Formation of 6-nitrotryptophan in purified proteins by reactive nitrogen species: A possible new biomarker

Exposure of tropoelastin to peroxynitrous acid gives high yields of nitrated tyrosine residues, di-tyrosine cross-links and altered protein structure and function

Elastin is an abundant extracellular matrix protein in elastic tissues, including the lungs, skin and arteries, and comprises 30-57% of the aorta by dry mass. The monomeric precursor, tropoelastin (TE), undergoes complex processing during elastogenesis to form mature elastic fibres. Peroxynitrous acid (ONOOH), a potent oxidising and nitrating agent, is formed in vivo from superoxide and nitric oxide radicals. Considerable evidence supports ONOOH formation in the inflamed artery wall, and a role for this species in the development of human atherosclerotic lesions, with ONOOH-damaged extracellular matrix implicated in lesion rupture. We demonstrate that TE is highly sensitive to ONOOH, with this resulting in extensive dimerization, fragmentation and nitration of Tyr residues to give 3-nitrotyrosine (3-nitroTyr). This occurs with equimolar or greater levels of oxidant and increases in a dose-dependent manner. Quantification of Tyr loss and 3-nitroTyr formation indicates extensive Tyr modification with up to two modified Tyr per protein molecule, and up to 8% conversion of initial ONOOH to 3-nitroTyr. These effects were modulated by bicarbonate, an alternative target for ONOOH. Inter- and intra-protein di-tyrosine cross-links have been characterized by mass spectrometry. Examination of human atherosclerotic lesions shows colocalization of 3-nitroTyr with elastin epitopes, consistent with TE or elastin modification in vivo, and also an association of 3-nitroTyr containing proteins and elastin with lipid deposits. These data suggest that exposure of TE to ONOOH gives marked chemical and structural changes to TE and altered matrix assembly, and that such damage accumulates in human arterial tissue during the development of atherosclerosis.

Importance of tryptophan nitration of carbonic anhydrase III for the morbidity of atopic dermatitis

The nitration of proteins results from the vigorous production of reactive nitrogen species in inflammatory disease. We previously reported the proteomic analysis of nitrated tryptophan residues in in vitro model cells for inflammatory diseases using a 6-nitrotryptophan-specific antibody. In this paper, we applied this method to the analysis of a disease model animal and identified the 6-nitrotryptophan-containing proteins in the skin of atopic dermatitis model mice (AD-NC/Nga mice). We found three nitrotryptophan-containing proteins, namely, carbonic anhydrase III (CAIII), α-enolase (α-ENO), and cytoskeletal keratin type II (KTII), and identified the positions of the nitrotryptophan residues in their amino acid sequences: Trp47 and Trp123 in CAIII, Trp365 in α-ENO, and Trp221 in KTII. Among these, the nitration of CAIII was increased not only in the lesional skin of AD-NC/Nga mice but also in the mice that did not present any symptoms. The in vitro nitration of purified CAIII by peroxynitrite reduced its CO2 hydratase activity in a dose-dependent manner. In addition, we found that CAIII was induced during the differentiation of normal human epidermal keratinocytes. Furthermore, we found the presence of CAIII and the formation of 6-nitrotryptophan-containing proteins in both the lesional and the nonlesional sections of the skin of patients with atopic dermatitis through immunohistochemical staining. This study provides the first demonstration of the formation of 6-nitrotryptophan in human tissues and disease.

Proteomic analysis of endogenous nitrotryptophan-containing proteins in rat hippocampus and cerebellum

Nitration of tryptophan residues is a novel post-translational modification. In the present study, we examined whether NO2Trp (nitrotryptophan)-containing proteins are produced in the hippocampus and cerebellum of the adult rat under physiological conditions in vivo. Using Western blot analysis with anti-6-NO2Trp-specific antibody, we found many similar immunoreactive spots in the protein extracts from both regions. These spots were subsequently subjected to trypsin digestion and LC-ESI-MS/MS (LC-electrospray ionization-tandem MS) analysis. We identified several cytoskeletal proteins and glycolytic enzymes as NO2Trp-containing proteins and determined the position of nitrated tryptophan residues with significant ion score levels (P<0.05) in several proteins in both regions. We also observed that the total amount of NO2Trp-containing proteins in the cerebellum was significantly greater than that in the hippocampus (P<0.05). Moreover, IP (immunoprecipitation) assays using anti-aldolase C antibody showed that the relative intensity of immunostaining for NO2Trp over aldolase C was much higher in cerebellum than in hippocampus. The amounts of nNOS (neuronal nitric oxide synthase) and eNOS (endothelial nitric oxide synthase) were much greater in cerebellum than in hippocampus. This is the first evidence of several specific sites of nitrated tryptophan in proteins under physiological conditions in vivo.

Protein nitrotryptophan: formation, significance and identification

Reactive nitrogen species are formed during a variety of disease states and have been shown to modify several amino acids on proteins. To date, the majority of research in this area has focused on the nitration of tyrosine residues to form 3-nitrotyrosine. However, emerging evidence suggests that another modification, nitration of tryptophan residues, to form nitrotryptophan (NO(2)-Trp), may also play a significant role in the biology of nitrosative stress. This review takes an in-depth look at NO(2)-Trp, presenting the current research about its formation, prevalence and biological significance, as well as the methods used to identify NO(2)-Trp-modified proteins. Although more research is needed to understand the full biological role of NO(2)-Trp, the data presented herein suggest a contribution to nitrosative stress-induced cell dysregulation and perhaps even in physiological cell processes.

Probing the conformation and orientation of adsorbed enzymes using side-chain modification

The bioactivity of enzymes that are adsorbed on surfaces can be substantially influenced by the orientation of the enzyme on the surface and adsorption-induced changes in the enzyme's structure. Circular dichroism (CD) is a powerful method for observing the secondary structure of proteins; however, it provides little information regarding the tertiary structure of a protein or its adsorbed orientation. In this study, we developed methods using side-chain-specific chemical modification of solvent-exposed tryptophan residues to complement CD spectroscopy and bioactivity assays to provide greater detail regarding whether changes in enzyme bioactivity following adsorption are due to adsorbed orientation and/or adsorption-induced changes in the overall structure. These methods were then applied to investigate how adsorption influences the bioactivity of hen egg white lysozyme (HEWL) and glucose oxidase (GOx) on alkanethiol self-assembled monolayers over a range of surface chemistries. The results from these studies indicate that surface chemistry significantly influences the bioactive state of each of these enzymes but in distinctly different ways. Changes in the bioactive state of HEWL are largely governed by its adsorbed orientation, while the bioactive state of adsorbed GOx is influenced by a combination of both adsorbed orientation and adsorption-induced changes in conformation.