxide is reversible; thus, cyclic oxidation and reduction of methionine residues leads to consumption of ROS and thereby increases the resistance of proteins to oxidation. The importance of protein oxidation in aging is supported by the observation that levels of oxidized proteins increase with animal age. The age-related accumulation of oxidized proteins may reflect age-related increases in rates of ROS generation, decreases in antioxidant activities, or losses in the capacity to degrade oxidized proteins.

Oxidative Stress - Clinical Diagnostic Significance

Elevated free radical production and/or insufficient antioxidative defense results in cellular oxidant stress responses. Sustained and/or intense oxidative insults can overcome cell defenses resulting in accumulated damage to macromolecules, leading to loss of cell function, membrane damage, and ultimately to cell death. Oxidative stress (OS) can result from conditions including excessive physical stress, exposure to environmental pollution and xenobiotics, and smoking. Oxidative stress, as a pathophysiological mechanism, has been linked to numerous pathologies, poisonings, and the ageing process. Reactive oxygen species and reactive nitrogen species, endogenously or exogenously produced, can readily attack all classes of macromolecules (proteins, DNA, unsaturated fatty acid). The disrupted oxidative-reductive milieu proceeds via lipid peroxidation, altered antioxidative enzyme activities and depletion of non-enzymatic endogenous antioxidants, several of which can de detected in the pre-symptomatic phase of many diseases. Therefore, they could represent markers of altered metabolic and physiological homeostasis. Accordingly, from the point of view of routine clinical-diagnostic practice, it would be valuable to routinely analyze OS status parameters to earlier recognize potential disease states and provide the basis for preventative advance treatment with appropriate medicines.

Hydroxylation of 3-Nitrotyrosine and Its Derivatives by Gamma Irradiation

Radiation-induced hydroxylation of 3-nitrotyrosine (3-NY) and its derivatives in aqueous solution were investigated as a function of gamma-radiation dose. Irradiated 3-NY, 3-nitrotyrosine ethyl ester (3-NYE) and 3-NY containing peptide Gly-nitroTyr-Gly were separated and analyzed with reverse-phase HPLC and UV-Vis absorption spectroscopy. The structures of the hydroxylated products were confirmed by electrospray ionization mass spectrometry and (1)H-NMR spectrometry. The amounts of the hydroxylated products in irradiated 3-NY and Gly-nitroTyr-Gly solutions increased with increasing radiation dose. Tandem electrospray ionization mass spectrometry (ESI-Mass(2)) was performed to investigate the hydroxylation of peptide Gly-nitroTyr-Gly. These studies showed that the hydroxylation occurred at 3-NY residue. We also found that the identification of 3-NY hydroxylation in peptide could be identified by ion scanning for the specific immonium ion at m/z 197.0.

Suitability of Tryptophan Radiation Products as Markers for the Detection of γ-Irradiated Protein Rich Food

During gamma-irradiation (5 kGy) of aqueous tryptophan (Trp) solutions small amounts of 5-, 6-, and 7-hydroxytryptophan (OH-Trp) (0.04-0.08 Mol-%) are formed. Protein rich food like shrimps contain reasonable amounts of non-protein bound Trp (100 mg/kg). In order to detect the treatment of shrimps with gamma-irradiation a method for the determination of OH-Trp in gamma-irradiated shrimps was developed. After homogenization, squeezing of shrimp samples and protein precipitation, a two-step-SPE-clean up was performed using a C18-cartridge and a propylsulfonic acid cation-exchange SPE followed by HPLC analysis with electrochemical detection (750 mV). Results showed that 5-OH-Trp contents in shrimp samples increased with applied doses up to 3 kGy and then decreased with higher doses. Other OH-Trp isomers were not detectable in the irradiated shrimps. Similarly no formation of 4-, 6-, and 7-OH-Trp was detected in model solutions containing the same amino acid composition as in shrimps. This indicates a suppression of the reaction of OH-radicals with Trp by the 300 fold molar excess of other amino acids acting as well as radical scavengers. Therefore, non-physiological OH-Trp isomers formed from free Trp are not suitable as markers for the detection of gamma-irradiated protein-rich foodstuff.

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.

Metabolism of protein-bound DOPA in mammals

Protein-bound 3,4-dihydroxyphenylalanine (DOPA) can be generated in mammalian cells by both controlled enzymatic pathways, and by uncontrolled radical reactions. Protein-bound DOPA (PB-DOPA) has reducing activity and the capacity to inflict secondary damage on other important biomolecules such as DNA. This may be mediated through replenishment of transition metals or from catechol-quinone-catechol redox cycles in the presence of cellular components such as ascorbate or cysteine, resulting in amplification of radical damaging events. The generation of PB-DOPA confers on protein the ability to chelate transition metals generating protein 'oxychelates'; this may be amongst the factors, which localise such damage. Tissue levels of PB-DOPA are increased in a number of age-related pathologies such as atherosclerosis and cataract formation. We discuss the detoxification, and the subsequent proteolysis and excretion of components of PB-DOPA. We contrast the fact that in marine organisms, and particularly in extracellular proteins, PB-DOPA and other DOPA-polymers can play important functional roles in adhesion and the provision of tensile properties.

Isolation and analysis of dityrosine from enzyme-catalyzed oxidation of tyrosine and X-irradiated peptide and proteins

Dityrosine (DT) was isolated in a single-step by reversed-phase HPLC in 25% yield from enzyme-catalyzed oxidation of N-acetyl tyrosine followed by deacetylation. The isolated product was characterized by 1H NMR. A three-step chromatographic procedure was reported to facilitate the preparation of DT from the enzyme-catalyzed oxidation of tyrosine in 26% yield of theoretical maximum. Upon irradiation at 284 nm in acidic and 315 nm alkaline conditions, DT exhibits strong fluorescence at 400 nm-range. However, when excited at 300 nm-range, contribution of similar fluorescence by Trp oxidation and other protein modifications cannot be overruled. In order to identify the formation of DT unequivocally, Tyr was subjected to X-irradiation under nitrogen at pH 4 and labeled with dansyl chloride. HPLC conditions were devised to resolve dansylated DT from dansylated standard amino acids. Radiation-induced DT was identified by cochromatography with a dansylated, authentic sample of DT isolated and characterized from enzyme-catalyzed oxidation of Tyr. The formation of DT in the irradiated samples, determined by the integrated peak area, increased with dose (0-600 Gy). HPLC analysis of dansylated hydrolysate of the major product from an irradiated tripeptide (Tyr Gly Gly) detected Gly and DT (2:0.5). Extension of the model study to irradiated BSA and RNase A also showed DT as the major oxidation product of Tyr under the experimental conditions. Fluorescence signal of dansylated DT was linear from 0.5 pmol to 1.5 nmol (correlation coefficient 0.999, n = 3). The detection limit 0.5 pmol per 5 microliters injection hydrolysate corresponds to one molecule of DT per 300 molecules of BSA (BSA at 1 mg/ml). DT can be used as a marker for assessing oxidative damage of proteins. Most standard amino acid analysis techniques are limited to detect normal residues of proteins. The assay reported in the present study has potential for low-level detection of DT unequivocally and may be useful for monitoring oxidative stress-related physiological and pathological processes.

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

We have previously shown that exposure of many proteins, and free aromatic amino acids (particularly tyrosine) to free radical fluxes generates a stable activity capable of reducing protein bound and free transition metal ions. Here we define the capacity of several radical generating systems (gamma irradiation of water, UV irradiation, metal-dependent sugar autoxidation and Haber-Weiss systems) to produce protein-bound reducing moieties (PBRedM), and also reducing derivatives of tyrosine. Under the defined conditions of the gamma radiolysis system, reductive activity was generated under both oxic and anoxic irradiations and specific gassing regimes as well as the inclusion of specific radical scavengers established that hydroxyl radicals were responsible. When BSA was irradiated anoxically in the presence of formate a reductive activity related to the exposure of protein thiol groups was generated; all other reductive activities we detected were not thiol-related. Incubations of tyrosinase with BSA or insulin also generated reductive activity. All the conditions we have studied can convert tyrosine into DOPA and we suspect that protein-bound DOPA is the main reductive activity generated on proteins.

OH radical-induced products of tyrosine peptides

Reactions of radiation-generated OH radicals with tyrosine and its homopeptides, i.e. L-Tyr-L-Tyr and L-Tyr-L-Tyr-L-Tyr, in N2O saturated solutions were shown to give crosslinks between the peptide chains with high yields. High-performance liquid chromatography, capillary gas chromatography and mass spectrometry were used for isolation and identification of the monomeric and dimeric products. Evidence is presented for the crosslinking to occur through C-C and C-O-C bonds. To the best of our knowledge, the formation of the ether type of crosslink is demonstrated for the first time. Mechanisms of product formation are also discussed, which involve radicals that were described in previous pulse radiolysis studies.

Radiation-induced dimerization of tyrosine and glycyltyrosine in aqueous solutions

Products of radiolysis of tyrosine and glycyl-L-tyrosine in oxygen-free nitrogen, N2O or air saturated water solution, pH 4.0 or 8.6 were analysed in an amino acid analyser and upon separation on DEAE cellulose or BioGel P-2 column with spectrofluorimetry. Apart from dihydroxyphenylalanine tyrosine solution irradiated in nitrogen or N2O contained dityrosine, while that of irradiated glycyl-L-tyrosine contained glycyl-dityrosine-glycine. Both dimeric products were formed with radiation yields, of about 0.15 at low pH value. In addition an unknown, nonfluorescent but ninhydrin positive product was found in irradiated tyrosine solution. Another, unidentified product of radiolysis was detected in N2O or air saturated solutions of tyrosine. The product had a blue-green fluorescence and a molecular weight of about 500.