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PubMed2002

Universal solid-phase approach for the immobilization, derivatization, and resin-to-resin transfer reactions of boronic acids.

Boronic acid-containing molecules are employed in a broad range of biological, medicinal, and synthetic applications. These compounds, however, tend to be difficult to handle by solution-phase methods. Herein, this problem is addressed with the development of the first general solid-phase approach for the derivatization of functionalized boronic acids. This approach is based on the use of a diethanolamine resin anchor that facilitates boronic acid immobilization by avoiding the need for exhaustive removal of water in the esterification process. The immobilization of a wide variety of boronic acids onto N,N-diethanolaminomethyl polystyrene (DEAM-PS, 1) can be performed within minutes by simple stirring in anhydrous solvents at room temperature. Evidence for the formation of a bicyclic diethanolamine boronate with putative N-B coordination was shown by (1)H NMR analysis of DEAM-PS-supported p-tolylboronic acid. The hydrolytic cleavage of the same model boronic acid from the DEAM-PS resin was studied by UV spectroscopy. Hydrolysis and attachment were shown to occur under a rapidly attained equilibrium, and a large excess of water (>32 equiv) is required to effect a practically quantitative release of boronic acids from DEAM-PS. Despite their relative sensitivity to water and alcohols, DEAM-PS-bound arylboronic acids functionalized with a formyl, a bromomethyl, a carboxyl, or an amino group can be transformed in good to excellent yields into a wide variety of amines, amides, anilides, and ureas, respectively. Ugi multicomponent reactions on DEAM-PS-supported aminobenzeneboronic acids, derivatization of multifunctional arylboronic acids, and sequential reactions can also be carried out efficiently. These new DEAM-PS-supported arylboronic acids can be employed directly into resin-to-resin transfer reactions (RRTR). This type of multiresin process helps eliminate time-consuming cleavage and transfer operations, thereby considerably simplifying the outlook of combinatorial library synthesis by manual or automated means. This concept was illustrated by a set of optimized procedures for the Suzuki cross-coupling and the borono-Mannich reactions.

Boronic Acids↗

PubMed1988

Proteus mirabilis urease. Partial purification and inhibition by boric acid and boronic acids.

Urease was purified 800-fold and partially characterized from Proteus mirabilis, the predominant microorganism associated with urinary stones. Boric acid is a rapid reversible competitive inhibitor of urease. The pH-dependence of inhibition exhibited pKa values of 6.25 and 9.3, where the latter value is probably due to the inherent pKa of boric acid. Three boronic acids also were shown to inhibit urease competitively.

Boric Acids↗

PubMed1983

The inhibition of class C beta-lactamases by boronic acids.

Aromatic boronic acids are reversible inhibitors of the recently classified class C beta-lactamases. The boronic acids studied include ortho-, meta- and para-methyl-, -hydroxymethyl- and -formyl-phenylboronic acid. The beta-lactamases were chromosomally-encoded enzymes, one from Pseudomonas aeruginosa, and the other specified by the ampC gene of Escherichia coli. The inhibition may be correlated with our finding that these beta-lactamases are serine enzymes, i.e. their function entails the hydroxy group of a serine residue acting as a nucleophile.

Binding Sites↗

PubMed1995

Boronic acid adducts of technetium dioxime (BATO) complexes derived from quinuclidine benzilate (QNB) boronic acid stereoisomers: syntheses and studies of their binding to the muscarinic acetylcholine receptor.

We have investigated the possibility of using BATO complexes derivatized with the muscarinic acetylcholine receptor (mAChR) antagonist, quinuclidinyl benzilate (QNB), for mAChR imaging. The BATO complexes, TcCl(DMG)3B-QNB, were prepared using QNB derivatives containing a 4'-boronic acid substituent on one of the benzilic benzene rings (QNB-boronic acid). The QNB-boronic acid molecule has two chiral centers, and all four QNB-BATO stereoisomers were made and evaluated. When studied using in vitro receptor binding assays based on tissue from rat brain caudate-putamen (which contains primarily M1 and M4 mAChR) and rat heart (M2 mAChR), the QNB-boronic acid stereoisomers had binding affinities (KA) in the range 2 x 10(5)-1 x 10(8), at least 10-fold lower than the KA for QNB (ca 2 x 10(9)). The stereochemistry of both centers had some influence on the affinity constant. When the TcCl(DMG)3B-QNB complexes were studied, none of the stereoisomeric complexes displayed measurable specific binding (KA < 10(6)), but all showed high non-specific binding. In vitro autoradiography with rat brain slices confirmed the absence of specific binding in these tracers. In vivo, the 99mTcCl(DMG)3B-QNB complexes displayed minimal brain uptake, and modest heart uptake; the latter was unlikely to be related to uptake by the mAChR. In light of these findings, we conclude that the interaction between the TcCl(DMG)3B-QNB complexes and biological membranes is dominated by the hydrophobicity of the BATO moiety. The TcCl(DMG)3B-QNB complexes, therefore, have little potential for mAChR imaging.

Animals↗

PubMed1997

Cysteine proteases such as papain are not inhibited by substrate analogue peptidyl boronic acids.

Peptidyl boronic acids that are close structural analogues of good substrates have been prepared and evaluated as potential transition state analogue inhibitors of the representative cysteine protease, papain. However, no inhibition could be detected at concentrations up to 10 mM. The reasons for the lack of inhibition were sought from molecular modeling. Molecular mechanics and semi-empirical quantum mechanics calculations indicated that the absence of inhibition was due to boronic acid-cysteine protease tetrahedral complexes being 0.79 kcal mol-1 less stable than their preceding noncovalent EI-complexes. In contrast, an analogous boronic acid-serine protease tetrahedral complex was calculated to be 2.74 kcal mol-1 more stable than its precursor Michaelis EI-complex. It thus appears that boronic acids are ineffective inhibitors of cysteine proteases due to the thermodynamic favoring of a weak EI-complex preceding tetrahedral intermediate formation, and that any oxyanion hole stabilization of the subsequent tetrahedral intermediate cannot overcome this energy handicap.

Boronic Acids↗

PubMed2003

Inhibition of the enzymatic activity of prostate-specific antigen by boric acid and 3-nitrophenyl boronic acid.

BACKGROUND: Prostate specific antigen (PSA) is a well-established marker of prostate cancer, but it can also degrade extracellular matrix proteins such as fibronectin and could be involved in tumor progression and metastasis. In this study, we have addressed the use of boric acid and 3-nitrophenyl boronic acid (NPBA) as PSA inhibitors in vitro. METHODS: The inhibition of PSA by boric acid was studied by using specific fluorogenic substrates. Fibronectin, a biologically relevant substrate for PSA, was used as a substrate in a zymographic assay, and the degradation of fibronectin by PSA in the presence of boric acid and NPBA was followed by Western Blot. RESULTS: Low concentrations of boric acid partially inhibited the proteolytic activity of PSA toward a synthetic fluorogenic substrate. Also, by Western blot, we have found significant inhibition in the proteolysis of fibronectin by PSA in the presence of boric acid as well as NPBA. Results indicate that the boronated compounds used in this study can be used for the modulation of PSA activity. CONCLUSION: PSA activity is inhibited in vitro by boric acid and NPBA. If degradation of fibronectin by PSA were, in fact, an important step in the progression of prostate cancer, then borate-induced inhibition of PSA activity should help reduce the development and proliferation of prostate carcinomas.

Blotting, Western↗

PubMed1997

Inhibition of dipeptidyl peptidase IV (CD26) by peptide boronic acid dipeptides.

Peptide boronic acid dipeptide compounds were analyzed for their ability to inhibit recombinant human dipeptidylpeptidase IV (CD26, DPPIV). Rate constants for the peptide boronates are difficult to obtain because the active boronic acid dipeptide exists in equilibrium with a cyclic inactive species in aqueous solution. Rate constants were determined for the inhibition of DPPIV using several peptide boronates at different pH values. Val-boroPro forms the most tightly bound complex with DPPIV; the first order half life for dissociation of the inactive enzyme-inhibitor complex at 23 degrees C is approximately 27 days.

Binding, Competitive↗

PubMed1989

Do inert beta-lactamase inhibitors act as synergizers of beta-lactam antibiotics? Utility of boric and boronic acids.

Boric and boronic acids were used as inhibitors of beta-lactamases produced by two Citrobacter diversus strains and by one strain of Pseudomonas aeruginosa; all strains were clinical isolates. The beta-lactamases produced by the two Citrobacter diversus strains were inhibited by both borates and boronates, using cephazolin as substrate. The enzyme from Pseudomonas aeruginosa was inhibited only by boronates, using benzylpenicillin as substrate. These inhibitors were also used in combination with selected beta-lactams so as to determine if a synergism of antimicrobial activity occurred. All data reported in the present paper indicate that the minimum inhibitory concentration (MIC) values were lowered in the presence of these inhibitors for the two Citrobacter diversus strains. In the Pseudomonas aeruginosa strains the MIC values were not significantly altered, thus indicating the presence of a permeability barrier for 3-aminophenylboronic acid.

Anti-Bacterial Agents↗

PubMedDate not supplied

Phenylboronic acid-salicylhydroxamic acid bioconjugates. 1. A novel boronic acid complex for protein immobilization.

A chemical affinity system exhibiting antibody-like properties is described. The system exploits bioconjugates with appended phenylboronic acid (PBA) moieties and a support-bound phenylboronic acid complexing reagent derived from salicylhydroxamic acid (SHA) for protein immobilization on a chromatographic support. The structure of the PBA.SHA complex was characterized by 11B NMR and mass spectrometry and compared with complexes derived from model compounds. Protein modification reagents were synthesized from 3-aminophenylboronic acid and utilized to prepare bioconjugates from alkaline phosphatase (AP) and horseradish peroxidase (HRP). AP obtained from one source afforded PBA bioconjugates exhibiting significant loss of enzymatic activity, whereas AP obtained from a second source afforded PBA bioconjugates exhibiting only a modest loss of enzymatic activity. Conversely, HRP afforded PBA bioconjugates exhibiting no loss of enzymatic activity. SHA-modified Sepharose was prepared by reaction of methyl 4-[(6-aminohexanoylamino)methyl]salicylate with CNBr-activated Sepharose 4B, followed by treatment with aqueous alkaline hydroxylamine. PBA-AP and PBA-HRP conjugates were efficiently immobilized on SHA-Sepharose at pH 8.3. PBA-AP conjugates were retained after washing with acidic buffers at pH 6.7, 4.2, and 2.5, whereas PBA-HRP conjugates were retained after washing with buffer at pH 6.7, but were eluted to some extent at and below pH 4.2. The results are interpreted in terms of multivalent interactions involving boronic acid complex formation between the enzyme bioconjugates and immobilized complexing reagent.

Alkaline Phosphatase↗

PubMed1980

Boronic acid inhibitors of porcine pancreatic lipase.

Porcine pancreatic lipase was inhibited by alkane and arene boronic acids. The inhibition by octadecane boronic acid was competitive when measured against the hydrolysis of dissolved tripropionin in the presence of siliconized glass beads. The value of Ki in this system was 1.34 x 10(3) molecules micron-2. The ratio of substrate to inhibitor concentrations giving 50% inhibition was in the range of 700 to 2200, indicating that lipase has a greater affinity for boronic acids than for tripropionin. Boronic acids did not interfere with the interaction of lipase with the siliconized glass/water interface, demonstrating that the binding of lipase to substrate interfaces, the first step in lipase action, was not the step at which inhibition occurred. The boronic acid binding site on lipase is at or near the active center serine since modification of this residue by diethyl p-nitrophenyl phosphate was prevented by boronic acids. Modification of the active center serine residue by diethyl p-nitrophenyl phosphate also prevented boronic acid binding. Binding of a chromophoric boronic acid, 7-nitrobenzo-2-oxa-1,3-diazolyl m-aminobenzene boronic acid, to lipase was demonstrated by equilibrium gel filtration on polyacrylamide beads (Bio-Gel P-60) in the presence of 4 mM sodium taurodeoxycholate. The complex contained 1 molecule of boronic acid per molecule of lipase and had a dissociation constant Kd of 5 x 10(-6) M. The boronic acid was not bound in the absence of taurodeoxycholate. Boronic acids are believed to be analogs of the tetrahedral intermediate in the action of lipase.

Animals↗

PubMed1998

Potent and selective inhibitors of the proteasome: dipeptidyl boronic acids.

Potent and selective dipeptidyl boronic acid proteasome inhibitors are described. As compared to peptidyl aldehyde compounds, boronic acids in this series display dramatically enhanced potency. Compounds such as 15 are promising new therapeutics for treatment of cancer and inflammatory diseases.

Boronic Acids↗

PubMed1988

Kinetic properties of the binding of alpha-lytic protease to peptide boronic acids.

The kinetic parameters for peptide boronic acids in their interaction with alpha-lytic protease were determined and found to be similar to those of other serine proteases [Kettner, C., & Shenvi, A. B. (1984) J. Biol. Chem. 259, 15106-15114]. alpha-Lytic protease hydrolyzes substrates with either alanine or valine in the P1 site and has a preference for substrate with a P1 alanine. The most effective inhibitors are tri- and tetrapeptide analogues that have a -boroVal-OH residue in the P1 site. At pH 7.5, MeOSuc-Ala-Ala-Pro-boroVal-OH has a Ki of 6.4 nM and Boc-Ala-Pro-boroVal-OH has a Ki of 0.35 nM. Ac-boroVal-OH and Ac-Pro-boroVal-OH are 220,000- and 500-fold less effective, respectively, than the tetrapeptide analogue. The kinetic properties of the tri- and tetrapeptide analogues are consistent with the mechanism for slow-binding inhibition, E + I in equilibrium EI in equilibrium EI*, while the less effective inhibitors are simple competitive inhibitors. MeO-Suc-Ala-Ala-Pro-boroAla-OH is a simple competitive inhibitor with a Ki of 67 nM at pH 7.5. Other peptide boronic acids, which are analogues of nonsubstrates, are less effective than substrate analogues but still are effective competitive inhibitors. For example, MeOSuc-Ala-Ala-Pro-boroPhe-OH has a Ki of 0.54 microM although substrates with a phenylalanine in the P1 position are not hydrolyzed. Binding for boronic acid analogues of both substrate and nonsubstrate analogues is pH dependent with higher affinity near pH 7.5. Similar binding properties have been observed for pancreatic elastase. Both enzymes have almost identical requirements for an extended peptide inhibitor sequence in order to exhibit highly effective binding and slow-binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗

PubMed1985

Acylamino boronic acids and difluoroborane analogues of amino acids: potent inhibitors of chymotrypsin and elastase.

A series of 1-acylamino boronic acids (IA-VA), analogues of the amino acids phenylalanine, phenylglycine, alanine, valine, and isoleucine, were prepared as potential transition-state inhibitors of the serine proteases alpha-chymotrypsin and elastase, by a boronate homologation reaction. The corresponding difluoroboranes (IB-VB), produced from the boronic acids by treatment with HF, were more easily purified than the boronic acids. Since the difluoroboranes readily hydrolyze in water, they proved to be convenient precursors for the boronic acids. The phenylalanine and phenylglycine analogues I and II were good competitive inhibitors of alpha-chymotrypsin (Ki = 0.3-8 microM), and the alanine, valine, and isoleucine analogues (III-IV) proved to be good inhibitors of elastase (Ki = 0.1-35 microM). On the basis of their high affinity and the tendency of boronic acids to form borate complexes, these acylamino boronic acids may be behaving as transition-state inhibitors.

Acylation↗

PubMed2003

Boronic acid compounds as potential pharmaceutical agents.

Boronic acid compounds have been used, because of their unique structural features, for the development of potent enzyme inhibitors, boron neutron capture agents for cancer therapy, and as antibody mimics that recognize biologically important saccharides. Consequently, there has been a surge of interests in boronic acid compounds. This study reviews the recent development in this area during the last six years.

Acids↗

PubMed1988

Peptide boronic acids, substrate analogs, inhibit chymase, and histamine release from rat mast cells.

Peptide boronic acids, such as methoxysuccinyl-Ala-Ala-Pro-(L)boro-Phe-OH, its pinacol ester, and t-butyloxycarbonyl-Phe-Pro-(L)boro-Phe-pinacol, inhibited the activity of chymase from connective tissue mast cells approximately 40- to 80-fold more than atypical chymase from mucosal mast cells, and did not inhibit trypsin. Only peptide boronic acids containing "L" forms of boronic acids were inhibitory. The Ki values of these peptide boronic acids for chymase were in the 60-170 nM concentration range, like those of the natural inhibitors tested, but all the natural inhibitors tested except Eglin C and chymostatin inhibited both chymase and trypsin. Thus these peptide boronic acids should be useful for selective inhibition of chymase with less inhibitory activity for atypical chymase and without inhibition of trypsin. These peptide boronic acids markedly inhibited histamine release induced by anti-rat immunoglobulin E, suggesting that chymase in connective tissue mast cells plays some role in the process of histamine release. These peptides are assumed to be therapeutically useful for treatment of allergic inflammations catalyzed by chymase.

Animals↗

PubMed1988

Beta-lactamase inhibitors. The inhibition of serine beta-lactamases by specific boronic acids.

Many beta-lactamases have active-site serine residues, and are competitively inhibited by boronic acids. Hitherto, the boronic acids used have lacked any structural resemblance to the substrates of beta-lactamases. Phenylacetamidomethaneboronic acid, trifluoroacetamidomethaneboronic acid and 2,6-dimethoxybenzamidomethaneboronic acid have now been synthesized. The first of these contains the side-chain moiety of penicillin G, and the last that of methicillin. The pH-dependence of binding of the first inhibitor to beta-lactamase I from Bacillus cereus revealed pK values of 4.7 and 8.2 for (presumably) active-site groups in the enzyme. The kinetics of inhibition were studied by cryoenzymology and by stopped-flow spectrophotometry. These techniques provided evidence for a two-step mechanism of binding of the first two boronic acids mentioned above to beta-lactamase I, and for benzeneboronic acid to a beta-lactamase from Pseudomonas aeruginosa. The slower step is probably associated with a change in enzyme conformation as well as the formation of an O-B bond between the active-site serine hydroxy group and the boronic acid.

Bacillus cereus↗

PubMed1992

Identification of serine and histidine adducts in complexes of trypsin and trypsinogen with peptide and nonpeptide boronic acid inhibitors by 1H NMR spectroscopy.

We have previously shown, in 15N NMR studies of the enzyme's active site histidine residue, that boronic acid inhibitors can form two distinct types of complexes with alpha-lytic protease. Inhibitors that are structural analogs of good alpha-lytic protease substrates form transition-state-like tetrahedral complexes with the active site serine whereas those that are not form complexes in which N epsilon 2 of the active site histidine is covalently bonded to the boron of the inhibitor. This study also demonstrated that the serine and histidine adduct complexes exhibit quite distinctive and characteristic low-field 1H NMR spectra [Bachovchin, W. W., Wong, W. Y. L., Farr-Jones, S., Shenvi, A. B., & Kettner, C. A. (1988) Biochemistry 27, 7689-7697]. Here we have used low-field 1H NMR diagnostically for a series of boronic acid inhibitor complexes of trypsin and trypsinogen. The results show that H-D-Val-Leu-boroArg and Ac-Gly-boroArg, analogs of good trypsin substrates, form transition-state-like serine adducts with trypsin, whereas the nonsubstrate analog inhibitors boric acid, methane boronic acid, butane boronic acid, and triethanolamine borate all form histidine adducts, thereby paralleling the previous results obtained with alpha-lytic protease. However, with trypsinogen, Ac-Gly-boroArg forms predominantly a histidine adduct while H-D-Val-Leu-boroArg forms both histidine and serine adducts, with the histidine adduct predominating below pH 8.0 and the serine adduct predominating above pH 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1983

A calorimetric investigation of the binding of indole and phenylethane boronic acid to chymotrypsin.

The heat of formation of the chymotrypsin-phenylethane boronic acid complex has been observed calorimetrically from pH 4 to 8 at 25 degrees C and is found to be pH-dependent, changing from near -6 kcal/mol at pH 4 to -13 kcal/mol at pH 8. The heat of formation of the chymotrypsin-indole complex is a nearly constant -6 kcal/mol over most of the same pH range. alpha-Chymotrypsin has been purified by pH gradient elution from an immobilized lima bean inhibitor column. Solutions of the enzyme up to 400 microM, prepared in this manner, have a zero heat of dilution from pH 5 to 8 in 0.1 M KCl, with or without added 0.05 M Tris, N-(tris[hydroxy-methyl]methyl-2-amino) ethanesulfonic acid, 4-morpholineethanesulfonic acid, or acetate buffers. Binding of phenylethane boronic acid causes a pH-dependent decrease in proton binding to chymotrypsin; the decrease in proton binding evoked by formation of the indole complex is much less, with a much smaller pH dependence. The calorimetric and proton-binding results are applied to a model for boronic acid binding (Hanai, K. (1976) J. Biochem. (Tokyo) 79, 107-116). We conclude that the thermodynamics of formation of the trigonal boronic acid complex are quite similar to those for the formation of the noncovalent complex formed by indole and related ligands. The trigonal-tetrahedral tautomerism in the boronic acid-chymotrypsin complex is characterized by thermodynamic changes similar to those accompanying the binding of virtual substrates to chymotrypsin.

Boronic Acids↗