Catalytic inhibition of gamma-aminobutyric acid - alpha-ketoglutarate transaminase of bacterial origin by 4-aminohex-5-ynoic acid, a substrate analog

Design and Mechanism of GABA Aminotransferase Inactivators. Treatments for Epilepsies and Addictions

When the brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) diminishes below a threshold level, the excess neuronal excitation can lead to convulsions. This imbalance in neurotransmission can be corrected by inhibition of the enzyme γ-aminobutyric acid aminotransferase (GABA-AT), which catalyzes the conversion of GABA to the excitatory neurotransmitter l-glutamic acid. It also has been found that raising GABA levels can antagonize the rapid elevation and release of dopamine in the nucleus accumbens, which is responsible for the reward response in addiction. Therefore, the design of new inhibitors of GABA-AT, which increases brain GABA levels, is an important approach to new treatments for epilepsy and addiction. This review summarizes findings over the last 40 or so years of mechanism-based inactivators (unreactive compounds that require the target enzyme to catalyze their conversion to the inactivating species, which inactivate the enzyme prior to their release) of GABA-AT with emphasis on their catalytic mechanisms of inactivation, presented according to organic chemical mechanism, with minimal pharmacology, except where important for activity in epilepsy and addiction. Patents, abstracts, and conference proceedings are not covered in this review. The inactivation mechanisms described here can be applied to the inactivations of a wide variety of unrelated enzymes.

A numerical method for analysis of in vitro time-dependent inhibition data. Part 1. Theoretical considerations

Inhibition of cytochromes P450 by time-dependent inhibitors (TDI) is a major cause of clinical drug-drug interactions. It is often difficult to predict in vivo drug interactions based on in vitro TDI data. In part 1 of these manuscripts, we describe a numerical method that can directly estimate TDI parameters for a number of kinetic schemes. Datasets were simulated for Michaelis-Menten (MM) and several atypical kinetic schemes. Ordinary differential equations were solved directly to parameterize kinetic constants. For MM kinetics, much better estimates of KI can be obtained with the numerical method, and even IC50 shift data can provide meaningful estimates of TDI kinetic parameters. The standard replot method can be modified to fit non-MM data, but normal experimental error precludes this approach. Non-MM kinetic schemes can be easily incorporated into the numerical method, and the numerical method consistently predicts the correct model at errors of 10% or less. Quasi-irreversible inactivation and partial inactivation can be modeled easily with the numerical method. The utility of the numerical method for the analyses of experimental TDI data is provided in our companion manuscript in this issue of Drug Metabolism and Disposition (Korzekwa et al., 2014b).

Effect of di-n-propylacetate and γ-acetylenic GABA on hyperbaric oxygen-induced seizures and GABA metabolism

The administration of γ-acetylenic GABA or di-n-propylacetate to mice delayed the onset of hyperbaric oxygen-induced seizures in the animals. The former compound caused large increases in brain GABA content and strong inhibition of glutamate decarboxylase activity, whereas the latter compound brough about only moderate increases in brain GABA level and had little or no effect on the enzyme activity. It is suggested that the GABA system is not involved in the anticonvulsant mechanism of γ-acetylenic GABA but may play a role in the action of di-n-propylacetate.

Clotrimazole Inhibits Hemoperoxidase of Plasmodium falciparum and Induces Oxidative Stress

The mechanism of antimalarial activity of clotrimazole was studied placing emphasis on its role in inhibiting hemoperoxidase for inducing oxidative stress in Plasmodium falciparum. Clotrimazole, in the presence of H2O2, causes irreversible inactivation of the enzyme, and the inactivation follows pseudo-first order kinetics, consistent with a mechanism-based (suicide) mode. The pseudo-first order kinetic constants are ki = 2.85 microM, k(inact) = 0.9 min(-1), and t(1/2) = 0.77 min. The one-electron oxidation product of clotrimazole has been identified by EPR spectroscopy as the 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) adduct of the nitrogen-centered radical (aN = 15 G), and as DMPO protects against inactivation, this radical is involved in the inactivation process. Binding studies indicate that the clotrimazole oxidation product interacts at the heme moiety, and the heme-clotrimazole adduct has been dissociated from the inactivated enzyme and identified (m/z 1363) by mass analysis. We found that the inhibition of hemoperoxidase increases the accumulation of H2O2 in P. falciparum and causes oxidative stress. Furthermore, the inhibition of hemoperoxidase correlates well with the inhibition of parasite growth. The results described herein indicate that the antimalarial activity of clotrimazole might be due to the inhibition of hemoperoxidase and subsequent development of oxidative stress in P. falciparum.

Kinetic values for mechanism-based enzyme inhibition: Assessing the bias introduced by the conventional experimental protocol

The in vitro characterisation of a mechanism-based enzyme inactivator (MBEI) includes determination of the maximum inactivation rate constant (k(inact)), the inactivator concentration that produces half-maximal rate of inactivation (K(I)), and the partition ratio (r). Conventional experimental protocols (CEPs) assume insignificant metabolism of the MBEI during the "pre-incubation" stage and negligible inactivation of enzyme during the "incubation" stage. The aim of this study was to evaluate the bias in the estimation of kinetic values as a consequence of these assumptions. Ranges of values of k(inact), K(I), and r for reported MBEIs were collated and data for 27 virtual compounds were generated by combining the median, high and low values of each parameter. The kinetics of the virtual compounds and of four reported MBEIs were simulated under CEP, but taking account of enzyme inactivation, metabolism of the MBEI and the probe substrate, and their interaction at relevant stages. The differences between the estimated and starting kinetic values reflect the bias introduced by the CEP in the absence of experimental error. Despite simulating a stringent experimental procedure, 19% of the estimated kinetic values of the 27 virtual MBEIs had greater than 100% bias. Simulations relating to two of the actual MBEIs indicated no bias in k(inact) and 8-33% bias in K(I). However, the bias in K(I) values of the two other compounds exceeded 98% and corresponding bias in k(inact) was greater than 300%. Thus, CEP may introduce substantial bias in estimated kinetic values for mechanism-based inhibition, and the validity of some of the reported kinetic parameters may be questionable.

Two interacting binding sites for quinacrine derivatives in the active site of trypanothione reductase: a template for drug design

Trypanothione reductase is a key enzyme in the trypanothione-based redox metabolism of pathogenic trypanosomes. Because this system is absent in humans, being replaced with glutathione and glutathione reductase, it offers a target for selective inhibition. The rational design of potent inhibitors requires accurate structures of enzyme-inhibitor complexes, but this is lacking for trypanothione reductase. We therefore used quinacrine mustard, an alkylating derivative of the competitive inhibitor quinacrine, to probe the active site of this dimeric flavoprotein. Quinacrine mustard irreversibly inactivates Trypanosoma cruzi trypanothione reductase, but not human glutathione reductase, in a time-dependent manner with a stoichiometry of two inhibitors bound per monomer. The rate of inactivation is dependent upon the oxidation state of trypanothione reductase, with the NADPH-reduced form being inactivated significantly faster than the oxidized form. Inactivation is slowed by clomipramine and a melarsen oxide-trypanothione adduct (both are competitive inhibitors) but accelerated by quinacrine. The structure of the trypanothione reductase-quinacrine mustard adduct was determined to 2.7 A, revealing two molecules of inhibitor bound in the trypanothione-binding site. The acridine moieties interact with each other through pi-stacking effects, and one acridine interacts in a similar fashion with a tryptophan residue. These interactions provide a molecular explanation for the differing effects of clomipramine and quinacrine on inactivation by quinacrine mustard. Synergism with quinacrine occurs as a result of these planar acridines being able to stack together in the active site cleft, thereby gaining an increased number of binding interactions, whereas antagonism occurs with nonplanar molecules, such as clomipramine, where stacking is not possible.

Structures of γ-Aminobutyric Acid (GABA) Aminotransferase, a Pyridoxal 5′-Phosphate, and [2Fe-2S] Cluster-containing Enzyme, Complexed with γ-Ethynyl-GABA and with the Antiepilepsy Drug Vigabatrin

Gamma-aminobutyric acid aminotransferase (GABA-AT) is a pyridoxal 5'-phosphate-dependent enzyme responsible for the degradation of the inhibitory neurotransmitter GABA. GABA-AT is a validated target for antiepilepsy drugs because its selective inhibition raises GABA concentrations in brain. The antiepilepsy drug, gamma-vinyl-GABA (vigabatrin) has been investigated in the past by various biochemical methods and resulted in several proposals for its mechanisms of inactivation. In this study we solved and compared the crystal structures of pig liver GABA-AT in its native form (to 2.3-A resolution) and in complex with vigabatrin as well as with the close analogue gamma-ethynyl-GABA (to 2.3 and 2.8 A, respectively). Both inactivators form a covalent ternary adduct with the active site Lys-329 and the pyridoxal 5'-phosphate (PLP) cofactor. The crystal structures provide direct support for specific inactivation mechanisms proposed earlier on the basis of radio-labeling experiments. The reactivity of GABA-AT crystals with the two GABA analogues was also investigated by polarized absorption microspectrophotometry. The spectral data are discussed in relation to the proposed mechanism. Intriguingly, all three structures revealed a [2Fe-2S] cluster of yet unknown function at the center of the dimeric molecule in the vicinity of the PLP cofactors.

Development of a Stable-Isotope Dilution Assay for γ-Aminobutyric Acid (GABA) Transaminase in Isolated Leukocytes and Evidence That GABA and β-Alanine Transaminases Are Identical

Background: Several methods have been published for measuring γ-aminobutyric acid transaminase (GABA-T) activity, but these methods are either impracticable because of the use of radioisotopes or insufficiently sensitive to determine small enzyme activities in leukocyte extracts. We developed a direct and sensitive enzyme method.

Methods: We developed a stable-isotope dilution method for the measurement of [15N]glutamic acid derived from [15N]GABA and α-ketoglutaric acid, catalyzed by GABA-T. The method for analysis of [15N]glutamic acid comprised a solid-phase extraction procedure to isolate this analyte from incubation samples. After derivatization, [15N]glutamic acid was quantified by gas chromatography–mass spectrometry relative to its 2H5-labeled internal standard. In addition to [15N]GABA, [15N]β-alanine was a cosubstrate.

Results: GABA-T-deficient lymphoblasts showed diminished enzyme activity, with both [15N]GABA and [15N]β-alanine as substrate. Vigabatrin inhibited the enzyme activity for both substrates.

Conclusions: The activity of GABA-T can be accurately determined by our procedure using 15N-labeled substrate, measuring the formation of [15N]glutamic acid. Our results with [15N]β-alanine indicate that GABA and β-alanine transaminases are identical.

Trichodiene synthase: mechanism-based inhibition of a sesquiterpene cyclase

The 10-cyclopropylidene analog of farnesyl diphosphate was shown to be a mechanism-based inhibitor of trichodiene synthase with an inactivation rate (k(inact)) of 0.010 +/- 0.0003 min(-1) and an apparent Ki of 663 +/- 75 nM. The presence of three anomalous sesquiterpene products detected in incubation mixtures indicate that the compound also serves as a substrate of the enzyme.

A beta-lactam inhibitor of cytosolic phospholipase A2 which acts in a competitive, reversible manner at the lipid/water interface

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 3,3-Dimethyl-6-(3-lauroylureido)-7-oxo-4-thia-1-azabicyclo[3,2,0] heptane-2-carboxylic acid (1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 72 microM). This beta-lactam did not inhibit other phospholipases, including the human nonpancreatic secreted phospholipase A2. The inhibition of cPLA2 was found not to be time-dependent. This, along with the observation that the degradation of the inhibitor was not catalyzed by the enzyme, demonstrates that the inhibition does not result from the formation of an acyl-enzyme intermediate with the active site serine residue. Moreover, the ring-opened form of 1 is also able to inhibit cPLA2 with near-equal potency. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphomethanol containing 6-10 mole percent of 1-palmitoyl-2-[1-14C]-arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition was defined by kinetic equations describing competitive inhibition at the lipid/water interface. The apparent dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.5 +/- 0.1 mole% versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.4 +/- 0.1 mole%. Thus, 1 represents a novel structural class of inhibitors of cPLA2 which partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site.

Mechanism-based inactivation of lacrimal-gland peroxidase by phenylhydrazine: a suicidal substrate to probe the active site

Humans are exposed to various hydrazine derivatives for therapeutic control of several diseases, and mammalian peroxidases are implicated in the oxidative metabolism of many drugs. The results presented here indicate that lacrimal-gland peroxidase is irreversibly inactivated in a mechanism-based way by phenylhydrazine, which acts as a suicidal substrate in the presence of H2O2. The pseudo-first-order kinetic constants for inactivation at pH 5.5 are Ki=18 microM, kinact=0.25 min-1 and tau50=2.75 min, with a second-order rate constant of 0.75x10(4) M-1.min-1. Approx. 27 mol of phenylhydrazine and 54 mol of H2O2 are required per mol of enzyme for complete inactivation. The pH-dependent inactivation kinetics indicate the involvement of an ionizable group on the enzyme with a pKa value of 5.4, protonation of which favours inactivation. SCN-, the plausible physiological electron donor of the enzyme, protects it from inactivation. Binding studies by optical difference spectroscopy indicate that phenylhydrazine interacts with the enzyme with a KD value of 60 microM, and its binding is prevented by the presence of SCN-. The enzyme is also protected by 5, 5-dimethyl-1-pyrroline N-oxide, a free-radical trap, suggesting the involvement of a radical species in the inactivation. ESR studies indicate the formation of a spin-trapped phenyl radical (aN=15.9G and abetaH=24.8G) generated on incubation of phenylhydrazine with the enzyme and H2O2. A 75% loss of the Soret spectrum is observed when the enzyme is completely inactivated. However, in the presence of the spin trap, spectral loss is prevented and the enzyme compound II is readily reduced to the native state by phenylhydrazine. The phenylhydrazine-inactivated enzyme reacts with H2O2 or CN- to form compound II or the cyanide complex with a characteristic spectrum, indicating that haem iron is protected from attack by the radical species. The inactivated enzyme binds SCN- with a KD value similar to that of the native enzyme (15+/-3 mM), suggesting that the donor-binding site remains unaffected. CD studies of the inactive enzyme show complete disappearance of the Soret band at 409 nm with the appearance of a new band at 275 nm. This indicates that the haem environment of the enzyme is perturbed in the inactive form. As benzene, the end product of phenylhydrazine oxidation, has no effect on the enzyme, we suggest that the phenyl radical formed by one-electron oxidation by catalytically active enzyme inactivates it by incorporation in the vicinity of its haem moiety. The data support the use of phenylhydrazine as a probe for structural and mechanistic analysis of the active site of the lacrimal-gland peroxidase.

7alpha-Arylaliphatic androsta-1,4-diene-3,17-diones as enzyme-activated irreversible inhibitors of aromatase

Inhibition of aromatase, the enzyme responsible for converting androgens to estrogens, may be therapeutically useful for the endocrine treatment of hormone-dependent breast cancer. Previous research on 7alpha-thiosubstituted androgens, especially 7alpha-(4'-aminophenylthio)-androsta-1,4-diene-3,17-di one, has shown that these compounds are potent enzyme-activated irreversible inhibitors of aromatase. Research on the synthesis of more metabolically stable inhibitors has focused on replacing the thioether linkage at the 7alpha position with a carbon-carbon linkage. Several 7alpha-arylaliphatic androst-4-ene-3,17-diones were previously shown to be potent competitive inhibitors of aromatase. The extension of the research on these 7alpha-arylaliphatic androgens includes the introduction of a C1-C2 double bond in the A-ring to provide enzyme-activated irreversible inhibitors. The desired 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were obtained from their corresponding 7alpha-arylaliphatic androst-4-ene-3,17-diones by oxidation using DDQ. A new improved synthesis of the 7alpha-arylaliphatic androst-4-ene-3,17-diones using an in situ preparation of the CuI-(n-Bu3)P complex was employed. The aryl ring of the 7alpha-phenethyl and 7alpha-phenpropyl derivatives were functionalized to their corresponding p-nitro and p-amino derivatives. These compounds were all potent inhibitors of aromatase with apparent K(i)s ranging between 7 and 19 nM. These inhibitors demonstrated enzyme-mediated inactivation of aromatase with apparent k(inact)s ranging from 4.4 x 10(-4) to 1.90 x 10(-3)/s. The best inactivator of the series was the 7alpha-phenpropylandrosta-1,4-diene-3,17-dione, which exhibited a T(1/2) of 6.08 min.

Mechanism-based inactivation of gastric peroxidase by mercaptomethylimidazole

The mechanism of inhibition of gastric peroxidase (GPO) activity by mercaptomethylimidazole (MMI), an inducer of gastric acid secretion, has been investigated. Incubation of purified GPO with MMI in the presence of H2O2 results in irreversible inactivation of the enzyme. No significant inactivation occurs in the absence of H2O2 or MMI, suggesting the involvement of peroxidase-catalysed oxidized MMI (MMIOX.) in the inactivation process. The inactivation follows pseudo-first-order kinetics consistent with a mechanism-based (suicide) mode. The pseudo-first-order kinetic constants at pH 8 are ki = 111 microM, k(inact.) = 0.55 min-1 and t1/2 = 1.25 min, and the second-order rate constant is 0.53 x 10(4) M-1 x min-1. Propylthiouracil also inactivates GPO activity in the same manner but its efficiency (k(inact./ki = 0.46 mM-1 x min-1) is about 10 times lower than that of MMI (k(inact./ki = 5 mM-1 x min-1). The rate of inactivation with MMI shows pH-dependence with an inflection point at 7.3, indicating the involvement in the inactivation process of an ionizable group on the enzyme with a pKa of 7.3. The enzyme is remarkably protected against inactivation by micromolar concentrations of electron donors such as iodide and bromide but not by chloride. Although GPO oxidizes MMI slowly, iodide stimulates it through enzymic generation of I+ which is reduced back to I- by MMI. Although MMIOX. is formed at a much higher rate in the presence of I-, a constant concentration of I- maintained via the reduction of I+ by MMI, protects the active site of the enzyme against inactivation. We suggest that MMI inactivates catalytically active GPO by acting as a suicidal substrate.

Identification of an isoprenylated cysteine methyl ester hydrolase activity in bovine rod outer segment membranes

Proteins from eucaryotic cells which have a carboxyl-terminal CAAX motif are posttranslationally modified by isoprenylation. The pathway involves the linkage of an all-trans-farnesyl (C15) or an all-trans-geranylgeranyl (C20) moiety to the cysteine residue followed by proteolysis which generates the modified cysteine as the carboxyl-terminal residue. Carboxylmethylation of the modified cysteine residue completes the pathway. This latter methylation reaction is the only potentially reversible reaction in the pathway and thus of possible regulatory significance. A specific esterase is required to reverse the methylation. It is demonstrated here that simple isoprenylated cysteine derivatives, such as N-acetyl-S-farnesyl-L-cysteine methyl ester (L-AFCM) and N-acetyl-S-geranylgeranyl-L-cysteine methyl ester (L-AGGCM), are substrates for a rod outer segment (ROS) membrane esterase activity. The KM and Vmax values for L-AFCM and L-AGGCM are 186 microM and 2.2 nmol mg-1 min-1 and 435 microM and 4.8 nmol mg-1 min-1, respectively. The enzyme(s) is stereoselective rather than stereospecific because D-AFCM is enzymatically hydrolyzed with KM and Vmax values of 157 microM and 0.46 nmol mg-1 min-1, respectively. The enzyme(s) does not process N-acetyl-L-cysteine methyl ester, demonstrating that the isoprenyl moiety is required for substrate activity. Ebelactone B is a potent mechanism-based inactivator of the enzyme with a KI = 42 microM and a kinh = 3.7 x 10(-3) s-1. Importantly, L-AFCM, L-AGGCM, and ebelactone B all inhibit the demethylation of the endogenous ROS substrates, showing that the same enzymatic activity is involved in the processing of the synthetic and physiological substrates.

Mechanism of inactivation and identification of sites of modification of ornithine aminotransferase by 4-aminohex-5-ynoate

The inactivation of ornithine aminotransferase by an enzyme-activated irreversible inhibitor 4-aminohex-5-ynoate was accompanied by stoichiometric binding of the radiolabeled compound. Distribution of radiolabel among separated tryptic peptides indicated that more than one amino acid residue had reacted. Lys-292 and Cys-388 were positively identified. Reduction with borohydride was necessary to stabilize the adduct formed with Lys-292, and the relevant peptide prepared after this treatment contained equimolar amounts of inhibitor and coenzyme. The coenzyme chromophore in this peptide showed strong negative circular dichroism. A mechanism consistent with these observations is proposed.

Effects of pharmacological manipulation on neurotransmitter and other amino acid levels in the CSF of the Senegalese baboon Papio papio

The concentrations of GABA, glutamate, aspartate, glycine, taurine, glutamine, asparagine and alanine were determined in the CSF of 10 Senegalese baboons (Papio papio) following initial ketamine anaesthesia and subsequent administration (4 h later) of different compounds known to alter either inhibitory or excitatory neurotransmission. Ketamine itself was apparently without effect as the administration of a second dose of ketamine did not significantly alter the levels of any of the amino acids studied, although GABA levels tended to decrease. The presence of haemolysed material in occasional samples was associated with high GABA, glutamate, aspartate, taurine and asparagine levels. Therefore only haemolysate-free samples were included for analysis. Of the compounds administered, gamma-vinyl GABA had the most evident effect on CSF amino acid levels, increasing GABA (greater than 5-fold) and decreasing glutamate (greater than 50%), aspartate (40-50%), asparagine (20%) and alanine (30-35%) levels. The changes in GABA, glutamate and aspartate were still apparent 24 h post-gamma-vinyl GABA administration. In contrast, sodium valproate did not significantly alter the CSF levels of any of the amino acids studied. Upon acute administration allylglycine decreased the CSF concentrations of GABA and alanine, but not glutamate. These alterations are unlikely related to the occurrence of allylglycine-induced convulsions (in 2 of 4 experiments) as electroconvulsive shock did not alter CSF amino acid levels. During the experimental period encompassing the allylglycine injection (8 weeks), basal (initial post-ketamine, pre-drug sample) amino acid levels were abnormal with large increases in glutamate, GABA, aspartate and taurine whereas asparagine levels were below the limit of detection. Diazepam administration was followed by a significant increase in taurine and a decrease in aspartate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

4-(Oxoalkyl)-substituted GABA analogues as inactivators and substrates of GABA aminotransferase

On the basis of the mechanism of inactivation of gamma-aminobutyric acid (GABA) aminotransferase by gamma-ethynyl GABA that was reported recently (Burke and Silverman, manuscript submitted), three 4-(oxoalkyl)-substituted GABA analogues, 4-amino-5-oxohexanoic acid (7), 4-amino-5-oxopentanoic acid (8), and 4-amino-6-oxohexanoic acid (9) were synthesized and tested as inactivators. Only 8 was an inactivator of the enzyme. A mechanism for the inactivation by 8 is proposed as well as rationalizations for the lack of inactivation by 7 and 9.

DL-canaline and 5-fluoromethylornithine. Comparison of two inactivators of ornithine aminotransferase

5-Fluoromethylornithine (5FMOrn) is an enzyme-activated irreversible inhibitor or ornithine aminotransferase (L-ornithine:2-oxo-acid 5-aminotransferase, OAT). For purified rat liver OAT, Ki(app.) was found to be 30 microM. and tau 1/2 = 4 min. Of the four stereomers of 5FMOrn only one reacts with OAT. The formation of a chromophore with an absorption maximum at 458 nm after inactivation of OAT by 5FMOrn suggests the formation of an enamine intermediate, which is slowly hydrolysed to release an unsaturated ketone. L-Canaline [(S)-2-amino-4-amino-oxybutyric acid] is a well-known irreversible inhibitor of OAT. Not only the natural L-enantiomer but also the D-enantiomer reacts by oxime formation with pyridoxal 5'-phosphate in the active site of the enzyme, although considerably more slowly. This demonstrates that the stereochemistry at C-2 of ornithine is not absolutely stringent. In vitro, canaline reacted faster than 5FMOrn with OAT. In vivo, however, only incomplete OAT inhibition was observed with canaline. Whereas intraperitoneal administration of 10 mg of 5FMOrn/kg body wt. to mice was sufficient to inactivate OAT in brain and liver by 90% for 24 h, 500 mg of DL-canaline/kg body wt. only produced a transient inhibition of 65-70%. The accumulation of ornithine in these tissues was considerably slower and the maximum concentrations lower than were achieved with 5FMOrn. It appears that DL-canaline, in contrast with 5FMOrn, is not useful as a tool in studies of biological consequences of OAT inhibition.

Electrical stimulation-evoked release of endogenous aspartate from rat medulla oblongata slices. Effects of inhibitors of aspartate aminotransferase and GABA transaminase

The effects of aminooxyacetic acid (AOAA), an aspartate aminotransferase (AAT) inhibitor, L-canaline, an ornithine aminotransferase inhibitor, and gamma-acetylenic GABA and gabaculine, both gamma-aminobutyric acid transaminase (GABA-T) inhibitors, on the release of aspartate from slices of rat medulla oblongata and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2(+)-dependent stimulus-evoked release of endogenous aspartate. AOAA (10(-4) and 10(-3) M) decreased the evoked release of aspartate in the medulla oblongata but not in the hippocampus. In addition, AOAA produced a decrease in the spontaneous efflux and tissue content of aspartate in the medulla oblongata. L-Canaline (5 x 10(-5) M), gamma-acetylenic GABA (10(-4) M) and gabaculine (10(-5) M) did not affect the evoked release of aspartate in the medulla oblongata, while these agents produced a decrease in spontaneous efflux and tissue content of aspartate. These findings suggest that AAT participates in the synthesis of transmitter aspartate in the medulla oblongata of the rat. It appears that there are the pools of transmitter aspartate and non-transmitter aspartate in the rat medulla oblongata.

Enzyme inactivation by potential metabolites of an aromatase-activated inhibitor (MDL 18,962)

MDL 18,962, 19-acetylenic androstenedione, is an enzyme-activated inhibitor of estrogen biosynthesis which is in Phase I clinical evaluations as a potential therapeutic agent for estrogen-dependent cancers. 19-Acetylenic analogs corresponding to the major metabolites of androstenedione were synthesized as potential metabolites of MDL 18,962. These compounds were 19-acetylenic testosterone, the product of 17 beta-hydroxy steroid oxidoreductase, 6 beta-hydroxy- and 6-oxo-19-acetylenic androstenedione, products of P450 steroid 6 beta-hydroxylase and alcohol dehydrogenase, respectively. All of these analogs showed time-dependent inactivation of human placental aromatase activity. The time-dependent Ki and t1/2 at infinite inhibitor concentration (tau 50) were 4.3 nM, 12.0 min for MDL 18,962; 28 nM, 7.8 min for 17-hydroxy analog; 13 nM, 37 min for 6 beta-hydroxy analog; and 167 nM, 6.1 min for the 6-oxo analog. The 19-acetylenic testosterone, a confirmed metabolite from primate studies, was 25% as efficient as MDL 18,962 for aromatase inactivation, while 6 beta-hydroxy- and 6-oxo analogs were 11% and 5%, respectively as efficient as their parent compound. These data indicate that first-pass metabolism of MDL 18,962 does not cause an obligatory loss of time-dependent inhibition of human aromatase activity.

Inhibition and time-dependent inactivation of human placental aromatase by 3-oxo-17 beta-carboxamido steroids

Several N,N-dialkyl-3-oxo-4-aza-17 beta-carboxamido steroids were found to be competitive inhibitors versus androstenedione (AND) and time-dependent inactivators of aromatase activity from human term placental microsomes. Inhibition constants (Kis) from dead-end inhibition analyses indicated interactions between these compounds and the enzyme over a 0.8-7 microM inhibitor concentration range. The affinity of these compounds for aromatase leading to the time-dependent loss of enzyme activity was several fold higher than that estimated by the steady-state kinetics, with rate constants of inactivation of 0.025-0.033 min-1. 3-Oxo-4-aza steroids lacking a 17 beta-carboxamide were found to be competitive inhibitors of AND for aromatase, but did not inactivate enzyme activity in a time-dependent manner. Steroids which did not contain a 4-aza substituent, but retained the 17 beta-carbamoyl functionality, were both inhibitors and inactivators of aromatase activity in the microsomes. The time-dependent loss of aromatase activity induced by these compounds was shown to require reducing equivalents as provided by NADPH. Hence, it is suggested that the inactivation of aromatase by compounds in this series is dependent on enzymatic activation in the presence of the N,N-dialkyl-17 beta-carbamoyl substituent.

gamma-Acetylenic-gamma-aminobutyrate as an enzyme-activated inhibitor of D-amino acid transaminase

Bacterial D-amino acid transaminase undergoes complete inactivation by gamma-acetylenic GABA. This inactivation is completely prevented by D-alanine and partially prevented by L-alanine. During inactivation the coenzyme portion of the enzyme undergoes significant spectral changes.

Inactivation of gamma-aminobutyric acid aminotransferase by (Z)-4-amino-2-fluorobut-2-enoic acid

(Z)-4-Amino-2-fluorobut-2-enoic acid (1) is shown to be a mechanism-based inactivator of pig brain gamma-aminobutyric acid aminotransferase. Approximately 750 inactivator molecules are consumed prior to complete enzyme inactivation. Concurrent with enzyme inactivation is the release of 708 +/- 79 fluoride ions; transamination occurs 737 +/- 15 times per inactivation event. Inactivation of [3H]pyridoxal 5'-phosphate ([3H]PLP) reconstituted GABA aminotransferase by 1 followed by denaturation releases [3H]PMP with no radioactivity remaining attached to the protein. A similar experiment carried out with 4-amino-5-fluoropent-2-enoic acid [Silverman, R. B., Invergo, B. J., & Mathew, J. (1986) J. Med. Chem. 29, 1840-1846] as the inactivator produces no [3H]PMP; rather, another radioactive species is released. These results support an inactivation mechanism for 1 that involves normal catalytic isomerization followed by active site nucleophilic attack on the activated Michael acceptor. A general hypothesis for predicting the inactivation mechanism (Michael addition vs enamine addition) of GABA aminotransferase inactivators is proposed.

Biochemical changes associated with alpha-difluoromethylornithine uptake and resistance in Trypanosoma brucei

Procyclic Trypanosoma brucei grown in semi-defined media are sensitive to alpha-difluoromethylornithine (DFMO) (EC50 100 microM), an inhibitor of ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis. Organisms resistant to 5 mM DFMO (EC50 greater than 20 mM) were obtained by passage in incremental amounts of drug. Resistant and wild-type cells accumulated DFMO by passive diffusion with a consequent decrease in polyamine levels, indicating inhibition of ODC in both cell types. The resistant phenotype was stable in the absence of DFMO, in which state there was no increase in ODC abundance or activity. By kinetic analysis, the ODC of resistant cells appeared normal. In wild-type and resistant cells, [3H]DFMO equally and uniquely affinity-labelled a 50 kDa polypeptide corresponding to the ODC subunit. Levels of ODC and tubulin mRNAs were elevated 4-fold in resistant cells grown in the presence of DFMO, although there was no indication of gene amplification. The intracellular concentration of dihydrotrypanothione (N1,N8-bis(glutathionyl)-spermidine), a redox intermediate unique to kinetoplastids, was unchanged in resistant cells growing in DFMO but was halved in wild-type cells exposed to DFMO for 48 h. The exceptionally elevated levels of ornithine found in DFMO-treated resistant cells most likely play a crucial role in cell survival by maintaining intracellular concentrations of dihydrotrypanothione by competing with DFMO for ODC.

Micro-vacuolation in rat brains after long term administration of GABA-transaminase inhibitors. Comparison of effects of ethanolamine-O-sulphate and vigabatrin

Two "suicide" inhibitors of GABA-aminotransferase which are known to raise the concentration of GABA in vivo and to have anti-convulsant properties, have been compared for the extent to which they produce micro-vacuoles in the brains of rats. The compounds gamma-vinyl-GABA (Vigabatrin) and ethanolamine-O-sulphate were administered orally for six months to rats at doses that produced the same increase in brain GABA levels. Micro-vacuolation was found to be present in the brains of animals treated with either compound but to be more severe in those treated with Vigabatrin. A quantitative assessment using computerised image analysis revealed that both the number of vacuoles, and the area occupied by them, was twice as high in the Vigabatrin treated animals as in those treated with ethanolamine-O-sulphate. This quantitative difference could be seen to be due to the fact that in the Vigabatrin treated animals the vacuoles extended into the white matter tracts between the cerebellar folia whereas in those animals treated with ethanolamine-O-sulphate it was confined to the roof nucleus.

Irreversible inhibition of GABA-T by halogenated analogues of beta-alanine

beta-Difluoromethyl-beta-alanine (3-amino-4,4-difluorobutanoic acid) is a potent in vitro and in vivo inhibitor of GABA-T. The rate of inhibition of GABA-T is concentration- and time-dependent. The inactivation is active-site directed. No reactive species escapes from the active site before reacting with the enzyme. The inhibition is irreversible and stereospecific. The use of beta-2H-beta-difluoromethyl-beta-alanine results in a marked primary isotope effect in vitro and in vivo. The use of differently substituted dihalogeno derivatives of beta-alanine suggests that the rate of inhibition is dependent on the nature and position of the leaving group. The mechanism of inhibition is discussed on the basis of spectral changes.

Anterior thalamic mediation of generalized pentylenetetrazol seizures

The effects of microinjection of various neuroactive compounds into the anterior thalamic nucleus (AN) and other selected subcortical regions of guinea pig brain on the expression of pentylenetetrazol (PTZ)-induced behavioral and electrical seizure activity were examined. Excitatory agents, kainic acid (KA), bicuculline (BIC) or PTZ, injected into the AN or other thalamic nuclei, striatum, but not the mammillary bodies (MB), facilitated the EEG convulsant action of systemically administered PTZ. Injection of muscimol into the AN protected against the expression of PTZ-induced repetitive high-voltage EEG seizure discharges and inhibited the facilitatory effects of subcortically applied KA or BIC. Injection of muscimol into the AN was also able to terminate established ongoing seizure discharges. Unilateral application of muscimol to the AN did not prevent the repetitive hypersynchronous EEG discharges following systemic PTZ but did result in the delay in the onset of cortical hypersynchrony in the ipsilateral hemisphere. Muscimol injections into other thalamic nuclei, MB, cortex, striatum or directly into the CSF space had no anticonvulsant effect, however. Microinjection of gamma-vinyl-gamma-aminobutyric acid, a selective GABA transaminase inhibitor, resulted in protection from the behavioral convulsant action and lethal effects of PTZ when administered into the thalamus, especially the AN, but not when injected into the striatum or CSF. These data demonstrate that the AN is an important subcortical nucleus for the mediation of both cortical EEG synchrony and behavioral seizure expression induced by PTZ. In light of previous results establishing a role for the brainstem and diencephalon in PTZ seizure expression, the AN may serve, in part, as a gating mechanism for the propagation of paroxysmal activity between subcortical areas and the cerebral cortex.

Catalytic irreversible inhibition of Trypanosoma brucei brucei ornithine decarboxylase by substrate and product analogs and their effects on murine trypanosomiasis

Ornithine decarboxylase from Trypanosoma brucei brucei was inhibited by several substrate (ornithine) and product (putrescine) analogs both in vitro and in vivo. Since alpha-difluoromethylornithine is effective for the treatment of experimental and clinical African trypanosomiasis, it was possible that the more potent ornithine and putrescine analogs might be more active in treating the disease. However, only alpha-monofluoromethyldehydroornithine methyl ester was more potent than alpha-difluromethylornithine against mouse trypanosomiasis and warrants further study in model infections.

MDL 72145, an enzyme-activated irreversible inhibitor with selectivity for monoamine oxidase type B

MDL 72145, (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine hydrochloride, was designed and synthesised as a potential enzyme-activated irreversible inhibitor of monoamine oxidase (MAO). In vitro, the compound displayed time-dependent pseudo-first-order irreversible inhibitory characteristics with high selectivity for the B form of rat brain mitochondrial MAO. At 10 degrees C the Ki and tau 50 values for the B enzyme were 40 microM and 1.7 min, respectively, while these same kinetic constants for the A enzyme were 131 microM and 14.5 min, respectively. Selective protection against inactivation of the two forms of MAO by MDL 72145 was obtained by preincubating the enzyme with suitable concentrations of the selective A and B substrates, 5-hydroxytryptamine and benzylamine.

Stereochemistry of the inactivation of 4-aminobutyrate: 2-oxoglutarate aminotransferase and L-glutamate 1-carboxylase by 4-aminohex-5-ynoic acid enantiomers

Incubation of rat brain or bacterial 4-aminobutyrate aminotransferase (EC 2.6.1.19) with both (S)-(+)- and (R)-(-)-enantiomers of 4- aminohex -5- ynoic acid results in a time-dependent irreversible loss of enzymatic activity. Rat brain glutamate decarboxylase (EC 4.1.1.15) is inactivated by the (S)-(+)-enantiomer while the bacterial glutamate decarboxylase is inactivated by the (R)-(-)-enantiomer. In addition, we demonstrate that (R)-(-)-4- aminohex -5- ynoic acid is a selective and effective inhibitor of rat brain 4-aminobutyrate aminotransferase in vivo.

Time-dependent inhibition of aromatase in trophoblastic tumor cells in tissue culture

Human choriocarcinoma trophoblast cells (JAr line) were utilized as whole cell preparations in tissue culture to evaluate the effects of mechanism-based inactivation or "suicide inhibition" of estrogen biosynthesis. A C-19 acetylenic analog (MDL 18,962) of the substrate, androstenedione, was evaluated in competitive and time-dependent assays. Product formation was determined by accumulation of 3H2O resulting from the stereo-specific elimination of 1 beta-tritium from the androgen substrate. Trophoblast cells exhibited initial linear kinetics for at least 3 h following addition of [1-3H]androstenedione. The Km for androstenedione was 35.1 nM with Vmax of 3.7 pmol/h/10(6) trophoblast cells. Kinetic analysis of time-dependent inhibition of aromatase in trophoblast cells revealed an apparent Ki of 0.6 nM for MDL 18,962 and at t 1/2 of inactivation of 26 min at infinite inhibitor concentration. These studies suggest that a suicide aromatase inhibitor can cause irreversible inhibition of estrogen biosynthesis in intact trophoblast cells. In the presence of 1 nM or 10 nM MDL 18,962, trophoblast cells exhibited initial linear kinetics for estrogen biosynthesis during the first hour following co-incubation with inhibitor and 33 nM substrate. During the subsequent 30 min the rate of estrogen biosynthesis precipitously declined from 104 +/- 24 fmol/min/10(6) cells in control cells to 24 +/- 13 and 8 +/- 4 fmol/min/10(6) trophoblasts treated with 1 or 10 nM MDL 18,962, respectively. This significant decrease in aromatase activity (P less than or equal to 0.01) implied irreversible inactivation, which was supported by prolonged inhibition of aromatase activity in trophoblast cells incubated for 6-48 h following removal of medium containing 3 nM or 30 nM MDL 18,962.

Mechanism of inactivation of mammalian L-histidine decarboxylase by (S)-alpha-fluoromethylhistidine

The mechanism of inactivation by (S)-alpha-fluoromethylhistidine (FMH) of L-histidine decarboxylase (HDC, L-histidine carboxy-lyase, EC 4.1.1.22) purified from whole bodies of fetal rats was studied. FMH inhibited the activities of HDC purified from fetal HDC as well as HDCs from the brain and stomach of adult rats. The activity was not restored by extensive dialysis, indicating that the inhibition was irreversible. The inactivation of HDC was time and concentration dependent and followed pseudo first-order kinetics. L-Histidine, a substrate, protected HDC against inactivation, but D-histidine did not. Apo-HDC was not inactivated by FMH. On labeling of HDC with [3H]FMH, a correlation was found between the extent of incorporation of radioactivity into the enzyme and the degree of inactivation. Two moles of the inhibitor were incorporated into one mole of HDC (108,000 daltons). Experiments with [carboxyl-14C]FMH and [ring 2-14C]FMH showed that decarboxylation was necessary for the inactivation and that one molecule of FMH moiety was incorporated into an HDC monomer during every three decarboxylations of FMH.

Changes in neurotransmitter metabolism following olfactory bulbectomy in the rat

Bilateral ablation of the olfactory bulbs caused marked changes in the 'turnover' of several neurotransmitters in the amygdaloid cortex and the mid-brain areas of the rat brain. Following tyrosine and tryptophan hydroxylase inhibition, the decrease in the concentration of noradrenaline and serotonin respectively in the amygdaloid cortex was not so marked in the bulbectomized rats as in their controls. This suggests that the 'turnover' of these biogenic amines is reduced following bulbectomy. Following GABA transaminase inhibition, the increase in the concentration of GABA in this region was increased compared to the controls thereby suggesting that the 'turnover' of the inhibitory neurotransmitter was enhanced, glutamate decarboxylase activity was also increased in the amygdaloid cortex. No changes were found in the 'turnover' of noradrenaline or serotonin in the mid-brain but that of dopamine was decreased as was the activity of glutamate decarboxylase. It is concluded that changes in neurotransmitter 'turnover' in these brain regions are attributable to the destruction of the olfactory bulbs and may contribute to the behavioural deficits which we, and others, have reported elsewhere.

The influence of GABAergic drugs on PGO activity in the cat

The effects of drugs modifying GABAergic neurotransmission have been examined on ponto-geniculo-occipital (PGO) waves induced in the encéphale isolé cat by reserpine (PGOres), Ro4-1284 (PGO1284) or PCPA (PGOPCPA). The GABA agonists muscimol and THIP both caused large increases in density of PGOPCPA. The PGO1284 and PGOres were less affected although, of these, a larger increase in PGO1284 density was recorded. None of the increases could be reversed by subsequent injection of bicuculline. Chlordiazepoxide brought about large increases in PGOPCPA density but was ineffective in altering PGOres or PGO1284. The GABA transaminase inhibitor gamma-acetylenic GABA increased the density of all PGO waves but was not effective in the case of PGOPCPA. These results confirm a role for GABA in modulating PGO activity. The pathways involved in this GABA modulation are discussed.

An investigation of the properties of ornithine aminotransferase after inactivation by the 'suicide' inhibitor aminohexynoate and use of the compound as a probe of intracellullar protein turnover

Ornithine aminotransferase is shown to bind 1 mol of amino[14C]hexynoate per mol of coenzyme in the 'suicide' inactivation process. At the same time the coenzyme pyridoxal phosphate becomes irreversibly bound to the enzyme protein. Apart from the inactivation, the labelled enzyme is indistinguishable from native ornithine aminotransferase by several separation techniques. Because the rate of degradation of the labelled enzyme is the same as that of the normal enzyme it is concluded that loss of coenzyme does not initiate turnover. Free aminohexynoate is rapidly eliminated from the liver, and 70% of the compound is excreted unchanged in 7.5 h. Inactivated ornithine aminotransferase accounts for 11% of the total labelled liver protein and significant amounts of label are found in aspartate aminotransferase which is also extensively inactivated. The rate of return of enzyme activity is determined and found to be more rapid than expected for a process in which the enzyme is synthesized at a constant rate and degraded in a single, first-order process.

Chronic elevation of brain GABA by gamma-vinyl GABA treatment does not alter the sensitivity of GABAergic or dopaminergic receptors in rat CNS

Rat brain GABA levels were elevated chronically by daily administration of gamma-vinyl GABA, an enzyme-activated, irreversible inhibitor of GABA:2-oxo-glutarate aminotransferase (GABA-T; EC2.6.1.19). Following various periods of drug treatment and withdrawal, the sensitivity of dopamine and GABA receptors in the CNS was determined by biochemical and behavioral evaluations. In contrast to chronic haloperidol treatment, none of the treatment schedules with gamma-vinyl GABA had any significant effect on parameters such as apomorphine induced locomotor activity, [3H] spiperone binding or dopamine-stimulated adenylate cyclase in the corpus striatum; nor did gamma-vinyl GABA treatment affect [3H] GABA binding or GABA-activated [3H] diazepam binding in the cerebral cortex. Moreover, co-administration of gamma-vinyl GABA and haloperidol did not alter the ability of the neuroleptic to induce supersensitivity in the striatal dopaminergic system. Thus, it appears that, in contrast to reported studies using chronic administration of other less specific GABA-T inhibitors such as gamma-acetylenic GABA, amino-oxyacetic acid and isonicotinic acid hydrazide or direct GABA agonists such as THIP (4,5,6,7-tetrahydroisoxazolo (5,4-c-)-pyridin-3-ol) or kojic amine, gamma-vinyl GABA does not alter the sensitivity of the striatal dopaminergic system.

alpha-Monofluoromethyl and alpha-difluoromethyl putrescine as ornithine decarboxylase inhibitors: in vitro and in vivo biochemical properties

In vitro, 5-fluoropentane-1,4-diamine and 5,5-difluoropentane-1,4-diamine are potent enzyme-activated inhibitors of rat liver ornithine decarboxylase (EC 4.1.1.17). The two alpha-fluoromethyl derivatives of putrescine activate to different degrees S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50). The difluoromethyl derivative differs from the monofluoromethyl derivative in that it is not a substrate of diamine oxidase (EC 1.4.3.6), but is a better substrate of mitochondrial monoamine oxidase (EC 1.4.3.4) than the monofluoromethyl derivative. In vivo, a single i.p. injection of 200 mg/kg of 5-fluoropentane-1,4-diamine to rats causes a marked decrease of the ornithine decarboxylase activity in the ventral prostate and to a lesser extent in the thymus, whereas 5,5-difluoropentane-1,4-diamine causes only a slight decrease of this enzyme activity in the prostate and does not affect it in the thymus. Both compounds produce a decrease of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19) activity in the brain. The differences observed between the biochemical properties of the two alpha-fluoromethyl derivatives of putrescine are discussed in relation to the pKa value of the alpha-amino group which decreases from 7.75 for 5-fluoropentane-1,4-diamine to 6.4 for 5,5-difluoropentane-1,4-diamine.