Study of the fragmentation of arginine isobutyl ester applied to arginine quantification in Aedes aegypti mosquito excreta

Structural determination of arginine-linked cisplatin complexes via IRMPD action spectroscopy: arginine binds to platinum via NO- binding mode

Cisplatin, (NH₃)₂PtCl₂, has been known as a successful metal-based anticancer drug for more than half a century. Its analogue, Argplatin, arginine-linked cisplatin, (Arg)PtCl₂, is being investigated because it exhibits reactivity towards DNA and RNA that differs from that of cisplatin. In order to understand the basis for its altered reactivity, the deprotonated and sodium cationized forms of Argplatin, [(Arg-H)PtCl₂]^- and [(Arg)PtCl₂ + Na]⁺, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy in the IR fingerprint and hydrogen-stretching regions. Complementary electronic structure calculations are performed using density functional theory approaches to characterize the stable structures of these complexes and to predict their infrared spectra. Comparison of the theoretical IR spectra predicted for various stable conformations of these Argplatin complexes to their measured IRMPD spectra enables determination of the binding mode(s) of Arg to the Pt metal center to be identified. Arginine is found to bind to Pt in a bidentate fashion to the backbone amino nitrogen and carboxylate oxygen atoms in both the [(Arg-H)PtCl₂]^- and [(Arg)PtCl₂ + Na]⁺ complexes, the NO^- binding mode. The neutral side chain of Arg also interacts with the Pt center to achieve additional stabilization in the [(Arg-H)PtCl₂]^- complex. In contrast, Na⁺ binds to both chlorido ligands in the [(Arg)PtCl₂ + Na]⁺ complex and the protonated side chain of Arg is stabilized via hydrogen-bonding interactions with the carboxylate moiety. These findings are consistent with condensed-phase results, indicating that the NO^- binding mode of arginine to Pt is preserved in the electrospray ionization process even under variable pH and ionic strength.

Statistical Modeling for Enhancing the Discovery Power of Citrullination from Tandem Mass Spectrometry Data

Citrullination is a post-translational modification implicated in various human diseases including rheumatoid arthritis, Alzheimer's disease, multiple sclerosis, and cancers. Due to a relatively low concentration of citrullinated proteins in the total proteome, confident identification of citrullinated proteome is challenging in mass spectrometry (MS)-based proteomic analysis. From these MS-based analyses, MS features that characterize citrullination, such as immonium ions (IMs) and neutral losses (NLs), called diagnostic ions, have been reported. However, there has been a lack of systematic approaches to comprehensively search for diagnostic ions and no statistical methods for the identification of citrullinated proteome based on these diagnostic ions. Here, we present a systematic approach to identify diagnostic IMs, internal ions (INTs), and NLs for citrullination from tandem mass (MS/MS) spectra. Diagnostic INTs mainly consisted of internal fragment ions for di- and tripeptides that contained two and three amino acids with at least one citrullinated arginine, respectively. A statistical logistic regression model was built for a confident assessment of citrullinated peptides that database searches identified (true positives) and prediction of citrullinated peptides that database searches failed to identify (false negatives) using the diagnostic IMs, INTs, and NLs. Applications of our model to complex global proteome data sets demonstrated the increased accuracy in the identification of citrullinated peptides, thereby enhancing the size and functional interpretation of citrullinated proteomes.

Revisiting Fragmentation Reactions of Protonated α-Amino Acids by High-Resolution Electrospray Ionization Tandem Mass Spectrometry with Collision-Induced Dissociation

Fragmentation reactions of protonated α-amino acids (AAs) were studied previously using tandem mass spectrometry (MS/MS) of unit mass resolution. Isobaric fragmentation products and minor fragmentation products could have been overlooked or misannotated. In the present study, we examined the fragmentation patterns of 19 AAs using high-resolution electrospray ionization MS/MS (HR-ESI-MS/MS) with collision-induced dissociation (CID). Isobaric fragmentation products from protonated Met and Trp were resolved and identified for the first time. Previously unreported fragmentation products from protonated Met, Cys, Gln, Arg, and Lys were observed. Additionally, the chemical identity of a fragmentation product from protonated Trp that was incorrectly annotated in previous investigations was corrected. All previously unreported fragmentation products and reactions were verified by pseudo MS³ experiments and/or MS/MS analyses of deuterated AAs. Clearer pictures of the fragmentation reactions for Met, Cys, Trp, Gln, Arg and Lys were obtained in the present study.

Positional stable isotope tracer analysis reveals carbon routes during ammonia metabolism of Aedes aegypti mosquitoes

In Aedes aegypti females, the ammonia released during blood meal digestion is partially metabolized to facilitate the disposal of excess nitrogen. In this study, we used low- and high-resolution liquid chromatography-mass spectrometry (LC/MS) techniques to investigate the role of glucose during ammonia detoxification. Mosquitoes were fed a blood meal supplemented with [1,2-¹³C₂]glucose, and downstream metabolites were measured for 24 h. Quantification of [¹³C] amino acids in the entire mosquito body was conducted without sample derivatization using selected reaction monitoring of mass transitions that are indicative of the structural position of [¹³C] atom incorporation. Identification of unlabeled and [¹³C] isotopologs of 43 compounds, including amino acids, amino acid derivatives, and organic acids, was performed by high-resolution LC/MS techniques. Blood-fed mosquitoes synthesized [¹³C] metabolites in mainly 2 carbon positions from [1,2-¹³C₂]glucose. [¹³C₂]Ala and [¹³C₂]Pro were the most abundant and rapidly labeled amino acids synthesized. Additional [¹³C] amino acids, [¹³C] amino acid derivatives, and [¹³C] organic acids in 1 or 2 carbon positions were also identified. Two kinetic routes were proposed based on the incorporation of a [¹³C] atom at position 1 in specific amino acids. Our findings provide evidence that glucose is used for ammonia detoxification and [¹³C] uric acid synthesis through multiple metabolic pathways, uncovering a metabolic link at the carbon atomic level in ammonia metabolism of A. aegypti-Horvath, T. D., Dagan, S., Lorenzi, P. L., Hawke, D. H., Scaraffia, P. Y. Positional stable isotope tracer analysis reveals carbon routes during ammonia metabolism of Aedes aegypti mosquitoes.

Factors Affecting the Production of Aromatic Immonium Ions in MALDI 157 nm Photodissociation Studies

Immonium ions are commonly observed in the high energy fragmentation of peptide ions. In a MALDI-TOF/TOF mass spectrometer, singly charged peptides photofragmented with 157 nm VUV light yield a copious abundance of immonium ions, especially those from aromatic residues. However, their intensities may vary from one peptide to another. In this work, the effect of varying amino acid position, peptide length, and peptide composition on immonium ion yield is investigated. Internal immonium ions are found to have the strongest intensity, whereas immonium ions arising from C-terminal residues are the weakest. Peptide length and competition among residues also strongly influence the immonium ion production. Quantum calculations provide insights about immonium ion structures and the fragment ion conformations that promote or inhibit immonium ion formation.

Effective disposal of nitrogen waste in blood-fed Aedes aegypti mosquitoes requires alanine aminotransferase

To better understand the mechanisms responsible for the success of female mosquitoes in their disposal of excess nitrogen, we investigated the role of alanine aminotransferase (ALAT) in blood-fed Aedes aegypti. Transcript and protein levels from the 2 ALAT genes were analyzed in sucrose- and blood-fed A. aegypti tissues. ALAT1 and ALAT2 exhibit distinct expression patterns in tissues during the first gonotrophic cycle. Injection of female mosquitoes with either double-stranded RNA (dsRNA)-ALAT1 or dsRNA ALAT2 significantly decreased mRNA and protein levels of ALAT1 or ALAT2 in fat body, thorax, and Malpighian tubules compared with dsRNA firefly luciferase-injected control mosquitoes. The silencing of either A. aegypti ALAT1 or ALAT2 caused unexpected phenotypes such as a delay in blood digestion, a massive accumulation of uric acid in the midgut posterior region, and a significant decrease of nitrogen waste excretion during the first 48 h after blood feeding. Concurrently, the expression of genes encoding xanthine dehydrogenase and ammonia transporter (Rhesus 50 glycoprotein) were significantly increased in tissues of both ALAT1- and ALAT2-deficient females. Moreover, perturbation of ALAT1 and ALAT2 in the female mosquitoes delayed oviposition and reduced egg production. These novel findings underscore the efficient mechanisms that blood-fed mosquitoes use to avoid ammonia toxicity and free radical damage.-Mazzalupo, S., Isoe, J., Belloni, V., Scaraffia, P. Y. Effective disposal of nitrogen waste in blood-fed Aedes aegypti mosquitoes requires alanine aminotransferase.

Urea synthesis and excretion in Aedes aegypti mosquitoes are regulated by a unique cross-talk mechanism

Aedes aegypti mosquitoes do not have a typical functional urea cycle for ammonia disposal such as the one present in most terrestrial vertebrates. However, they can synthesize urea by two different pathways, argininolysis and uricolysis. We investigated how formation of urea by these two pathways is regulated in females of A. aegypti. The expression of arginase (AR) and urate oxidase (UO), either separately or simultaneously (ARUO) was silenced by RNAi. The amounts of several nitrogen compounds were quantified in excreta using mass spectrometry. Injection of mosquitoes with either dsRNA-AR or dsRNA-UO significantly decreased the expressions of AR or UO in the fat body (FB) and Malpighian tubules (MT). Surprisingly, the expression level of AR was increased when UO was silenced and vice versa, suggesting a cross-talk regulation between pathways. In agreement with these data, the amount of urea measured 48 h after blood feeding remained unchanged in those mosquitoes injected with dsRNA-AR or dsRNA-UO. However, allantoin significantly increased in the excreta of dsRNA-AR-injected females. The knockdown of ARUO mainly led to a decrease in urea and allantoin excretion, and an increase in arginine excretion. In addition, dsRNA-AR-injected mosquitoes treated with a specific nitric oxide synthase inhibitor showed an increase of UO expression in FB and MT and a significant increase in the excretion of nitrogen compounds. Interestingly, both a temporary delay in the digestion of a blood meal and a significant reduction in the expression of several genes involved in ammonia metabolism were observed in dsRNA-AR, UO or ARUO-injected females. These results reveal that urea synthesis and excretion in A. aegypti are tightly regulated by a unique cross-talk signaling mechanism. This process allows blood-fed mosquitoes to regulate the synthesis and/or excretion of nitrogen waste products, and avoid toxic effects that could result from a lethal concentration of ammonia in their tissues.

Low mass MS/MS fragments of protonated amino acids used for distinction of their 13C-isotopomers in metabolic studies

Glu, Gln, Pro, and Ala are the main amino acids involved in ammonia detoxification in mosquitoes. In order to develop a tandem mass spectrometry method (MS(2)) to monitor each carbon of the above isotopically-labeled (13)C-amino acids for metabolic studies, the compositions and origins of atoms in fragments of the protonated amino acid should be first elucidated. Thus, various electrospray (ESI)-based MS(2) tools were employed to study the fragmentation of these unlabeled and isotopically-labeled amino acids and better understand their dissociation pathways. A broad range of fragments, including previously-undescribed low m/z fragments was revealed. The formulae of the fragments (from m/z 130 down to m/z 27) were confirmed by their accurate masses. The structures and conformations of the larger fragments of Glu were also explored by ion mobility mass spectrometry (IM-MS) and gas-phase hydrogen/deuterium exchange (HDX) experiments. It was found that some low m/z fragments (m/z 27-30) are common to Glu, Gln, Pro, and Ala. The origins of carbons in these small fragments are discussed and additional collision induced dissociation (CID) MS(2) fragmentation pathways are proposed for them. It was also found that small fragments (≤m/z 84) of protonated, methylated Glu, and methylated Gln are the same as those of the underivatized Glu and Gln. Taken together, the new approach of utilizing low m/z fragments can be applied to distinguish, identify, and quantify (13)C-amino acids labeled at various positions, either in the backbone or side chain.

Neighboring effect in fragmentation pathways of cage guanylhydrazones in the gas phase

ESI-MS/MS investigation of the mono- and bis(guanylhydrazone) derivatives 1-5 based on adamantane and pentacycloundecane (PCU) skeleton was described. Elimination of neutral guanidine is the most abundant reaction channel in the case of 2,4-adamantyl and PCU derivatives 4 and 5, while the elimination of CH2N2 fragment is preferred for other compounds. This was attributed to the cage opening of adamantane or PCU skeletons in the former case leading to the formation of the cyclohexyl- or cyclopropylcarbinyl carbocation stabilized by the conjugation with the guanylhydrazone subunit. The main fragmentation pathways observed experimentally were analyzed by using DFT calculations. All investigated bis(guanylhydrazone)s formed dications and their abundances were found to be proportional to the interguanidine distance in the considered ions. Calculation of the first and the second proton affinities supported qualitative interpretation of the dication abundance. Close contact of two guanidine subunits is thus confirmed to be crucial in determining preferential fragmentation pathway and to suppress formation of the dication.