Use of homoarginine to obtain attenuated cationic membrane lytic peptides

Our research group has been studying the design of intracellular delivery peptides based on cationic lytic peptides. By placing negatively charged amino acids on potentially hydrophobic faces of the peptides, membrane lytic activity is attenuated on the cell surface, whereas it recovers in endosomes, enabling cytosolic delivery of proteins including antibodies. These lytic peptides generally contain multiple lysines, facilitating cell surface interaction and membrane perturbation. This study evaluated the effect of lysine-to-homoarginine substitution using HAad as a model delivery peptide. The resulting peptide had a comparable or better delivery efficacy for Cre recombinase, antibodies, and the Cas9/sgRNA complex with one-quarter of the concentration of HAad, implying that a subtle structural difference can affect delivery activity.

O-Methylisourea Can React with the α-Amino Group of Lysine: Implications for the Analysis of Reactive Lysine

The specificity of O-methylisourea (OMIU) to bind to the ε-amino group of Lys, an important supposition for the OMIU-reactive Lys analysis of foods, feeds, ingredients, and digesta, was investigated. Crystalline l-Lys incubated under standard conditions with OMIU resulted in low homoarginine recoveries. The reaction of OMIU with the α-amino group of Lys was confirmed by MS analysis, with double derivatized Lys being identified. None of the changes in reaction conditions (OMIU pH, OMIU to Lys ratio, and reaction time) with crystalline l-Lys resulted in 100% recovery of homoarginine. The average free Lys content in ileal digesta of growing pigs and broilers was found to be 13% of total Lys, which could result in a significant underestimation of the reactive Lys content. The reaction of OMIU with α-amino groups may necessitate analysis of free Lys to accurately quantify reactive lysine in samples containing a large proportion of Lys with a free α-amino group.

Solid-phase N-terminal peptide enrichment study by optimizing trypsin proteolysis on homoarginine-modified proteins by mass spectrometry

RATIONALE

Proteolytic cleavages generate active precursor proteins by creating new N-termini in the proteins. A number of strategies have recently been published regarding the enrichment of original or newly formed N-terminal peptides using guanidination of lysine residues and amine-reactive reagents. For effective enrichment of N-terminal peptides, the efficiency of trypsin proteolysis on homoarginine (guanidinated) modified proteins must be understood and simple and versatile solid-phase N-terminal capture strategies should be developed.

METHODS

We present here a mass spectrometry (MS)-based study to evaluate and optimize the trypsin proteolysis on a guanidinated-modified protein. Trypsin proteolysis was studied using different amounts of trypsin to modified protein ratios. To capture the original N-termini, after guanidination of proteins, original N-termini were acetylated and the proteins were digested with trypsin. The newly formed N-terminal tryptic peptides were captured with a new amine reactive acid-cleavable solid-phase reagent. The original N-terminal peptides were then collected from the supernatant of the solution.

RESULTS

We demonstrated a detailed study of the efficiency of enzyme trypsin on homoarginine-modified proteins. We observed that the rate of hydrolysis of homoarginine residues compared to their lysine/arginine counterparts were slower but generally cleaved after an overnight digestion period depending on the protein to protease concentration ratios. Selectivity of the solid-phase N-terminal reagent was studied by enrichment of original N-terminal peptides from two standard proteins, ubiquitin and RNaseS.

CONCLUSIONS

We found enzyme trypsin is active in the guanidinated form of the protein depending on the enzyme to protein concentrations, time and the proximity of arginine residues in the sequence. The novel solid-phase capture reagent also successfully enriched N-terminal peptides from the standard protein mixtures. We believe this trypsin proteolysis study on homoarginine-modified proteins and our simple and versatile solid-phase capture strategy could be very useful for enrichment and sequence determination of proteins N-termini by MS.

Determination of homoarginine, arginine, NMMA, ADMA, and SDMA in biological samples by HPLC-ESI-mass spectrometry

N^G,N^G-dimethyl-l-arginine (ADMA) and N^G-methyl-l-arginine (NMMA) are endogenous inhibitors of nitric oxide synthase (NOS). In contrast, N^G,N′^G-dimethyl-Larginine (SDMA) possesses only a weak inhibitory potency towards neuronal NOS and it is known to limit nitric oxide (NO) production by competing with l-arginine for cellular uptake. The inhibition of NOS is associated with endothelial dysfunction in cardiovascular diseases as well in chronic renal failure. l-Homoarginine (HArg), a structural analog of l-arginine (Arg), is an alternative but less efficient substrate for NOS. Besides, it inhibits arginase, leading to an increased availability of l-arginine for NOS to produce NO. However, its relation with cardiovascular disease remains unclear. To date, several analytical methods for the quantitative determination of Arg, HArg, NMMA, AMDA, and SDMA in biological samples have been described. Here, we present a simple, fast, and accurate HPLC-ESI-MS/MS method which allows both the simultaneous determination and quantification of these compounds without needing derivatization, and the possibility to easily modulate the chromatographic separation between HArg and NMMA (or between SDMA and ADMA). Data on biological samples revealed the feasibility of the method, the minimal sample preparation, and the fast run time which make this method very suitable and accurate for analysis in the basic and clinical settings.

Synthesis and biological activity of homoarginine-containing opioid peptides

Two tris-alkoxycarbonyl homoarginine derivatives, Boc-Har{omega,omega'-[Z(2Br)]2}-OH and Boc-Har{omega,omega'-[Z(2Cl)]2}-OH, were prepared by guanidinylation of Boc-Lys-OH, and used for the synthesis of neo-endorphins and dynorphins. The results were compared with that obtained in the synthesis in which Boc-Lys(Fmoc)-OH was incorporated into the peptide chain, and after removing Fmoc protection, the resulting peptide-resin was guanidinylated with N,N'-[Z(2Br)]2- or N,N'-[Z(2Cl)]2-S-methylisourea. The peptides were tested in the guinea-pig ileum (GPI) and mouse vas deferens (MVD) assays. The results indicated that replacement of Arg by Har may be a good avenue for the design of biologically active peptides with increased resistance to degradation by trypsin-like enzymes.

Chemical and biological investigations of beta-oligoarginines

In view of the important role arginine plays in living organisms as the free amino acid and, especially, as a residue in peptides and proteins, the homologous beta-homoarginines are central in our investigations of beta-peptides (Fig. 1). The preparation of beta2-homoarginine derivatives suitably protected for solution- or solid-phase peptide syntheses is described with full experimental detail (9 and 12 in Scheme 1). The readily available Fmoc-beta3 hArg(Boc)2-OH is used for manual solid-phase synthesis of beta3-oligoarginines (on Rink amide or Rink amide AM resin) either by single amino acid coupling (Scheme 3) or, much better, by dimer-fragment coupling (Scheme 4). In this way, beta3-oligoarginine amides composed of 4, 6, 7, 8, and 10 residues, both with and without fluorescein labelling, were synthesized (Schemes 2-4), purified by preparative HPLC and identified by high-resolution mass spectrometry. The free amino acids (R)- and (S)-H-beta2 hArg-OH and (S)-H-beta3 hArg-OH were tested for their ability to function as substrates for NO synthase (iNOS); the beta3-oligoarginine amides (5, 6, and 7 residues) were tested for antibacterial (against six pathogens) and hemolytic (against rat and human erythrocytes) activities. All test results were negative: none of the free beta-homoarginines induced NO formation (Fig. 3), and there was no lysis of erythrocytes (concentrations up to 100 microM; Table 1), and no significant antibiotic activity (MIC > or = 64 microg/ml; Table 2). Cell-penetration studies with the fluorescence-labelled, peptidase-resistant beta3-oligoarginine amides were carried out with HeLa cells and human foreskin keratinocytes (HFKs). The results obtained with fluorescence microscopy are: i) the longer-chain beta-oligoarginine amides (8 and 10 residues; Figs. 4-6) enter the cells and end up in the nuclei, especially in the nucleoli, irrespective of temperature (37 degrees and 4 degrees with HFKs) or pretreatment with NaN3 (with HFKs), indicating a non-endocytotic and non-energy-dependent uptake mechanism; ii) the beta-tetraarginine derivative occupies the cell surface but does not enter the cells (with HeLa); iii) the cell-growth rate of the HFKs is not affected by a 1-microM concentration of the fluorescence-labelled beta-octaarginine amide (Fig. 7), i.e., there is no antiproliferative effect. In vivo experiments with mouse skin and the beta-octaarginine derivative show migration of the beta-peptide throughout the epidermis (Fig. 8). As a contribution to understanding the mechanism, we have also studied the behavior of fluorescence-labelled beta-octa- and beta-decaarginine amides (TFA salts) towards giant unilamellar vesicles (GUVs) built of neutral (POPC) or anionic (POPC/POPG mixtures) phospholipids: the beta-oligoarginine amides bind tightly to the surface of anionic GUVs but do not penetrate the lipid bilayer (Fig. 9) as they do with living cells. In contrast, a beta-heptapeptide FL-22, which had been used as a negative control sample for the cell-penetration experiments, entered the GUVs of negative surface charge. Thus, the mechanisms of cell and GUV-model penetration appear to be different. Finally, the possible applications and implications of the 'protein transduction' by beta-oligoarginines are discussed.

Role of lysine versus arginine in enzyme cold-adaptation: modifying lysine to homo-arginine stabilizes the cold-adapted alpha-amylase from Pseudoalteramonas haloplanktis

The cold-adapted alpha-amylase from Pseudoalteromonas haloplanktis (AHA) is a multidomain enzyme capable of reversible unfolding. Cold-adapted proteins, including AHA, have been predicted to be structurally flexible and conformationally unstable as a consequence of a high lysine-to-arginine ratio. In order to examine the role of low arginine content in structural flexibility of AHA, the amino groups of lysine were guanidinated to form homo-arginine (hR), and the structure-function-stability properties of the modified enzyme were analyzed by transverse urea gradient-gel electrophoresis. The extent of modification was monitored by MALDI-TOF-MS, and correlated to changes in activity and stability. Modifying lysine to hR produced a conformationally more stable and less active alpha-amylase. The k(cat) of the modified enzyme decreased with a concomitant increase in deltaH# and decrease in K(m). To interpret the structural basis of the kinetic and thermodynamic properties, the hR residues were modeled in the AHA X-ray structure and compared to the X-ray structure of a thermostable homolog. The experimental properties of the modified AHA were consistent with K106hR forming an intra-Domain B salt bridge to stabilize the active site and decrease the cooperativity of unfolding. Homo-Arg modification also appeared to alter Ca2+ and Cl- binding in the active site. Our results indicate that replacing lysine with hR generates mesophilic-like characteristics in AHA, and provides support for the importance of lysine residues in promoting enzyme cold adaptation. These data were consistent with computational analyses that show that AHA possesses a compositional bias that favors decreased conformational stability and increased flexibility.

Computational docking of L-arginine and its structural analogues to C-terminal domain of Escherichia coli arginine repressor protein (ArgRc)

The arginine repressor (ArgR) of Escherichia coli binds to six L-arginine molecules that act as its co-repressor in order to bind to DNA. The binding of L-arginine molecules as well as its structural analogues is compared by means of computational docking. A grid-based energy evaluation method combined with a Monte Carlo simulated annealing process was used in the automated docking. For all ligands, the docking procedure proposed more than one binding site in the C-terminal domain of ArgR (ArgRc). Interaction patterns of ArgRc with L-arginine were also observed for L-canavanine and L-citrulline. L-lysine and L-homoarginine, on the other hand, were shown to bind poorly at the binding site. Figure A general overview of the sites found from docking the various ligands into ArgRc ( grey ribbons). Red coloured sticks: residues in binding site H that was selected for docking

Oriented versus random protein immobilization

Immobilization of proteins on solid surfaces plays an important role in all the fields of modern biology. Two approaches are used in the immobilization of proteins: a random and an oriented mode of binding to solid matrices. In this note, we show that there is not much difference in using either mode of immobilization, since proteins usually bind to a matrix through only one or two bonds. This is demonstrated by the attachment of several proteins to CNBr-activated Sepharose through their lysines and the consequent conversion of those lysines to homoarginine upon treatment with ammonium hydroxide.

Synthesis and biological characterization of L-N(6)-(1-iminoethyl)lysine 5-tetrazole-amide, a prodrug of a selective iNOS inhibitor

The 5-tetrazole amide of L-N(6)-(1-iminoethyl)lysine (L-NIL), L-N(6)-(1-iminoethyl)lysine 5-tetrazole amide (1), has been prepared and evaluated. In contrast to L-NIL, 1 is a stable, nonhygroscopic, crystalline solid. Unlike L-NIL, 1 has minimal inhibitory activity in vitro on human inducible nitric oxide synthase (iNOS). However, it is rapidly converted in vivo to L-NIL and produces dose-dependent inhibition of iNOS in acute and chronic models of inflammation in the rodent with efficacy comparable to L-NIL. In addition, both 1 and L-NIL exhibit significant and comparable in vivo selectivity for the inhibition of iNOS vs endothelial NOS. Doses approximately 80-fold greater than those that inhibited inflammation do not elevate systemic blood pressure. In summary, both the physical properties and the pharmacological profile of 1 make it an ideal molecule for preclinical and clinical studies on the role of selective iNOS inhibitors in mediating inflammatory disease processes.

Investigation of the electrospray response of lysine-, arginine-, and homoarginine-terminal peptide mixtures by liquid chromatography/mass spectrometry

Guanidination has been used to investigate how modification of the lysine eta-amino group into the corresponding guanidino group affects response in electrospray (ES) mass spectrometry (MS). Selected ion monitoring (SIM) analysis of equimolar mixtures containing arginine-, lysine- and the corresponding homoarginine-terminal peptides following liquid chromatography (LC) showed differences in ES response. The ionisation behaviour of the standard peptides is in accordance with the postulated higher stability of the guanidino group present on arginine- and homoarginine-terminal peptides. Modification of the separation conditions employed during LC demonstrates that relative abundances of electrosprayed peptides ions rely mostly on peptide structure.

Increased sensitivity of tryptic peptide detection by MALDI-TOF mass spectrometry is achieved by conversion of lysine to homoarginine

Mass spectrometric techniques for identification of proteins by "mass fingerprinting" (matching the masses of tryptic peptides from a protein digest to the theoretical peptides in a database) such as matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) are rapidly growing in popularity as the demand for high throughput analysis of the proteome increases. This is due, in part, to the ability to automate the technique and the rapid rate with which mass spectra may be acquired. An important factor in the accuracy of the technique is the number of tryptic peptides that are identified in the various searching algorithms that exist. The greater sequence coverage of the parent protein that is obtained, the higher the level of confidence in the identification that is determined. One impediment to high levels of sequence coverage is the bias of MALDI-TOF mass spectrometry to arginine-containing peptides. Increasing the sensitivity to lysine-containing peptides should increase the sequence coverage obtained. In order to achieve this result we have developed conditions to modify the epsilon-amine group of lysine in tryptic peptides with O-methylisourea. The conditions utilized result in the conversion of lysine to homoarginine with no modification of the amine terminus of the peptides. The sensitivity of MALDI-TOF mass spectrometry detection of peptides was increased dramatically following modification. The modification chemistry may be applied to tryptic peptide mixtures prior to desalting and spotting onto MALDI-TOF plates. This technique will be particularly useful for identifying proteins with a high lysine/arginine ratio.

Synthesis of the marine natural product nalpha-(4-bromopyrrolyl-2-carbonyl)-L-homoarginine, a putative biogenetic precursor of the pyrrole-imidazole alkaloids

Lysine is proposed as an alternative biosynthetic precursor of the pyrrole-imidazole alkaloids frequently found in marine sponges. As a putative key intermediate, the natural product Nalpha-(4-bromopyrrolyl-2-carbonyl)-L-homoarginine (1) from the sponge Agelas wiedenmayeri was synthesized in the solid phase starting from Fmoc/Pmc-protected L-homoarginine and in solution starting from readily available L-lysine methyl ester.

Substrate specificity of NO synthases: detailed comparison of L-arginine, homo-L-arginine, their N omega-hydroxy derivatives, and N omega-hydroxynor-L-arginine

A detailed comparison of the oxidation of five compounds closely related to L-arginine (Arg) by purified recombinant neuronal and macrophage NO synthases (NOS I and NOS II) was performed. Homo-L-arginine (homo-Arg) is oxidized by both NOSs in the presence of NADPH with major formation of NO and homo-L-citrulline, with a molar ratio of close to 1, and minor formation of N omega-hydroxyhomo-L-arginine (homo-NOHA). Oxidation of homo-NOHA by the two NOSs also leads to NO and homocitrulline in a 1:1 molar ratio. On the contrary, N omega-hydroxynor-L-arginine (nor-NOHA) is a very poor substrate of NOS I and II, which fails to produce significant amounts of nitrite. The catalytic efficiency of both NOSs markedly decreases in the order Arg > NOHA > homo-Arg > homo-NOHA, as shown by the 20- and 10-fold decrease of kcat/Km observed for NOS I and NOS II, respectively, when comparing Arg to homo-NOHA. The greater loss of catalytic efficiency for homo-Arg, when compared to that for Arg, appears to occur at the first step (N-hydroxylation) of the reaction. In that regard, it is noteworthy that the Vm values for NOHA and homo-NOHA oxidation are very similar (about 1 and 2 micromol of NO min-1 mg of protein-1 for NOS I and II, respectively). In fact, lengthening of the Arg chain by one CH2 leads not only to markedly decreased kcat/Km but also to clear disturbances in NOS functioning. This is shown by a greater accumulation of the N omega-hydroxyguanidine intermediate (homo-NOHA:homocitrulline ratio between 0.2 and 0.4) and an increased consumption of NADPH for NO formation (between 2.0 and 2.6 mol of NADPH consumed for the formation of 1 mol of NO in the case of homo-Arg, instead of 1.5 mol in the case of Arg). Most of the above results could be interpreted by comparing the possible positionings of the various substrates relative to the two NOS active oxygen species which are believed to be responsible for the two steps of the reaction.

The ligand recognition specificity of beta3 integrins

AlphaIIbbeta3 (platelet membrane glycoprotein IIb-IIIa) and alphavbeta3 are members of the beta3 subfamily of integrin adhesion receptors. A cyclic peptide, KYGC(s-s)HarGDWPC(s-s) (cHarGD), originally described by Scarborough et al. (Scarborough, R. M., Naughton, M. A., Teng, W., Rose, J. W., Phillips, D. R., Nannizzi, L., Arsten, A., Campbell, A. M., and Charo, I. F.(1993) J. Biol. Chem. 268, 1066-1073) has been employed as a high affinity ligand for alphaIIbbeta3 to examine the specificity of the beta3 integrins. cHarGD interacted with high affinity with purified alphaIIbbeta3 (Kd = 10 nM) or with platelets (Kd = 120 nM). While cHarGD was specific for alphaIIbbeta3 in the presence of Ca2+, it bound to both beta3 integrins in the presence of Mn2+. Barbourin, a snake venom disintegrin containing a reactive KGD sequence, remained alphaIIbbeta3-specific, even in the presence of Mn2+. cHarGD became cross-linked to a site in beta3 of alphaIIb beta3, which is distinct from that of RGD peptides. These results allow identification of at least four classes of beta3 ligands: Class I, represented by RGD peptides and vitronectin, react similarly with alphaIIbbeta3 and alphavbeta3; Class II, represented by cHarGD, gamma-chain peptides and fibrinogen, react with both receptors in the presence of Mn2+ but only with alphaIIbbeta3 in the presence of Ca2+; Class III, represented by barbourin, are alphaIIbbeta3-specific under all cation conditions; Class IV, represented by osteopontin, bind primarily to alphavbeta3.

L-homoarginine studies provide insight into the antimetabolic properties of L-canavanine

A method for the chemical synthesis of L-homoarginine, based on the guanidination of L-lysine with O-methylisourea, has been developed; this procedure provides radiochemically pure L-[guanidino-14C]homoarginine in high yield. Radiolabeled homoarginine is incorporated readily into the newly synthesized hemolymphic proteins of larvae of the tobacco hornworm, Manduca sexta without adversely affecting larval growth and development. This finding stands in sharp contrast to the effect of L-canavanine, another L-arginine analog, which is markedly deleterious to these larvae. Homoarginine is incorporated into M. sexta lysozyme, and the antibacterial proteins of the fly, Phormia terranovae with impunity. In contrast, the comparable canavanine-containing enzymes are inhibited severely. Experimental evidence is presented that the innocuous nature of homoarginine results from the elevated pKa value of its guanidino group which arguably exceeds even that of arginine. As a result, homoarginine does not disrupt essential residue interactions. In contrast canavanine, which is much less basic than arginine, does adversely affect R group interactions forming the requisite three-dimensional conformation of the protein.

A protein less sensitive to trypsin, guanidinated casein, is a potent stimulator of exocrine pancreas in rats

Previously, we have shown that, in rats that have had bile-pancreatic juice (BPJ) diverted from the proximal small intestine for 7 days, the exocrine pancreatic secretion was enhanced after they were fed a casein, fat-free diet. This demonstrates that the pancreatic secretion is stimulated by dietary protein with a pancreatic protease-independent pathway. To examine the chemical structure of casein responsible for the enhancement of pancreatic secretion, we prepared chemically modified casein in which lysine residues were guanidinated. Secretion of protein, amylase, and chymotrypsin in the chronic BPJ-diverted rat was increased much more after the rats were fed a diet containing guanidinated casein (250 g/kg diet) than after they were fed a diet containing intact casein (250 g/kg diet). In normal rats whose diverted BPJ was returned to the duodenum, the increases in the pancreatic secretion after consuming the guanidinated casein diet were comparable to those after consuming the intact casein diet. In vitro digestibility of guanidinated casein by trypsin and chymotrypsin was much lower than that of intact casein. Also, guanidinated casein inhibited tryptic hydrolysis of benzoyl-L-arginine p-nitroanilide to a lesser extent than did intact casein as determined by an in vitro assay. These results demonstrate that guanidinated casein is less sensitive to trypsin than is intact casein and that the structure that is sensitive to trypsin is not involved in the stimulation of pancreatic secretion in diverted rats. The results evidence that masking luminal trypsin activity does not predominantly contribute to the enhancement of pancreatic secretion in 7-day BPJ-diverted rats. Also, in normal rats, the luminal protease-independent mechanism may play a role partly in increasing the pancreatic secretion by dietary protein.

A new gonadotropin-releasing hormone (GnRH) superagonist in goldfish: influence of dialkyl-D-homoarginine at position 6 on gonadotropin-II and growth hormone release

The two native forms of gonadotropin-releasing hormones (GnRH) present in goldfish, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), stimulate gonadotropin-II (GTH-II) and growth hormone (GH) release both in vivo and in vitro. In our previous study using perifused goldfish pituitary fragments, many mammalian GnRH antagonists, especially those with D-Arg6, showed weak to strong stimulation of GTH-II and GH release. In the present study, the dose-related stimulation of GTH-II and GH release by [Ac-D(2)-Nal1, 4Cl-D-Phe2, D-Trp3, D-Arg6, Trp7, D-Ala10] mGnRH (analog J) and [Ac-D(2)-Nal1, 4Cl-D-Phe2, D-Trp3, D-hArg(Et2)6, D-Ala10] mGnRH (analog K) was demonstrated; the stimulatory potency of both analogs was significantly lower than that of native sGnRH. In the presence of analogs J and K, cGnRH-II stimulated GTH-II release was significantly suppressed. Further, GTH-II and GH stimulation by 2 microM of analog K was significantly suppressed by a 'true' GnRH antagonist, [Ac-delta 3-Pro1, 4FD-Phe2, D-Trp3,6] mGnRH (analog E). These results indicate that analogs J and K increase GTH-II and GH release in goldfish by acting on GnRH receptors on gonadotrophs and somatotrophs. Since analog K, having [D-hArg(Et2)6], strongly stimulated GTH-II release, the potency of [D-hArg(Et2)6] or [D-hArg(CH2CF3)2(6)] substituted analogs to stimulate GTH-II and GH release from the perifused goldfish pituitary fragments was tested. Among the peptides tested, [D-hArg(Et2)6, Pro9-NHEt] sGnRH had a higher potency in stimulating GTH-II release than any other analog tested in the present or in previous studies. For stimulation of GH release, [D-hArg(Et2)6, Pro9-NHEt] sGnRH and [D-Arg6, Pro9-NHEt] sGnRH were the most potent analogs tested; analogs of mGnRH were less potent than sGnRH, indicating the importance of Trp7, Leu8 residues in the native peptide. These results suggest the importance of [D-Arg6] or alkylated [D-Arg6] in determining the intrinsic activity and potency of GnRH peptides in goldfish.