Mechanism of inactivation of chymotrypsin by 3-benzyl-6-chloro-2-pyrone

The mechanism of inactivation of chymotrypsin by 3-benzyl-6-chloro-2-pyrone has been studied. Chloride analysis of the inactivated enzyme suggests that the complex does not contain intact chloropyrone or an acid chloride. 13C NMR studies of the enzyme inactivated with 13C-enriched chloropyrones show that (1) the pyrone ring is no longer intact, (2) C-6 becomes a carboxylate group and C-2 becomes esterified to the enzyme, probably to serine-195, and (3) a double bond is present adjacent to the serine ester. The inactivated enzyme slowly regains catalytic activity with the concomitant release of (E)-4-benzyl-2-pentenedioic acid. It is concluded that double bond migration occurs during reactivation since the position of the double bond in the released diacid product is different than in the inactivator-enzyme complex. When the reactivation is carried out in [18O]H2O-enriched water, a single oxygen-18 is incorporated into the released product and is further evidence that the inactivator is bound to the enzyme only through a single ester linkage. A deuterium isotope effect on reactivation is observed when a chloropyrone deuterated at C-5 is used. This result demonstrates that removal of a proton from C-5 is required for reactivation and that isomerization of the double bond and not hydrolysis of the acyl-enzyme is rate determining. A variety of amines accelerate the rate of reactivation by functioning as general bases and not as nucleophiles. A reaction scheme is presented that accounts for the formation of the stable inactivator-enzyme complex as well as the production of two products derived from enzymatic hydrolysis of the chloropyrone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Novel inactivators of serine proteases based on 6-chloro-2-pyrone

The interaction of serine protease (esterases) with 6-chloro-2-pyrones was investigated. Time-dependent inactivation of chymotrypsin, alpha-lytic protease, pig liver elastase, and cholinesterase was found with 3- and 5-benzyl-6-chloro-2-pyrone, as well as 3- and 5-methyl-6-chloro-2-pyrone. No inactivation was observed with the unsubstituted 6-chloro-2-pyrone. The substituted pyrones did not inactivate papain or carboxypeptidase A, as well as a number of other nonproteolytic enzymes. The substituted chloropyrones, therefore, show considerable selectivity toward serine proteases. Analogues in which the 6-chloro substituent is replaced by H or OH do not inactivate. The presence of the halogen is, therefore, essential for inactivation. Chymotrypsin catalyzes the hydrolysis of 3-benzyl-6-chloro-2-pyrone. At pH 7.5, (E)-4-benzyl-2-pentenedioic acid is the major product, and 2-benzyl-2-pentenedioic anhydride is a minor product. The ration of hydrolysis product found to the number of enzyme molecules inactivated varies from 14 to 40. The enzyme inactivated with the 3-benzyl compound does not show a spectrum characteristic of the pyrone ring. This suggests that inactivation by 3-benzyl-6-chloro-2-pyrone occurs in a mechanism-based fashion after enzymatic lactone hydrolysis. When the enzyme is inactivated with the 5-benzyl compound, absorbance due to the pyrone ring is observed. We suggest that inactivation occurs through an active site directed mechanism involving a 1,6-conjugate addition of an active site nucleophile to the pyrone ring.     

X-ray diffraction analysis of the inactivation of chymotrypsin by 3-benzyl-6-chloro-2-pyrone

The inactivation of chymotrypsin by 3-benzyl-6-chloro-2-pyrone has been studied. A covalent adduct is formed that deacylates slowly with a half-life of 23 h. X-ray diffraction analysis at 1.9-A resolution of the inactivator-enzyme complex shows that the gamma-oxygen of the active-site serine (serine-195) is covalently attached to C-1 of (Z)-2-benzylpentenedioic acid, the benzyl group of the inactivator is held in the hydrophobic specificity pocket of the enzyme, and the free carboxylate forms a salt bridge with the active-site histidine (histidine-57). The conformational changes that occur in the protein as a result of complexation are described. It is proposed that formation of the salt bridge prevents access of water and, therefore, hydrolysis of the acyl-enzyme.     

Adenosine deaminase converts purine riboside into an analogue of a reactive intermediate: a 13C NMR and kinetic study

The 13C NMR spectra of [2-13C]- and [6-13C]purine ribosides have been obtained free in solution and bound to the active site of adenosine deaminase. The positions of the resonances of the bound ligand are shifted relative to those of the free ligand as follows: C-2, -3.7 ppm; C-6, -73.1 ppm. The binary complexes are in slow exchange with free purine riboside on the NMR time scale, and the dissociation rate constant is estimated to be 13.5 s-1 from the slow exchange broadening of the free signal. In aqueous solution, protonation of purine riboside at N-1 results in changes in 13C chemical shift relative to those of the free base as follows: C-2, -4.9 ppm; C-6, -7.9 ppm. The changes in chemical shift that occur when purine riboside binds to the enzyme indicate that the hybridization of C-6 changes from sp2 to sp3 in the binary complex with formation of a new bond to oxygen or sulfur. A change in C-2 hybridization can be eliminated as can protonation at N-1 as the sole cause of the chemical shift changes. The kinetic constants for the adenosine deaminase catalyzed hydrolysis of 6-chloro- and 6-fluoropurine riboside have been compared, and the reactivity order implies that carbon-halogen bond breaking does not occur in the rate-determining step. These observations support a mechanism for the enzyme in which formation of a tetrahedral intermediate is the most difficult chemical step. Enzymic stabilization of this intermediate may be an important catalytic strategy used by the enzyme to lower the standard free energy of the preceding transition state.     

Inhibition of yeast lipase (CRL1) and cholesterol esterase (CRL3) by 6-chloro-2-pyrones: comparison with porcine cholesterol esterase

Previously, it was demonstrated that pancreatic cholesterol esterase is selectively inhibited by 6-chloro-2-pyrones with cyclic aliphatic substituents in the 3-position. Inhibition is reversible and is competitive with substrate. Pancreatic cholesterol esterase is a potential target for treatment of hypercholesterolemia. In the present study, yeast cholesterol esterase from Candida cylindracea (also called C. rugosa CRL3) was compared to porcine pancreatic cholesterol esterase for inhibition by a series of 3-alkyl- or 5-alkyl-6-chloro-2-pyrones. In addition, CRL3 was compared with the related yeast lipase CRL1. Inhibition of CRL3 by substituted 6-chloro-2-pyrones was competitive with binding of the substrate p-nitrophenyl butyrate. Inhibition constants ranged from 0.2 microM to >90 microM. Small changes in the alkyl group had profound effects on binding. The pattern of inhibition of CRL3 is quite distinct from that observed with porcine cholesterol esterase. Molecular modeling studies suggest that the orientation of binding of these inhibitors at the active site of CRL3 can vary but that the pyrone ring consistently occupies a position close to the active site serine. CRL1 is highly homologous to CRL3. Nevertheless, patterns of inhibition of CRL1 by substituted 6-chloro-2-pyrones differ markedly from patterns observed with CRL3. The substituted 6-chloro-2-pyrones are slowly hydrolyzed in the presence of CRL1 and are pseudosubstrates of CRL3, but are simple reversible inhibitors of pancreatic cholesterol esterase     

Synthesis and testing of 5-benzyl-2,4-diaminopyrimidines as potential inhibitors of leishmanial and trypanosomal dihydrofolate reductase

Dihydrofolate reductase is a drug target that has not been thoroughly investigated in leishmania and trypanosomes. Work has previously shown that 5-benzyl-2,4-diaminopyrimidines are selective inhibitors of the leishmanial and trypanosome enzymes. Modelling predicted that alkyl/aryl substitution on the 6-position of the pyrimidine ring should increase enzyme activity of 5-benzyl-2,4-diaminopyrimidines as inhibitors of leishmanial and trypanosomal dihydrofolate reductase. Various compounds were prepared and evaluated against both the recombinant enzymes and the intact organisms. The presence of a substituent had a small or negative effect on activity against the enzyme or intact parasites compared to unsubstituted compounds.     

Mechanism-based inactivation of dihydropyrimidine dehydrogenase by 5-ethynyluracil.

Uracil analogues with appropriate substituents at the 5-position inactivated dihydropyrimidine dehydrogenase (DHPDHase). The efficiency of these inactivators was highly dependent on the size of the 5-substituent. For example, 5-ethynyluracil inactivated DHPDHase with an efficiency (kinact/Ki) that was 500-fold greater than that for 5-propynyluracil. 5-Ethynyluracil inactivated DHPDHase by initially forming a reversible complex with a Ki of 1.6 +/- 0.2 microM. This initial complex yielded inactivated enzyme with a rate constant of 20 +/- 2 min-1 (kinact). Thymine competitively decreased the apparent rate constant for inactivation of DHPDHase by 5-ethynyluracil. The absorbance spectrum of 5-ethylnyluracil-inactivated DHPDHase was different from that of reduced enzyme. These optical changes were correlated with the loss of enzymatic activity. 5-Ethynyluracil inactivated DHPDHase with a stoichiometry of 0.9 mol of inactivator per mol of active site. Enzyme inactivated with [2-14C]5-ethynyluracil retained all of the radiolabel after denaturation in 8 M urea, but lost radiolabel under acidic conditions. These results suggested that inactivation was due to covalent modification of an amino acid residue and not due to modification of a noncovalently bound prosthetic group. A radiolabeled peptide was isolated from a tryptic digest of the enzyme inactivated with [2-14C]5-ethynyluracil. The sequence of this peptide was Lys-Ala-Glu-Ala-Ser-Gly-Ala-Y-Ala-Leu-Glu-Leu-Asn-Leu-Ser-X-Pro-His-Gly- Met-Gly-Glu-Arg, where X and Y were unidentified amino acids. Since the radiolabel was lost from the peptide during the first cycle on the amino acid sequenator, the position of the radiolabeled amino acid was not determined. The amino acid residue designated by X was identified as a cysteine from previous work with DHPDHase inactivated with 5-iodouracil. In contrast to 5-ethynyluracil, 5-cyanouracil was a reversible inactivator of the enzyme. 5-Cyanouracil-inactivated enzyme slowly regained activity (t1/2 = 1.8 min) after dilution into the standard assay. DHPDHases isolated from rat, mouse, and human liver had similar sensitivities to inactivation by 5-alkynyluracils.     

Synthesis and Testing of 5-Benzyl-2,4-diaminopyrimidines as Potential Inhibitors of Leishmanial and Trypanosomal Dihydrofolate Reductase

Dihydrofolate reductase is a drug target that has not been thoroughly investigated in leishmania and trypanosomes. Work has previously shown that 5-benzyl-2,4-diaminopyrimidines are selective inhibitors of the leishmanial and trypanosome enzymes. Modelling predicted that alkyl/aryl substitution on the 6-position of the pyrimidine ring should increase enzyme activity of 5-benzyl-2,4-diaminopyrimidines as inhibitors of leishmanial and trypanosomal dihydrofolate reductase. Various compounds were prepared and evaluated against both the recombinant enzymes and the intact organisms. The presence of a substituent had a small or negative effect on activity against the enzyme or intact parasites compared to unsubstituted compounds.     

Solid-state NMR observation of cysteine and lysine Michael adducts of inactivated estradiol dehydrogenase

The inactivation of estradiol dehydrogenase by enzyme-generated 3-hydroxy-14,15-secoestra-1,3,5(10)-trien-15-yn-17-one is accompanied by the formation of a lysine enaminone. The experiments leading to this conclusion involved degradation of the inactivated enzyme with Pronase and subsequent analysis by solution-state 13C NMR. The present paper reports solid-state 13C NMR experiments on lyophilized intact inactivated enzyme which are free from problems due to Pronase digestion. These experiments combine conventional cross-polarization and magic-angle spinning with selective irradiation of resonances arising from a 13C double label in the steroid. Magnetization transfer between neighboring 13C nuclei is used to simplify the spectra and to identify peaks due to label. The formation of cysteine and lysine Michael adducts of the enzyme is established by comparisons with chemical shifts of solid model adducts.     

The action of plasma amine oxidase on beta-haloamines. Evidence for proton abstraction in the oxidative reaction.

The action of plasma amine oxidase upon beta-Br-ethylamine beta-Cl-ethylamine, beta-OH-phenylethylamine, and beta-Cl-phenylethylamine was examined. Beta-Br-ethylamine is a substrate and irreversible inactivator of the enzyme. The enzyme becomes covalently labeled by the inactivator. Approximately 2 mol of inactivator are incorporated per mol of enzyme (MW 170,000). The reduced enzyme is not inactivated. The enzyme catalyzes the elimination of HCl from beta-Cl-phenylethylamine to produce phenylacetaldehyde. The rate of the elimination reaction is comparable to the normal oxidative reaction. We conclude that the occurrence of this elimination reaction establishes the ability of the enzyme to catalyze proton abstraction from C-1 of the substrate and that proton abstraction occurs during the catalytic oxidation normally catalyzed by plasma amine oxidase. Beta-Cl-ethylamine is only oxidized to corresponding aldehyde. Beta-OH-phenylethylamine is neither oxidized, nor does elimination occur. It is a competitive inhibitor in the oxidation of benzylamine and in the elimination of HCl from beta-Cl-phenylethylamine.     

A 13C-n.m.r. investigation of ionizations within a trypsin-inhibitor complex. Evidence that the pKa of histidine-57 is raised by interaction with the hemiketal oxyanion.

The 13C-n.m.r. titration shifts of the alpha-methylene group of N-alkylated imidazoles are shown to be a sensitive probe of the ionization of the imidazolium ion. The 13C-n.m.r. titration shifts of both the intact and denatured/autolysed 2-13C- and 1-13C-enriched trypsin-7-amino-3-benzyloxycarbonylamino-1-chloroheptan-2-one (Z-Lys-CH2Cl) complexes are compared. The titration shift for the denatured/autolysed complex confirms that this ionization is due to deprotonation of the N-alkylated imidazolium ring of histidine-57. In the intact trypsin-inhibitor complex the titration shift due to the 1-13C-enriched carbon is anomalous. We conclude that this titration shift cannot arise solely from the ionization of the imidazolium ion of histidine-57 and that the pKa of the imidazolium ion of histidine-57 is raised in the trypsin-inhibitor complex. The relevance of these studies to the mechanism of action of the serine proteinases is discussed.     

Carbon-13 nuclear magnetic resonance studies of adenosylcobalamin and alkylcorrinoids, selectively enriched with carbon-13.

The carbon-13 nuclear magnetic resonance spectra of a series of alkylcorrinoids, selectively enriched with 13C in the alkyl ligand, were recorded at 25.2 MHz and 25 degrees. The nature of the axial ligands markedly affects the chemical shift of the labeled alkyl moiety (trans effect) as well as the 13C resonances of selected carbon atoms of the corrin ring (cis effect). Although a number of factors appear to influence the trans effect on the chemical shift of the alkyl ligand (important among them being electric field effects), the cis effect appears to be dominated by changes in charge density (at the methine bridge carbon atoms, C-5, C-10, C-15) and by steric effects (at the methyl groups at C-1, C-5, and C-15) accompanying axial ligation. Spin-latice relaxation times of several organocorrinoids, selectively labeled with 13C in the ligands attached to cobalt, were also measured. The T1 values of the methylene carbons of [5'-13C]adenosylcobalamin and [2-13C]carboxymethylcobalamin are very similar to that of the methine bridge carbon atom C-10 of the corrin ring, indicating that rotation about the carbon-cobalt bond of these two corrinoids is severely restricted. On the other hand, internal rotation about the carbon-cobalt bond of methylcobalamin is rapid.     

The novel pyrimidine and purine derivatives of l-ascorbic acid: synthesis, one- and two-dimensional 1H and 13C NMR study, cytostatic and antiviral evaluation

The syntheses of the novel C-5 substituted pyrimidine derivatives of l-ascorbic acid containing free hydroxy groups at C-2' (6-10) or C-2' and C-3' (11-15) positions of the lactone ring are described. Debenzylation of the 6-chloro- and 6-(N-pyrrolyl)purine derivatives of 2,3-O,O-dibenzyl-l-ascorbic acid (16 and 17) gave the new compounds containing hydroxy groups at C-2' (18) and C-2' and C-3' (19 and 20). Z- and E-configuration of the C4'C5' double bond and position of the lactone ring of the compounds 6-9 were deduced from their one- and two-dimensional (1)H and (13)C NMR spectra and connectivities in NOESY and HMBC spectra. Compounds 15 and 18 showed the best inhibitory activities of all evaluated compounds in the series. The compound 15 containing 5-(trifluoromethyl)uracil showed marked inhibitory activity against all human malignant cell lines (IC(50): 5.6-12.8 microM) except on human T-lymphocytes. Besides, this compound influenced the cell cycle by increasing the cell population in G2/M phase and induced apoptosis in SW 620 and MiaPaCa-2 cells. The compound 18 containing 6-chloropurine ring expressed the most pronounced inhibitory activities against HeLa (IC(50): 6.8 microM) and MiaPaCa-2 cells (IC(50): 6.5 microM). The compound 20 with 6-(N-pyrrolyl)purine moiety showed the best differential inhibitory effect against MCF-7 cells (IC(50): 35.9 microM).     

Reaction of serine proteases with substituted 3-alkoxy-4-chloroisocoumarins and 3-alkoxy-7-amino-4-chloroisocoumarins: new reactive mechanism-based inhibitors

The time-dependent inactivation of several serine proteases including human leukocyte elastase, cathepsin G, rat mast cell proteases I and II, and human skin chymase by a number of 3-alkoxy-4-chloroisocoumarins, 3-alkoxy-4-chloro-7-nitroisocoumarins, and 3-alkoxy-7-amino-4-chloroisocoumarins at pH 7.5 and the inactivation of several trypsin-like enzymes including human thrombin and factor XIIa by 7-amino-4-chloro-3-ethoxyisocoumarin and 4-chloro-3-ethoxyisocoumarin are reported. The 3-alkoxy substituent of the isocoumarin is likely interacting with the S1 subsite of the enzyme since the most reactive inhibitor for a particular enzyme had a 3-substituent complementary to the enzyme's primary substrate specificity site (S1). Inactivation of several enzymes including human leukocyte elastase by the 3-alkoxy-7-amino-4-chlorisocoumarins is irreversible, and less than 3% activity is regained upon extensive dialysis of the inactivated enzyme. Addition of hydroxylamine to enzymes inactivated by the 3-alkoxy-7-amino-4-chloroisocoumarins results in a slow (t1/2 greater than 6.7 h) and incomplete (32-57%) regain in enzymatic activity at pH 7.5. Inactivation by the 3-alkoxy-4-chloroisocoumarins and 3-alkoxy-4-chloro-7-nitroisocoumarins on the other hand is transient, and full enzyme activity is regained rapidly either upon standing, after dialysis, or upon the addition of buffered hydroxylamine. The rate of inactivation by the substituted isocoumarins is decreased when substrates or reversible inhibitors are present in the incubation mixture, which indicates active site involvement. The inactivation rates are dependent upon the pH of the reaction mixture, the isocoumarin ring system is opened concurrently with inactivation, and the reaction of 3-alkoxy-7-amino-4-chloroisocoumarins with porcine pancreatic elastase is shown to be stoichiometric. The results are consistent with a scheme where 3-alkoxy-7-amino-4-chloroisocoumarins react with the active site serine of a serine protease to give an acyl enzyme in which a reactive quinone imine methide can be released. Irreversible inactivation could then occur upon alkylation of an active site nucleophile (probably histidine-57) by the acyl quinone imine methide. The finding that hydroxylamine slowly catalyzes partial reactivation indicates that several inactivated enzyme species may exist. The 3-alkoxy-substituted 4-chloroisocoumarins and 4-chloro-7-nitroisocoumarins are simple acylating agents and do not give stable inactivated enzyme structures.(ABSTRACT TRUNCATED AT 400 WORDS)     

NMR and ESR studies on human pregnancy zone protein. Comparison with human alpha 2-macroglobulin.

NMR and ESR spectroscopies have been used to examine the plasma protease inhibitor pregnancy zone protein (PZP) and its complex with chymotrypsin. The 1H NMR spectrum of PZP shows relatively few sharp resonances, which, by analogy with human alpha 2-macroglobulin, probably arise from the proteolytically sensitive bait region. Upon reaction with chymotrypsin to form a 1:1 protease.PZP tetramer complex, there is a large increase in the intensity of sharp resonances due to an increase in mobility of these residues. 35Cl NMR has been used to follow binding of zinc and manganese to apo-PZP. Zinc binding causes a linear broadening of the bulk Cl-, consistent with access of Cl- to PZP-bound zinc. Since zinc in the two highest affinity sites in human alpha 2-macroglobulin causes no broadening of Cl-, it is concluded that these zinc sites are absent from PZP. The mobility of chymotrypsin in the PZP.chymotrypsin complex was examined by covalently attaching a nitroxide spin label at the serine residue in the active site of the enzyme and examining the appearance of the ESR spectrum. The chymotrypsin is rigidly held by the PZP to which it is covalently bound. In an analogous experiment performed previously on alpha 2-macroglobulin, chymotrypsin, bound in the presence of methylamine and therefore largely noncovalently bound, was found to be free to rotate inside the cage formed by the protease inhibitor.     

Chromophore structure in the photocycle of the cyanobacterial phytochrome Cph1

The chromophore conformations of the red and far red light induced product states "Pfr" and "Pr" of the N-terminal photoreceptor domain Cph1-N515 from Synechocystis 6803 have been investigated by NMR spectroscopy, using specific 13C isotope substitutions in the chromophore. 13C-NMR spectroscopy in the Pfr and Pr states indicated reversible chemical shift differences predominantly of the C(4) carbon in ring A of the phycocyanobilin chromophore, in contrast to differences of C15 and C5, which were much less pronounced. Ab initio calculations of the isotropic shielding and optical transition energies identify a region for C4-C5-C6-N2 dihedral angle changes where deshielding of C4 is correlated with red-shifted absorption. These could occur during thermal reactions on microsecond and millisecond timescales after excitation of Pr which are associated with red-shifted absorption. A reaction pathway involving a hula-twist at C5 could satisfy the observed NMR and visible absorption changes. Alternatively, C15 Z-E photoisomerization, although expected to lead to a small change of the chemical shift of C15, in addition to changes of the C4-C5-C6-N2 dihedral angle could be consistent with visible absorption changes and the chemical shift difference at C4. NMR spectroscopy of a 13C-labeled chromopeptide provided indication for broadening due to conformational exchange reactions in the intact photoreceptor domain, which is more pronounced for the C- and D-rings of the chromophore. This broadening was also evident in the F2 hydrogen dimension from heteronuclear 1H-13C HSQC spectroscopy, which did not detect resonances for the 13C5-H, 13C10-H, and 13C15-H hydrogen atoms whereas strong signals were detected for the (13)C-labeled chromopeptide. The most pronounced 13C-chemical shift difference between chromopeptide and intact receptor domain was that of the 13C4-resonance, which could be consistent with an increased conformational energy of the C4-C5-C6-N2 dihedral angle in the intact protein in the Pr state. Nuclear Overhauser effect spectroscopy experiments of the 13C-labeled chromopeptide, where chromophore-protein interactions are expected to be reduced, were consistent with a ZZZssa conformation, which has also been found for the biliverdin chromophore in the x-ray structure of a fragment of Deinococcus radiodurans bacteriophytochrome in the Pr form.     

Mechanism-based isocoumarin inhibitors for trypsin and blood coagulation serine proteases: new anticoagulants

Trypsin, porcine pancreatic kallikrein, and several blood coagulation enzymes, including bovine thrombin, bovine factor Xa, human factor Xa, human plasma factor XIa, human plasma factor XIIa, and human plasma kallikrein, were inactivated by a number of substituted isocoumarins containing basic functional groups (aminoalkoxy, guanidino, and isothiureidoalkoxy). 3-Alkoxy-4-chloro-7-guanidinoisocoumarins were found to be the most potent inhibitors for the coagulation enzymes tested with kobsd/[I] values in the range of 10(3)-10(5) M-1 s-1. 4-Chloro-3-isothiureidoalkoxyisocoumarins show high inhibitory potency toward porcine pancreatic kallikrein, human plasma kallikrein, human factor XIa, human factor XIIa, and trypsin with kobsd/[I] values of the order of 10(4)-10(5) M-1 s-1. The inhibition of these serine proteases by the substituted isocoumarins are time dependent, and the inactivation of trypsin by 3-alkoxy-4-chloro-7-guanidinoisocoumarins and 7-amino-4-chloro-3-(3-isothiureidopropoxy)isocoumarin occured concurrently with the loss of the isocoumarin absorbance. The complex formed from inactivation of trypsin by these two types of inhibitors was very stable and regained less than 4% activity in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (pH 7.5) after 1 day at 25 degrees C and regained 8-45% activity upon addition of buffered 0.29 M hydroxylamine. Trypsin inactivated by other inhibitors regained full activity upon standing or addition of hydroxylamine. Thrombin inactivated by 3-alkoxy-4-chloro-7-guanidinoisocoumarins was also quite stable and only regained 9-15% activity under similar conditions. These results are consistent with a proposed mechanism, where serine proteases inactivated by aminoalkoxyisocoumarins or isothiureidoalkoxyisocoumarins form acyl enzymes that will deacylate upon standing or addition of hydroxylamine. However, the acyl enzymes formed from 3-alkoxy-4-chloro-7-guanidinoisocoumarins or 7-amino-4-chloro-3-(3-isothiureidopropoxy)-isocoumarin will decompose further, probably through a quinone imine methide, to give an irreversibly inactivated enzyme by reaction with an active-site nucleophile such as His-57. The quinone imine methide intermediate may also react with a solvent nucleophile to give an acyl enzyme that can be reactivated by hydroxylamine. The inhibitors 4-chloro-7-guanidino-3-methoxyisocoumarin and 4-chloro-3-ethoxy-7-guanidinoisocoumarin have been tested as anticoagulants in human plasma and were effective at prolonging the prothrombin time. However, they are unstable in plasma (t1/2 = 4-8 min), and their in vivo utility may be limited.     

Kinetic studies on the inactivation of L-lactate oxidase by [the acetylenic suicide substrate] 2-hydroxy-3-butynoate.

2-Hydroxy-3-butynoate is both a substrate and an irreversible inactivator of the flavoenzyme L-lactate oxidase. The partitioning between catalytic oxidation of 2-hydroxy-3-butynoate and inactivation of the enzyme is determined by the concentration of the second substrate, O2. Rapid reaction studies show the formation of an intermediate which is common to both the oxidation and inactivation pathways. This intermediate appears to be a charge-transfer complex between enzyme-reduced flavin and 2-keto-3-butynoate. It is characterized by a long-wavelength absorbing band (gamma(max) 600 nm) and lack of fluorescence, making it easily distinguished from the subsequently formed inactivated enzyme, which has no long wavelength absorption (gamma(max) 318, 368 nm) and which is strongly fluorescent. Inactivation is also accomplished by reaction of the reduced enzyme with 2-keto-3-butynoate. The absorbance and fluorescence characteristics of the inactivated enzyme are similar to those of a model compound, C(4a), N(5)-propano-bridged FMN bound to apolactate oxidase. That the modified chromophore of the inactivated enzyme is an adduct involving both the C(4a) and N5 positions is further supported by the spectral and fluorescence changes resulting from treatment of the inactivated enzyme with borohydride.     

The roles of Glu-327 and His-446 in the bisphosphatase reaction of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase probed by NMR spectroscopic and mutational analyses of the enzyme in the transient phosphohistidine intermediate complex

The bisphosphatase domain derived from the rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was studied by 1H-13C HMQC NMR spectroscopy of the histidine C2' and H2' nuclei. The bacterially expressed protein was specifically labeled with 13C at the ring C2' position of the histidines. Each of the seven histidine residues gave rise to a single cross-peak in the HMQC spectra, and these were assigned by use of a series of histidine-to-alanine point mutants. His-304, His-344, and His-469 exhibit 13C and 1H resonances that titrated with pH, while the remaining histidine-associated resonances did not. The 13C and 1H chemical shifts indicate that at neutral pH, His-304 and His-446 are deprotonated, while His-469 is protonated. The pKa of His-344 was determined to be 7.04. The 13C chemical shifts suggest that the deprotonated His-258 exists as the N1' tautomer, while His-392 and His-419 are protonated in the resting, wild-type enzyme. Mutation of the remaining member of the catalytic triad, Glu-327, to alanine in the resting enzyme caused an upfield shift of 1.58 and 1.30 ppm in the 1H and 13C dimensions, respectively, and significant narrowing of the His-258 cross-peak. Mutation of His-446 to alanine produced perturbations of the His-258 cross-peak that were similar to those detected in the E327A mutant. The His-392 resonances were also shifted by the E327A and H446A mutations. These observations strongly suggest that residues His-258, Glu-327, His-392, and His-446 exist within a network of interacting residues that encompasses the catalytic site of the bisphosphatase and includes specific contacts with the C-terminal regulatory region of the enzyme. The specifically 13C-labeled bisphosphatase was monitored during turnover by HMQC spectra acquired from the transient N3' phosphohistidine intermediate complex in the wild-type enzyme, the E327A mutant, and the H446A mutant. These complexes were formed during reaction with the physiological substrate fructose-2, 6-bisphosphate. Upon formation of the phosphohistidine at His-258, the 13C and 1H resonances of this residue were shifted downfield by 1.7 and 0.31 ppm, respectively, in the wild-type enzyme. The upfield shifts of the His-258 resonances in the E327A and H446A mutant resting enzymes were reversed when the phosphohistidine was formed, generating spectra very similar to that of the wild-type enzyme in the intermediate complex. In contrast, the binding of fructose-6-phosphate, the reaction product, to the resting enzyme did not promote significant changes in the histidine-associated resonances in either the wild-type or the mutant enzymes. The interpretation of these data within the context of the X-ray crystal structures of the enzyme is used to define the role of Glu-327 in the catalytic mechanism of the bisphosphatase and to identify His-446 as a putative link in the chain of molecular events that results in activation of the bisphosphatase site by cAMP-dependent phosphorylation of the hepatic bifunctional enzyme.     

Inactivation of gamma-glutamyl transpeptidase by phenylmethanesulfonyl fluoride, a specific inactivator of serine enzymes.

Rat kidney γ-glutamyl transpeptidase was found to be inactivated by phenylmethanesulfonyl fluoride, a specific inactivator of serine enzymes. The inactivation occurred only in the presence of maleate which was known to enhance the hydrolytic activity of this enzyme. The concentration of phenylmethanesulfonyl fluoride giving a half maximum rate of inactivation was 1.1 mM. The presence of S-methyl glutathione, a substrate for this enzyme, prevented the inactivation in a competitive fashion. These findings indicate that phenylmethanesulfonyl fluoride acts as an active site directed reagent for γ-glutamyl transpeptidase. A possible identity of the labeled site with that for 6-diazo-5-oxo-L-norleucine, another affinity label for this enzyme, was discussed.