Modified methods for measuring citrulline and carbamoyl-beta-alanine with reduced light sensitivity and sucrose interference

The formation of an anti-restenotic/anti-thrombotic surface by immobilization of nitric oxide synthase on a metallic carrier

Coronary stenosis due to atherosclerosis, the primary cause of coronary artery disease, is generally treated by balloon dilatation and stent implantation, which can result in damage to the endothelial lining of blood vessels. This leads to the restenosis of the lumen as a consequence of migration and proliferation of smooth muscle cells (SMCs). Nitric oxide (NO), which is produced and secreted by vascular endothelial cells (ECs), is a central anti-inflammatory and anti-atherogenic player in the vasculature. The goal of the present study was to develop an enzymatically active surface capable of converting the prodrug l-arginine, to the active drug, NO, thus providing a targeted drug delivery interface. NO synthase (NOS) was chemically immobilized on the surface of a stainless steel carrier with preservation of its activity. The ability of this functionalized NO-producing surface to prevent or delay processes involved in restenosis and thrombus formation was tested. This surface was found to significantly promote EC adhesion and proliferation while inhibiting that of SMCs. Furthermore, platelet adherence to this surface was markedly inhibited. Beyond the application considered here, this approach can be implemented for the local conversion of any systemically administered prodrug to the active drug, using catalysts attached to the surface of the implant.

Methods for the detection of peptidylarginine deiminase (PAD) activity and protein citrullination

The post-translational conversion of peptidylarginine to peptidylcitrulline, a process also known as citrullination, is catalyzed by the enzyme family of peptidylarginine deiminases (PADs) and has been demonstrated to be involved in many physiological processes, including the regulation of gene expression. In addition, citrullination has been shown to be associated with several diseases, such as cancer, multiple sclerosis, rheumatoid arthritis, and Alzheimer's disease. To get more insight into the role of PAD enzymes and citrullination in both health and disease, experimental strategies to study PAD activity and to characterize citrullinated proteins in complex biological samples are crucial. Here, we describe the chemical, proteomic and antibody-based procedures that are currently available and discuss their applicability for the analysis of complex samples. The methods that have been developed can be used to provide more insight in the substrate specificity of PAD enzymes. Because the evidence that PADs play a pathophysiological role in the diseases mentioned above is increasing, they become attractive targets for therapeutic interventions. More knowledge of PAD specificity and the availability of reliable, high-throughput assays for PAD activity will facilitate the development of highly specific PAD inhibitors.

Targeted analysis of protein citrullination using chemical modification and tandem mass spectrometry

Protein citrullination originates from enzymatic deimination of polypeptide-bound arginine and is involved in various biological processes during health and disease. However, tools required for a detailed and targeted proteomic analysis of citrullinated proteins in situ, including their citrullination sites, are limited. A widely used technique for detection of citrullinated proteins relies on antibody staining after specific derivatization of citrulline residues by 2,3-butanedione and antipyrine. We have recently reported on the details of this reaction. Here, we show that this chemical modification can be utilized to specifically detect and identify citrullinated peptides and their citrullination sites by liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis. Using model compounds, we demonstrate that in collision-induced dissociation (CID) a specific, modification-derived fragment ion appears as the dominating signal at m/z 201.1 in the MS/MS spectra. When applying electron transfer dissociation (ETD), however, the chemical modification of citrulline remained intact and extensive sequence coverage allowed identification of peptides and their citrullination sites. Therefore, LC/MS/MS analysis with alternating CID and ETD has been performed, using CID for specific, signature ion-based detection of derivatized citrullinated peptides and ETD for sequence determination. The usefulness of this targeted analysis was demonstrated by identifying citrullination sites in myelin basic protein deiminated in vitro. Combining antibody-based enrichment of chemically modified citrulline-containing peptides with specific mass spectrometric detection will increase the potential of such a targeted analysis of protein citrullination in the future.

Specific modification of peptide-bound citrulline residues

Immune reactions to citrulline-containing proteins appear to be central in the immunopathogenesis of rheumatoid arthritis. Citrulline residues are introduced into proteins by deimination of arginine residues, likely by an enzymatic process. There is a need to characterize which proteins in the inflamed joints of rheumatoid patients contain citrulline in situ. The characterization of deiminated proteins will be greatly facilitated by specific modification of peptide-bound citrulline residues that will enable specific enrichment and detection of citrulline-containing peptides. This study presents the details of such a modification method. The chemistry behind the reaction of the ureido group of citrulline with 2,3-butanedione in the presence of antipyrine is unraveled. Parameters for optimization of the reaction with respect to specificity and completeness, including the testing of different acids, reactant concentrations, and reaction time, are presented. This modification reaction is specific for citrulline residues. The modified product shows a characteristic mass shift of +238Da, as demonstrated by mass spectrometry. The product absorbs UV-Vis radiation at 464nm, and it is demonstrated that this can be used to selectively monitor citrulline-containing peptides during the separation of protein digests. Finally, the structure of the product of modified citrulline is solved by nuclear magnetic resonance spectroscopy using N-butylurea as a model substance. The results presented should facilitate the development of tags that can be used for the enrichment and subsequent detection of citrulline-containing protein fragments by mass spectrometry.

Serum Ornithine Carbamoyl Transferase as a Surrogate Marker in Malaria

Ornithine carbamoyl transferase (OCT) activity and other liver function tests were studied in a total of 50 patients of clinical malaria and 15 controls. They were grouped as group I (positive for malarial parasite on peripheral blood smear, n=18), group II (negative for malarial parasite on peripheral blood smear (PBS) but responded to antimalarials, n=17) and group III (peripheral blood smear negative and did not respond to antimalarial therapy, n=15). The mean OCT levels were significantly raised in group I (6.79 ± 1.84 IU/L, p value = 0.006) and group II (5.0 ± 1.15 IU/L, p value = 0.014) as compared to controls (2.5 ± 1.13 IU/L) and returned to normal after treatment In contrast, group III had normal levels except in a case of kala azar and septicemia where OCT levels were high and increased further on treatment. Taking PBS positivity as a gold standard of diagnostic criteria, OCT had a sensitivity of 83% and specificity of 86% with a high positive predictive value of 88% as compared to ALT which had a lower sensitivity of 55% and specificity of 80%. The clinical response rate in PBS negative cases of fever having high OCT level was 83% as compared to 35% in cases with normal OCT level, making OCT a good surrogate marker of malaria. OCT levels could also be of prognostic significance as 2 cases of cerebral malaria had high OCT levels of 11.1 UAL and 10.7 IU/L, respectively.

A colorimetric 96-well microtiter plate assay for the determination of enzymatically formed citrulline

l-Citrulline constitutes a product of a number of enzymatic reactions. In the past a number of colorimetric methods for the determination of l-citrulline, upon its chemical modification with diacetyl monoxime at 95 degrees C, have been reported. However, all these methods are time- and material-consuming. In this work, using the same chemical reaction, a new method for the use in 96-well polystyrene microtiter plates was developed. The method is fast and requires substantially less material as the enzymatic reaction is performed in a volume of 60 microl. The applicability of this enzymatic assay was established using l-N(omega), N(omega)-dimethylarginine dimethylaminohydrolase, which generates l-citrulline from side-chain methylated derivatives of l-arginine. The detection limit for l-citrulline is about 0.2 nmol. In addition, our studies show that most commonly used biochemical buffers and buffer additives do not affect the assay. This method may prove useful in the studies of other l-citrulline producing enzymes including nitric oxide synthase.

Colorimetric determination of citrulline residues in proteins

A method is described for the direct colorimetric determination of citrulline residues in proteins based on the reaction with diacetylmonoxime in the presence of lower concentrations of sulfuric acid. The reduced sensitivity due to the lower acid concentration was overcome by the addition of ferric chloride which also contributed to the color stabilization. Insoluble proteins or proteins resulting in turbidity can be analyzed following partial hydrolysis of those with enzyme or acid. The molar absorption coefficient (epsilon) for citrulline at 464 nm was 2.8 x 10(4). The method enabled us to determine low levels of protein-bound citrulline which are beyond the limitations of conventional methods using an amino acid analyzer. We determined the citrulline contents in the cornified cells of the epidermis of newborn rats, as well as soybean trypsin inhibitor.

Amplified nitric oxide production by pulmonary alveolar macrophages of newborn rats

Oxygen (O2)-dependent and O2-independent antimicrobial mechanisms are used by alveolar macrophages (AM) to maintain lung sterility, but these mechanisms are underdeveloped in neonatal AM. Nitric oxide (NO(.)), a more recently described antimicrobial and immunomodulating molecule, has not been studied in neonatal AM. Lavaged AM from 3-day-old, 10-day-old, maternal and adult rats were treated with or without lipopolysaccharide (LPS) and/or interferon-γ (IFN-γ) and NO(.) synthase activity was measured as its L-arginine metabolites: NO2(-), NO3(-), and citrulline. Superoxide anion (O2(.-)) production by suspended macrophages, initiated by either opsonized zymosan or phorbol, was used as a marker of O2-dependent antimicrobial activity. Lysozyme content of AM was measured as a component of O2-independent antimicrobial activity. Unstimulated 3-day-old macrophages generated >10-fold more NO2(-) + NO3(-) than did 10-day-old, maternal or adult AM. Twenty hours after LPS and IFN-γ stimulation, 3-day-old AM produced > 2 times more NO2(-) and NO3(-) than did the more mature macrophages. Basal and stimulated O2(.-) release was similar among 3-day-old, 10-day-old and adult AM, while lysozyme concentrations were > 4-fold higher in adult macrophages compared to AM from 3-day-old pups. Rather than having a role in NO(.)-dependent antimicrobial activity, we propose that newborn AM have amplified NO(.) production to modulate their own differentiation and replication after birth. The age-dependent differences in NO(.) synthase expression by AM may lend insight into the regulation of this important enzyme.

Age-related increase in peptidylarginine deiminase in the male rat pituitary

We have measured the activity of peptidylarginine deiminase (EC 3.5.3.15) in male Wistar rat pituitaries at various ages. Pituitaries obtained from 3- and 9-month-old rats showed negligible activities. The mean enzyme activity increased appreciably by 18 months and markedly by 24 months accompanied with actual increases in the enzyme content. The peptidylarginine deiminase mRNA content showed a similar but more gradual increase appreciable from 9 months. Many enzyme-positive cells were present in the pars distalis of 24-month-old male pituitaries. Most of the enzyme-positive cells coincided with lactotrophs. The pituitary prolactin content showed a gradually increasing profile resembling that of the enzyme mRNA, but the serum prolactin concentration did not increase significantly. Neither the serum 17 beta-estradiol content nor the pituitary estrogen receptor content showed significant variation that could account for the marked increase in the pituitary enzyme content between 18 and 24 months of age. These data suggest possible presence of other factors regulating the enzyme content in old male pituitaries.

Fluorescence lifetimes of the tryptophan residues in ornithine transcarbamoylase

Multifrequency (2-230 MHz) phase-modulation fluorescence measurements and site-directed mutagenesis have been employed to assign fluorescence lifetimes, quantum yields, and emission maxima to the four tryptophans in the enzyme ornithine transcarbamoylase from Escherichia coli (OTCase) (Trp-125, -92, -233, and -243). OTCase displays two apparent fluorescence lifetimes, 7.2 and 3.2 ns. Results on specific mutants show that Trp-233 has a lifetime of 7.1 ns, while TRP-125, -192, and -243 have lifetimes of 4.0, 3.6, and 4.9 ns, respectively. Thus, the specific conformational changes of the polypeptide segment involving Trp-233 may be monitored conveniently in the wild-type enzyme. On the basis of quantum yield values, Trp-233 is calculated to contribute approximately 43% of the fluorescence intensity of the enzyme, while direct measurements of the enzyme show that up to 65% of the total intensity is really emitted by this tryptophan. The discrepancy may arise from energy transfer from Trp-125 to Trp-233, with an efficiency of 20%. Application of the assigned tryptophan lifetimes to probe ligand-induced protein conformational changes has also been demonstrated.

Detection of citrulline residues in deiminated proteins on polyvinylidene difluoride membrane

We have developed a new detection method of deiminated proteins on polyvinylidene difluoride membranes. Citrulline residues in enzymatically deiminated histones were modified by incubating with diacetyl monoxime and antipyrine in a strong acid mixture. The products were injected to rabbits, and the antibodies obtained were affinity-purified using a modified citrulline column. Sample proteins blotted to the membrane were modified in a similar manner and incubated successively with the purified antibody and an alkaline phosphatase-conjugated second antibody. Detection was performed using a chemiluminescent substrate. The method enabled detection of 3-10 fmol of citrulline residues dot blotted as deiminated model proteins. It visualized numerous rat pituitary soluble proteins that had been enzymatically deiminated and Western blotted to the membrane. The data suggest usefulness of the method for detecting deiminated proteins regardless of the backbone protein molecules. Search for deiminated proteins on the Western blots of various rat tissue homogenates detected a single band on that of spinal cord, another band on that of uterus, and multiple bands on those of skin and hair root. The bands in the former two tissue homogenates comigrated with glial fibrillary acidic protein and vimentin, respectively.