On-plate self-desalting and matrix-free LDI MS analysis of peptides with a surface patterned sample support

A hydrophobic-hydrophilic-hydrophobic pattern has been produced on the surface of a silicon substrate for selective enrichment, self-desalting, and matrix-free analysis of peptides in a single step. Upon sample application, the sample solution is first confined in a small area by a hydrophobic F-SAM outer area, after which salt contaminants and peptides are selectively enriched in the hydrophilic and hydrophobic areas, respectively. Simultaneously, matrix background noise is significantly reduced or eliminated because of immobilization of matrix molecules. As a result, the detection sensitivity is enhanced 20-fold compared with that obtained using the usual MALDI plate, and interference-free detection is achieved in the low m/z range. In addition, peptide ions can be identified unambiguously in the presence of NH₄HCO₃ (100 mM), urea (1 M), and NaCl (1 M). When the device was applied to the analysis of BSA digests, the peptide recovery and protein identification confidence were greatly improved.

Differential mechanisms of activation of the Ang peptide receptors AT1, AT2, and MAS: using in silico techniques to differentiate the three receptors

The renin-angiotensin system is involved in multiple conditions ranging from cardiovascular disorders to cancer. Components of the pathway, including ACE, renin and angiotensin receptors are targets for disease treatment. This study addresses three receptors of the pathway: AT1, AT2, and MAS and how the receptors are similar and differ in activation by angiotensin peptides. Combining biochemical and amino acid variation data with multiple species sequence alignments, structural models, and docking site predictions allows for visualization of how angiotensin peptides may bind and activate the receptors; allowing identification of conserved and variant mechanisms in the receptors. MAS differs from AT1 favoring Ang-(1–7) and not Ang II binding, while AT2 recently has been suggested to preferentially bind Ang III. A new model of Ang peptide binding to AT1 and AT2 is proposed that correlates data from site directed mutagenesis and photolabled experiments that were previously considered conflicting. Ang II binds AT1 and AT2 through a conserved initial binding mode involving amino acids 111 (consensus 325) of AT1 (Asn) interacting with Tyr (4) of Ang II and 199 and 256 (consensus 512 and 621, a Lys and His respectively) interacting with Phe (8) of Ang II. In MAS these sites are not conserved, leading to differential binding and activation by Ang-(1–7). In both AT1 and AT2, the Ang II peptide may internalize through Phe (8) of Ang II propagating through the receptors’ conserved aromatic amino acids to the final photolabled positioning relative to either AT1 (amino acid 294, Asn, consensus 725) or AT2 (138, Leu, consensus 336). Understanding receptor activation provides valuable information for drug design and identification of other receptors that can potentially bind Ang peptides.

Mass spectrometry for the molecular imaging of angiotensin metabolism in kidney

To better understand the tissue distribution and activity of enzymes involved in angiotensin II (Ang II) processing, we developed a novel molecular imaging method using matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. Mouse kidney sections (12 μm) were incubated with 10-1,000 μmol/l Ang II for 5-15 min at 37°C. The formed peptides Ang III and Ang-(1-7) were identified by MALDI-TOF/TOF. A third metabolite, Ang-(1-4), was generated from further degradation of Ang-(1-7). Enzymatic processing of Ang II was dose and time dependent and absent in heat-treated kidney sections. Distinct spatial distribution patterns (pseudocolor images) were observed for the peptides. Ang III was localized in renal medulla, whereas Ang-(1-7)/Ang-(1-4) was present in cortex. Regional specific peptide formation was confirmed using microdissected cortical and medullary biopsies. In vitro studies with recombinant enzymes confirmed activity of peptidases known to generate Ang III or Ang-(1-7) from Ang II: aminopeptidase A (APA), Ang-converting enzyme 2 (ACE2), prolyl carboxypeptidase (PCP), and prolyl endopeptidase (PEP). Renal medullary Ang III generation was blocked by APA inhibitor glutamate phosphonate. The ACE2 inhibitor MLN-4760 and PCP/PEP inhibitor Z-pro-prolinal reduced cortical Ang-(1-7) formation. Our results establish the power of MALDI imaging as a highly specific and information-rich analytical technique that will further aid our understanding of the role and site of Ang II processing in cardiovascular and renal pathologies.

Formation of anionic peptide radicals in vacuo

In this paper, we demonstrate for the first time the formation of radical anionic peptides [M - 2H]*- through a one-electron transfer mechanism upon low-energy collision-induced dissociation (CID) of gas-phase singly charged [Mn(III)(salen)(M - 2H)]*- complex ions [where salen is N,N'-ethylenebis(salicylideneiminato) and M is an angiotensin III derivative]. The types of fragment ions formed from [M - 2H]*- share some similarities with those from the cationic radical peptides M*+ and [M + H]*2+, but differ significantly from those of the corresponding deprotonated peptides [M - H]-. Fragmentation of [M - 2H]*- radical anionic angiotensin III derivatives leads preferentially to product ions of side-chain cleavage of amino acid residues, z-type and minor x-type fragment ions, most of which are types rarely observed in low-energy CID spectra of deprotonated analogs. The degree of competitive dissociation of the complexes is highly dependent on the nature of the substituted salen derivatives. The yields of anionic peptide radicals were enhanced to the greatest extent when electron withdrawing groups were positioned at the 5 and 5' positions, but the effect was rather modest when such groups resided at the 3 and 3' positions. Substituting a cyclohexyl unit of a salen with phenyl or naphthyl moieties at the 8 and 8' positions also facilitated electron-transfer pathways.

Lack of angiotensin II conversion to angiotensin III increases water but not alcohol consumption in aminopeptidase A-deficient mice

Elevated central concentrations of the vasopressor octapeptide angiotensin (Ang) II increase the water intake in mammals. Recently, we showed that central AngII is also crucial in alcohol-consuming behavior. Since the heptapeptide AngIII, an AngII metabolite, is discussed to mediate AngII-related effects, we investigated water and alcohol consumption in mice, genetically deficient in aminopeptidase A (APA), a peptidase responsible for AngII conversion to AngIII. Sixteen male APA-deficient mice and their age matched wild-type controls were monitored on their water intake under basal conditions and total fluid and alcohol intake before and after social stress in a two-bottle free-choice paradigm. Alterations were connected to the regulation in activity of Ang-related peptidases (APA, ACE; ACE2) in brain regions involved in alcohol intake and peripheral organs. In comparison to their wild-type controls, APA-deficient mice drank significantly more water but not more alcohol at all investigated time points. A reduction in water intake, as observed in wild-type animals after social stress, did not occur in knockout mice. However, the reduction in alcohol consumption after social stress was significantly reduced in both strains. Alcohol consumption upregulated all three peptidases in the kidney, but not in lung. Notable, renal ACE2 activity was significantly higher in APA-deficient mice under basal condition. While the inhibition of AngII metabolism to AngIII does not influence the alcohol intake, water consumption in mice deficient for APA was significantly elevated. These differences induced by an altered AngII/AngIII ratio oppose the hypothesis that central AngII and AngIII act in a congruent pattern.

Isotope ratio monitoring of small molecules and macromolecules by liquid chromatography coupled to isotope ratio mass spectrometry

In the field of isotope ratio mass spectrometry, the introduction of an interface allowing the connection of liquid chromatography (LC) and isotope ratio mass spectrometry (IRMS) has opened a range of new perspectives. The LC interface is based on a chemical oxidation, producing CO2 from organic molecules. While first results were obtained from the analysis of low molecular weight compounds, the application of compound-specific isotope analysis by irm-LC/MS to other molecules, in particular biomolecules, is presented here. The influence of the LC flow rate on the CO2 signal and on the observed delta13C values is demonstrated. The limits of quantification for angiotensin III and for leucine were 100 and 38 pmol, respectively, with a standard deviation of the delta13C values better than 0.4 per thousand. Also, accuracy and precision of delta13C values for elemental analyser-IRMS and flow injection analysis-IRMS (FIA-LC/MS) were compared. For compounds with molecular weights ranging from 131 to 66,390 Da, precision was better than 0.3 per thousand, and accuracy varied from 0.1 to 0.7 per thousand. In a second part of the work, a two-dimensional (2D)-LC method for the separation of 15 underivatised amino acids is demonstrated; the precision of delta13C values for several amino acids by irm-LC/MS was better than 0.3 per thousand at natural abundance. For labelled mixtures, the coefficient of variation was between 1% at 0.07 atom % excess (APE) for threonine and alanine, and around 10% at 0.03 APE for valine and phenylalanine. The application of irm-LC/MS to the determination of the isotopic enrichment of 13C-threonine in an extract of rat colon mucosa demonstrated a precision of 0.5 per thousand, or 0.001 atom %.

A mechanistic investigation of the enhanced cleavage at histidine in the gas-phase dissociation of protonated peptides

Enhanced gas-phase cleavage of peptides adjacent to histidine was investigated. The peptides examined were angiotensins III (RVYIHPF) and IV (VYIHPF) as well as synthetic peptide analogues with altered key residues ((R)VYI-X-Z-F; X = F or H and Z = A, P, or Sar) or a fixed charge M3P(+)CH(2)C(O)-VYIHPF. While all singly protonated peptide ions containing both histidine and arginine fragment nonselectively, the doubly protonated peptide ions with arginine and histidine, and the singly protonated peptides containing histidine but not arginine, cleave in a selective manner. In particular, dominant complementary b+/y+ product ions resulting from cleavage between the HP amide bond are observed. For the fixed-charge derivative, selective cleavage occurs only if a proton is added to produce a doubly charged precursor. The results are consistent with involvement of a protonated histidine in the selective cleavage. The ratio of b+/y+ is determined by the identity of the residue C-terminal to histidine and by the ability of protonated histidine to transfer a proton to the C-terminal leaving fragment. This was probed further by systematically changing the residue C-terminal to histidine and by alkylating histidine. The results indicate that while b+/y+ complementary ion pairs dominate in doubly protonated RVYIHPF, b5(2+) and b6(2+) product ions dominate the spectra of doubly protonated RVYIHAF. Also, dominant b5(2+) product ions are observed when the histidine side chain is alkylated (H) in doubly protonated RVYIHPF. Based on all of the results, a selective fragmentation mechanism for enhanced cleavage at histidine involving an atypical b ion structure is proposed.

Primary and secondary locations of charge sites in angiotensin II (M + 2H)2+ ions formed by electrospray ionization

High-energy tandem mass spectrometry and molecular dynamics calculations are used to determine the locations of charge in metastably decomposing (M + 2H)2+ ions of human angiotensin II. Charge-separation reactions provide critical information regarding charge sites in multiple charged ions. The most probable kinetic energy released (Tm.p.) from these decompositions are obtained using kinetic energy release distributions (KERDs) in conjunction with MS/MS (MS2), MS/MS/MS (MS3), and MS/MS/MS/MS (MS4) experiments. The most abundant singly and doubly charged product ions arise from precursor ion structures in which one proton is located on the arginine (Arg) side chain and the other proton is located on a distal peptide backbone carbonyl oxygen. The MS3 KERD experiments show unequivocally that neither the N-terminal amine nor the aspartic acid (Asp) side chain are sites of protonation. In the gas phase, protonation of the less basic peptide backbone instead of the more proximal and basic histidine (His) side chain is favored as a result of reduced coulomb repulsion between the two charge sites. The singly and doubly charged product ions of lesser abundance arise from precursor ion structures in which one proton is located on the Arg side chain and the other on the His side chain. This is demonstrated in the MS3 and MS4 mass-analyzed ion kinetic energy spectrometry experiments. Interestingly, (b7" + OH)2+ product ions, like the (M + 2H)2+ ions of angiotensin II, are observed to have at least two different decomposing structures in which charge sites have a primary and secondary location.

Combined NMR and molecular modeling study of an iduronic acid-containing trisaccharide related to antithrombotic heparin fragments

An iduronic acid-containing trisaccharide, methyl-O-(4-O-methyl-2,3,6-tri-O-sulfo-alpha-D-glucopyranosyl-(1-->4)-O- (2-O-sulfo-alpha-L-idopyranosyluronic acid)-(1-->4)-O-2,6-di-O-sulfo-alpha-D-glucopyranoside, related to antithrombotic heparin fragments has been subjected to a combined NMR and molecular modeling investigation. The conformational behavior of the two constituting disaccharide segments was investigated using a systematic grid search approach with the MM3 force field along with the proper parameters for the sulfate ester group. The exploration of the potential energy surfaces of the trisaccharide was performed through the use of the CICADA methods interfaced with the MM3 force field. In all cases, the 2-O-sulfo-alpha-L-iduronate moiety was given the three favored ring conformations (1)C4, (4)C1, and (2)S0. Conformations were clustered into families, four of which are likely to exhibit significant occupancy in solution. The different low-energy conformational families display different orientations at the glycosidic linkages and/or different ring shapes for the iduronate ring. The (2)S0 conformation is the major one for the 2-O-sulfo-alpha-L-iduronate but is still in equilibrium with the (1)C4 ring shape. The occurrence of such a conformational equilibrium in solution was probed via high-resolution NMR spectroscopy through measurements of coupling constants and NOE build-up. These results are in keeping with the observation that 2-O-sulfated pentasaccharides display a similar affinity for antithrombin III as their 2-N-sulfated counterparts.

Determination of endogenous peptides with in vitro ACE inhibitory activity in normotensive human plasma by the fluorometric HPLC method

An in vitro degradation test of angiotensin (ANG) II or III in normotensive supine human plasma from 9 healthy male subjects confirmed the production of smaller ANG metabolites with angiotensin I-converting enzyme inhibitory activity. These metabolites were identified as ANG (3-8), ANG (5-8), and ANG (3-4), whose respective peptide concentrations were determined by our proposed naphthalene-2,3-dialdehyde (NDA)-HPLC method to be 64 +/- 9, 39 +/- 5, 176 +/- 22, and 197 +/- 35 fmol/ml of plasma.

Conformational analysis of neuropeptide Y-[18-36] analogs in hydrophobic environments

The interactive and conformational behavior of a series of neuropeptide Y-[18-36] (NPY-[18-36]) analogs in hydrophobic environments have been investigated using reversed-phase high-performance liquid chromatography (RP-HPLC) and circular dichroism (CD) spectroscopy. The peptides studied comprised a series of 16 analogs of NPY-[18-36], each containing a single D-amino acid substitution. The influence of these single L-->D substitutions on the alpha-helical conformation of the NPY-[18-36] analogs in different solvent environments was determined by CD spectroscopy. Retention parameters related to the hydrophobic contact area and the affinity of interaction were determined with an n-octadecyl (C18) adsorbent. Structural transitions for all peptides were manifested as significant changes in the hydrophobic binding domain and surface affinity between 4 degrees C and 37 degrees C. The results indicated that the central region of NPY-[18-36] (residues 23-33) is important for maintenance of the alpha-helical conformation. Moreover, L-->D amino acid residue substitutions within the N- and C-terminal regions, as well as Asn29 and Leu30, do not appear to affect the secondary structure of the peptide. These studies demonstrate that RP-HPLC provides a powerful adjunct for investigations into the induction of stabilized secondary structure in peptides upon their interaction with hydrophobic surfaces.

Effect of the position of a basic amino acid on C-terminal rearrangement of protonated peptides upon collision-induced dissociation

Internal rearrangement involving the loss of the C-terminal amino acid residue upon collision-induced dissociation (CID) or metastable decomposition was studied for protonated peptides. To investigate the structural characteristics of peptides responsible for this rearrangement, a series of synthetic peptides were prepared and subjected to B/E-linked scan or tandem mass spectrometric analyses using a four-sector instrument. The results showed that the position of a basic amino acid in the peptide sequence and its basicity have a significant influence on the rearrangement. Arginine (Arg) located at the n-1 position facilitates the rearrangement with about twice as many rearrangement ions as is observed for the other Arg-containing peptides. This can be attributed to the interaction of a positively charged guanidino group of Arg with its own carbonyl group via a salt bridge which is tightly formed in vacuo between a guanidino and carboxylate groups, the mechanism of which is analogous to that previously proposed for the formation of similar rearrangement ions observed in the spectra of metal-cationized peptides. This association would result in the facile attack of the C-terminal hydroxyl group on the penultimate carbonyl group, leading to the rearrangement. In addition, the rearrangement ion was observed both in metastable decomposition and high-energy CID spectra obtained by B/E-linked scan analyses without or with gas, respectively, but in a sequence dependent manner.

Sequence-specific fragmentation generated by matrix-assisted laser desorption/ionization in a quadrupole ion trap/reflectron time-of-flight device

Sequence-specific fragmentation for structural analysis has been generated by activation of ions via matrix-assisted laser desorption/ionization (MALDI) in an ion trap storage/reflectron time-of-flight device (IT/reTOF). The key to this work is that ion decay can be induced by MALDI activation but requires an extended period of time to occur in large peptides. This extended decay period, which may be in excess of 20 ms, is provided in these experiments using the long storage times of the ion trap device. The ions are stored until decay is complete and then rapidly pulsed into a reflectron TOF for analysis. Since the ions decay within the trap, they ultimately appear as stable ions in the reTOF rather than as metastable decay peaks. The ion fragmentation was found to depend strongly on laser power and the rf voltage placed on the ring electrode of the trap. The fragmentation obtained was shown to be similar to but different from that observed in FAB-low-energy CAD. In particular, enhanced fragmentation was obtained in the lower mass range and large species could be more easily fragmented than with FAB-low-energy CAD. The types of fragmentation for several target peptides are discussed.

Angiotensin-I- and -III-mediated cardiovascular responses in the freshwater North American eel, Anguilla rostrata: effect of Phe8 deletion

Cardiovascular responses to synthetic eel [Asn1, Val5, Gly9]-ANG-I, ANG-III (2-8), and ANG-I (1-7) were measured in conscious and resting freshwater North American eels. Indwelling Doppler flow probes were placed on the ventral and dorsal aortas, a pressure catheter in the lienomesenteric artery, and a peptide delivery catheter in the caudal vein. Twenty-five and 150 ng.kg-1 ANG-III increased baseline cardiac output (CO) (15.23 +/- 0.31 ml.min-1.kg-1; n = 5) by 23 and 47%, respectively, by increasing stroke volume (SV) but not heart rate (HR). ANG-I (150 ng.kg-1) also elevated CO (62%) by increasing both SV (44%) and HR (14%). Estimated branchial shunting (BS) was increased by 150 ng.kg-1 ANG-I and -III suggesting that more blood perfused the arteriovenous pathway in the gills. Dorsal aortic blood pressure (PDA) (3.08 +/- 0.12 kPa) was elevated by 150 ng.kg-1 ANG-I (67%) and -III (52%). Pressor responses temporally preceded the blood flow increases and there was a significant increase in systemic vascular resistance (RSYS) at the peak pressor responses. At the peak flow responses, increased CO was solely responsible for the increase in PDA; RSYS had returned to baseline values. Pressor responses to ANG-III decayed more rapidly (18.6 min) compared with those of ANG-I and -II (36 min). ANG-I (1-7) had no measurable effect on cardiovascular function indicating that the carboxyl-terminal 8-phenylalanine is an absolute requirement for the hormonal activity of angiotensin in fishes.

Angiotensin II--derived peptides devoid of phenylalanine in position 8 have full psychotropic activity of the parent hormone

In this work we compared in rats the influence of heptapeptide 1-7-angiotensin II, hexapeptide 2-7-angiotensin II, pentapeptide 3-7-angiotensin II and angiotensin II on motility, stereotypy, learning of conditioned avoidance responses and recall of passive avoidance behaviour allowing to avoid aversive stimulation. The 4 peptides administered 15 min before the experiment, tended to increase the number of crossings, rearings and bar approaches in open field, significantly accelerated acquisition of conditioned avoidance responses and improved recall of the passive avoidance. All the peptides applied immediately before the experiment intensified stereotypy evoked by apomorphine in the dose 1 mg/kg and amphetamine in the dose 6.5 mg/kg given intraperitoneally. These results show full psychotropic activity of the examined fragments of angiotensin II, comparable with the activity of the parent octapeptide. Our previous hypothesis that the Val-Tyr-Ile-His-Pro fragment of angiotensin II is responsible for the psychotropic activity evoked by angiotensins in rats is thus confirmed.

Angiotensin II receptors in Xenopus oocytes

Electrical recordings were used to study the sensitivity of native Xenopus oocytes to the octapeptide angiotensin II (AII). AII elicited oscillatory currents associated with an increase in membrane conductance to Cl-. Responsiveness to AII varied greatly between oocytes taken from different frogs, and to a lesser extent between oocytes from the same ovary. Oocytes from frogs showing high sensitivity had response thresholds between 0.5-1.0 nM AII, and at a holding potential of -60 mV, responded to 1 microM AII with currents greater than 3 microA. In contrast, oocytes from some frogs gave no response, even to 10 microM AII. A total of 618 oocytes from 79 frogs were tested for sensitivity to AII, and oocytes from 85% of frogs gave detectable electrical responses. Oscillatory Cl- currents elicited by AII were largely independent of extracellular Ca2+, were abolished by chelation of intracellular Ca2+ using EGTA and were mimicked by intraoocyte injection of inositol 1,4,5-trisphosphate (IP3). In addition to oscillatory Cl- currents, AII also evoked an influx of extracellular Ca2+, giving rise to a transient inward Cl- current on membrane hyperpolarizing steps. These experiments all suggested that AII responses were elicited through activation of an intracellular messenger pathway triggered by hydrolysis of inositolphospholipids, mobilization of intracellular Ca2+ by inositol polyphosphates, and activation of Ca(2+)-gated Cl- channels. The effect of manual or enzymic defolliculation on AII responses was studied in nine separate experiments recording from 70 defolliculated oocytes. Efficacy of defolliculation procedures was assayed using scanning electron microscopy, which confirmed removal of 90 to greater than 98% of follicular cells.(ABSTRACT TRUNCATED AT 250 WORDS)