Functional expression of plant acetolactate synthase genes in Escherichia coli

Acetolactate synthase (ALS; EC 4.1.3.18) is the first common enzyme in the biosynthetic pathways leading to leucine, isoleucine, and valine. It is the target enzyme for three classes of structurally unrelated herbicides, the sulfonylureas, the imidazolinones, and the triazolopyrimidines. A cloned ALS gene from the small cruciferous plant Arabidopsis thaliana has been fused to bacterial transcription/translation signals and the resulting plasmid has been used to transform Escherichia coli. The cloned plant gene, which includes sequences encoding the chloroplast transit peptide, is functionally expressed in the bacteria. It is able to complement genetically a strain of E. coli that lacks endogenous ALS activity. An ALS gene cloned from a line of Arabidopsis previously shown to be resistant to sulfonylurea herbicides has been similarly expressed in E. coli. The herbicide-resistance phenotype is expressed in the bacteria, as assayed by both enzyme activity and the ability to grow in the presence of herbicides. This system has been useful for purifying substantial amounts of the plant enzyme, for studying the sequence parameters involved in subcellular protein localization, and for characterizing the interactions that occur between ALS and its various inhibitors.

Role of taurine in regulation of intracellular calcium level and neuroprotective function in cultured neurons

Glutamate-induced excitotoxicity has been implicated as an important mechanism underlying a variety of brain injuries and neurodegenerative diseases. Previously we have shown that taurine has protective effects against glutamate-induced neuronal injury in cultured neurons. Here we propose that the primary underlying mechanism of the neuroprotective function of taurine is due to its action in preventing or reducing glutamate-induced elevation of intracellular free calcium, [Ca(2+)](i). This hypothesis is supported by the following findings. First, taurine transport inhibitors, e.g., guanidinoethyl sulfonate and beta-alanine, have no effect on taurine's neuroprotective function, suggesting that taurine protects against glutamate-induced neuronal damage through its action on the extracellular membranes. Second, glutamate-induced elevation of [Ca(2+)](i) is reduced to the basal level upon addition of taurine. Third, pretreatment of cultured neurons with taurine prevents or greatly suppresses the elevation of [Ca(2+)](i) induced by glutamate. Furthermore, taurine was found to inhibit the influx but not the efflux of (45)Ca(2+) in cultured neurons. Taurine has little effect on the binding of [(3)H]glutamate to the agonist binding site and of [(3)H]MDL 105,519 to the glycine binding site of the N-methyl-D-aspartic acid receptors, suggesting that taurine inhibits (45)Ca(2+) influx through other mechanisms, including its inhibitory effect on the reverse mode of the Na(+)/Ca(2+) exchangers (Wu et al. [2000] In: Taurine 4: taurine and excitable tissues. New York: Kluwer Academic/Plenum Publishers. p 35-44) rather than serving as an antagonist to the N-methyl-D-aspartic acid receptors.

Oxygen-induced seizures and inhibition of human glutamate decarboxylase and porcine cysteine sulfinic acid decarboxylase by oxygen and nitric oxide

The recombinant forms of the two human isozymes of glutamate decarboxylase, GAD65 and GAD67, are potently and reversibly inhibited by molecular oxygen (Ki = 0.46 and 0.29 mM, respectively). Inhibition of the vesicle-associated glutamate decarboxylase (GAD65) by molecular oxygen is likely to result in incomplete filling of synaptic vesicles with gamma-aminobutyric acid (GABA) and may be a contributing factor in the genesis of oxygen-induced seizures. Under anaerobic conditions, nitric oxide inhibits both GAD65 and GAD67 with comparable potency to molecular oxygen (Ki = 0.5 mM). Two forms of porcine cysteine sulfinic acid decarboxylase (CSADI and CSADII) are also sensitive to inhibition by molecular oxygen (Ki = 0.30 and 0.22 mM, respectively) and nitric oxide (Ki = 0.3 and 0.2 mM, respectively). Similar inhibition of glutamate decarboxylase and cysteine sulfinic acid decarboxylase by two different radical-containing compounds (O2 and NO) is consistent with the notion that these reactions proceed via radical mechanisms.

Interaction of the herbicide glyphosate with its target enzyme 5-enolpyruvylshikimate 3-phosphate synthase in atomic detail

Biosynthesis of aromatic amino acids in plants, many bacteria, and microbes relies on the enzyme 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase, a prime target for drugs and herbicides. We have identified the interaction of EPSP synthase with one of its two substrates (shikimate 3-phosphate) and with the widely used herbicide glyphosate by x-ray crystallography. The two-domain enzyme closes on ligand binding, thereby forming the active site in the interdomain cleft. Glyphosate appears to occupy the binding site of the second substrate of EPSP synthase (phosphoenol pyruvate), mimicking an intermediate state of the ternary enzyme.substrates complex. The elucidation of the active site of EPSP synthase and especially of the binding pattern of glyphosate provides a valuable roadmap for engineering new herbicides and herbicide-resistant crops, as well as new antibiotic and antiparasitic drugs.

Neurotoxic effect of acamprosate, n-acetyl-homotaurine, in cultured neurons

Acamprosate (AC), N-acetyl-homotaurine, has recently been introduced for treating alcohol craving and reducing relapses in weaned alcoholics. AC may exert its action through the taurine system rather than the glutamatergic or GABAergic system. This conclusion is based on the observations that AC strongly inhibits the binding of taurine to taurine receptors while it has little effect on the binding of glutamate to glutamate receptors or muscimol to GABA(A) receptors. In addition, AC was found to be neurotoxic, at least in neuronal cultures, triggering neuronal damage at 1 mM. The underlying mechanism of AC-induced neuronal injury appears to be due to its action in increasing the intracellular calcium level, [Ca2+](i). Both AC-induced neurotoxicity and elevation of [Ca2+](i) can be prevented by taurine suggesting that AC may exert its effect through its antagonistic interaction with taurine receptors.

S-methyl-N,N-diethylthiocarbamate sulfoxide elicits neuroprotective effect against N-methyl-D-aspartate receptor-mediated neurotoxicity

Glutamatergic neurotransmission, particularly of the NMDA receptor type, has been implicated in the excitotoxic response to several external and internal stimuli. In the present investigation, we report that S-methyl-N,N-diethylthiocarbamate sulfoxide (DETC-MeSO) selectively and specifically blocks the NMDA receptor subtype of the glutamate receptors, and attenuates glutamate-induced neurotoxicity in rat-cultured primary neurons. Other major ionotropic glutamate receptor subtypes, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate, were insensitive to DETC-MeSO both in vitro and in vivo. Disulfiram, the parent compound of DETC-MeSO, also inhibits glutamate receptors partially in vivo; however, it fails to inhibit glutamate receptors in mice pretreated with N-butyl imidazole, a cytochrome P450 enzyme inhibitor, implicating the need for bioactivation of disulfiram to be an effective antagonist. We showed that glutamate-induced increase in (45)Ca2+ was attenuated in rat-cultured primary neurons following pretreatment with DETC-MeSO. The Ca2+ influx into primary neurons, studied by confocal microscopy of the fluorescent Ca2+ dye fura-2, demonstrated a complete attenuation of NMDA-induced Ca2+ influx. Similarly, DETC-MeSO attenuated NMDA-induced (45)Ca2+ uptake. Glutamate-induced (45)Ca2+ uptake and Ca2+ influx, however, were partially blocked by DETC-MeSO, and this is consistent with both in vitro and in vivo studies in which DETC-MeSO partially blocked mouse brain glutamate receptors. In addition, DETC-MeSO pretreatment effectively prevented seizures in mice induced either by NMDA, ammonium acetate, or ethanol-induced kindling seizures, all of which are believed to be mediated by NMDA receptors. These data demonstrate that DETC-MeSO produces the neuroprotective effect through antagonism of NMDA receptors in vivo.

Polyhydroxylated C(60), fullerenols, as glutamate receptor antagonists and neuroprotective agents

Derivatives of C(60) have been shown to be effective free radical scavengers. Hence, many of the biological functions of fullerene are believed to be due to their antioxidant properties. Here we present evidence to show that fullerenols, that are caged fullerene oxides, exert their neuroprotective functions by blocking glutamate receptors and lowering the intracellular calcium, [Ca(2+)](i). In neuronal cultures, fullerenols reduce glutamate-induced neurotoxicity by about 80% at 50microM. No significant effect was observed on H(2)O(2)/Fe(2+)-induced neurotoxicity under the same conditions. Fullerenols were found to inhibit glutamate receptor binding in a dose-dependent manner inhibiting 50% of glutamate binding at 50 microM. Furthermore, AMPA receptors were found to be more sensitive to fullerenols than NMDA and KA receptors. On the other hand, GABA(A) receptors and taurine receptors were not significantly affected by fullerenols at the same concentrations used, suggesting that fullerenols inhibit primarily the glutamate receptors. In addition, fullerenols were also found to lower glutamate (Glu) receptor-induced elevation of [Ca(2+)](i), suggesting that the underlying mechanism of neuronal protective function of fullerenols is likely due to its ability to block the glutamate receptors and to reduce the level of [Ca(2+)](i).

Association of L-glutamic acid decarboxylase to the 70-kDa heat shock protein as a potential anchoring mechanism to synaptic vesicles

Recently we have reported that the membrane-associated form of the gamma-aminobutyric acid-synthesizing enzyme, l-glutamate decarboxylase (MGAD), is regulated by the vesicular proton gradient (Hsu, C. C., Thomas, C., Chen, W., Davis, K. M., Foos, T., Chen, J. L., Wu, E., Floor, E., Schloss, J. V., and Wu, J. Y. (1999) J. Biol. Chem. 274, 24366-24371). In this report, several lines of evidence are presented to indicate that l-glutamate decarboxylase (GAD) can become membrane-associated to synaptic vesicles first through complex formation with the heat shock protein 70 family, specifically heat shock cognate 70 (HSC70), followed by interaction with cysteine string protein (CSP), an integral protein of the synaptic vesicle. The first line of evidence comes from purification of MGAD in which HSC70, as identified from amino acid sequencing, co-purified with GAD. Second, in reconstitution studies, HSC70 was found to form complex with GAD(65) as shown by gel mobility shift in non-denaturing gradient gel electrophoresis. Third, in immunoprecipitation studies, again, HSC70 was co-immunoprecipitated with GAD by a GAD(65)-specific monoclonal antibody. Fourth, HSC70 and CSP were co-purified with GAD by specific anti-GAD immunoaffinity columns. Furthermore, studies here suggest that both GAD(65) and GAD(67) are associated with synaptic vesicles along with HSC70 and CSP. Based on these findings, a model is proposed to link anchorage of MGAD to synaptic vesicles in relation to its role in gamma-aminobutyric acid neurotransmission.

A novel method for expression and large-scale production of human brain l-glutamate decarboxylase

l-Glutamate decarboxylase (GAD; EC 4.1.1.15) is the rate-limiting enzyme involved in the synthesis of gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the mammalian brain. Imbalance in the conversion of glutamate to GABA has been implicated in a host of human diseases. Studies on the structure, function, and therapeutic use of GAD have been precluded by insufficient quantities of purified active enzyme. Here we report a novel methodology for the expression and large-scale production of enzymatically active, pure, recombinant human GAD65 and GAD67. This method circumvents the sequestering of expressed protein into insoluble inclusion bodies and reduces production of truncated proteins. The availability of sufficient quantities of purified HGAD65 and HGAD67 has allowed for the production of specific polyclonal antibodies that discriminate between the two isoforms. This methodology, in addition to providing key human brain enzymes, may be generally applicable to other systems.

Role of synaptic vesicle proton gradient and protein phosphorylation on ATP-mediated activation of membrane-associated brain glutamate decarboxylase

Previously, we have shown that the soluble form of brain glutamic acid decarboxylase (GAD) is inhibited by ATP through protein phosphorylation and is activated by calcineurin-mediated protein dephosphorylation (Bao, J., Cheung, W. Y., and Wu, J. Y. (1995) J. Biol. Chem. 270, 6464-6467). Here we report that the membrane-associated form of GAD (MGAD) is greatly activated by ATP, whereas adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP), a non-hydrolyzable ATP analog, has no effect on MGAD activity. ATP activation of MGAD is abolished by conditions that disrupt the proton gradient of synaptic vesicles, e.g. the presence of vesicular proton pump inhibitor, bafilomycin A1, the protonophore carbonyl cyanide m-chorophenylhydrazone or the ionophore gramicidin, indicating that the synaptic vesicle proton gradient is essential in ATP activation of MGAD. Furthermore, direct incorporation of (32)P from [gamma-(32)P]ATP into MGAD has been demonstrated. In addition, MGAD (presumably GAD65, since it is recognized by specific monoclonal antibody, GAD6, as well as specific anti-GAD65) has been reported to be associated with synaptic vesicles. Based on these results, a model linking gamma-aminobutyric acid (GABA) synthesis by MGAD to GABA packaging into synaptic vesicles by proton gradient-mediated GABA transport is presented. Activation of MGAD by phosphorylation appears to be mediated by a vesicular protein kinase that is controlled by the vesicular proton gradient.

The sulfonimidamide as a novel transition state analog for aspartic acid and metallo proteases

We have developed a novel strategy for the preparation of tetrahedral transition state analogs for aspartic acid and metallo-proteases based upon the sulfonimidamide functional group. Our best alpha-des-amino dipeptide analog binds at least 100-fold tighter than the corresponding ground state structure (i.e., amide). A previously unpublished five-membered cyclic sulfonimidamide was also synthesized.

New targets for the treatment of vascular diseases and diabetes

This symposium was organized by Dan Flynn (Monsanto Life Sciences, USA) and Timothy M Willson (Glaxo Wellcome, USA). PPARs (peroxisome proliferator-activated receptors), are nuclear hormone receptors that govern glucose and lipid homeostasis. There are several subtypes of receptors that share activation by unsaturated fatty acids and work in combination with retinoic acid receptors (RXR), which were a topic covered in an earlier symposium. Two classes of chemistry were discussed at the symposium: (i) thiazolidinediones, that interact directly with PPARs and alter lipid metabolism; and, (ii) benzothiepines or benzothiazepines, that inhibit the ileal bile acid transporter (IBAT) and reduce cholesterol levels by increasing bile acid excretion.

Abeles symposium

This symposium was the inspiration of Joanne Stubbe (MIT, MA, USA), who organized an impressive tribute to the career of Robert H Abeles (Brandeis University, MA, USA). Professor Abeles is an influential figures in the development of 'rational drug design' and is one of the pioneers in the development of suicide substrates. His work in transition-state or reaction-intermediate analogs, most notably trifluoromethyl ketones, has had a substantial impact on the design of novel pharmaceutical compounds. Despite retirement and failing health, Professor Abeles has managed to contribute to the development of combinatorial chemistry, which is currently of interest as a strategy in drug design. Another facet of his career has been a fascination with biosynthetic pathways or enzyme catalyzed reactions of an unusual or obscure nature. Professor Abeles's approach has always had a distinctly chemical emphasis resulting in unraveling detailed, and often surprising, mechanisms. He has served as an inspiration to countless academic and industrial scientists around the world. Evidence of this is the impressive attendance at the Abeles Symposium and the standing ovation given to Professor Abeles. Former associates gave most of the oral presen-tations in the morning and afternoon sessions of the symposium. Professor Joanne Stubbe gave the introductory talk for the morning's session. An overview of the various research areas by Professor Abeles, including his early work on ribonucleotide reductase that led, in part, to Professor Stubbe's own research in this area, was presented.

Inhibitors of branched-chain amino acid biosynthesis as potential antituberculosis agents

Leucine auxotrophs of Mycobacterium bovis (BCG) were found to have a reduced ability to survive in spleens and lungs of mice. This indicated that inhibitors of branched-chain amino acid biosynthesis could possibly be used as antituberculosis agents. Herbicides that inhibit plant branched-chain amino acid biosynthetic enzymes were tested for inhibition of Mycobacterium tuberculosis growth in vitro. Sulphometuron methyl (SM) and metsulphuron methyl, inhibitors of acetolactate synthase (ALS), had a modest effect on growth of M. tuberculosis strain ATCC 35801 (inhibitory concentrations <20 microM). Two inhibitors of ketol acid reductoisomerase (KARI) were ineffective against growth of strain ATCC 35801 in vitro. On the other hand, ALS and KARI inhibitors were more effective against growth of clinical drug-resistant isolates than against strain ATCC 35801. Mouse model studies of tuberculosis infection showed that high doses of SM significantly prevented growth of M. tuberculosis strain ATCC 35801 in the lungs but did not affect the level of infection in the spleen. The results suggest that inhibitors of branched-chain amino acid biosynthesis may be useful as new antituberculosis agents.

Biosynthesis of 2-aceto-2-hydroxy acids: acetolactate synthases and acetohydroxyacid synthases

Two groups of enzymes are classified as acetolactate synthase (EC 4. 1.3.18). This review deals chiefly with the FAD-dependent, biosynthetic enzymes which readily catalyze the formation of acetohydroxybutyrate from pyruvate and 2-oxobutyrate, as well as of acetolactate from two molecules of pyruvate (the ALS/AHAS group). These enzymes are generally susceptible to inhibition by one or more of the branched-chain amino acids which are ultimate products of the acetohydroxyacids, as well as by several classes of herbicides (sulfonylureas, imidazolinones and others). Some ALS/AHASs also catalyze the (non-physiological) oxidative decarboxylation of pyruvate, leading to peracetic acid; the possible relationship of this process to oxygen toxicity is considered. The bacterial ALS/AHAS which have been well characterized consist of catalytic subunits (around 60 kDa) and smaller regulatory subunits in an alpha2beta2 structure. In the case of Escherichia coli isozyme III, assembly and dissociation of the holoenzyme has been studied. The quaternary structure of the eukaryotic enzymes is less clear and in plants and yeast only catalytic polypeptides (homologous to those of bacteria) have been clearly identified. The presence of regulatory polypeptides in these organisms cannot be ruled out, however, and genes which encode putative ALS/AHAS regulatory subunits have been identified in some cases. A consensus sequence can be constructed from the 21 sequences which have been shown experimentally to represent ALS/AHAS catalytic polypeptides. Many other sequences fit this consensus, but some genes identified as putative 'acetolactate synthase genes' are almost certainly not ALS/AHAS. The solution of the crystal structures of several thiamin diphosphate (ThDP)-dependent enzymes which are homologous to ALS/AHAS, together with the availability of many amino acid sequences for the latter enzymes, has made it possible for two laboratories to propose similar, reasonable models for a dimer of catalytic subunits of an ALS/AHAS. A number of characteristics of these enzymes can now be better understood on the basis of such models: the nature of the herbicide binding site, the structural role of FAD and the binding of ThDP-Mg2+. The models are also guides for experimental testing of ideas concerning structure-function relationships in these enzymes, e.g. the nature of the substrate recognition site. Among the important remaining questions is how the enzyme suppresses alternative reactions of the intrinsically reactive hydroxyethylThDP enamine formed by the decarboxylation of the first substrate molecule and specifically promotes its condensation with 2-oxobutyrate or pyruvate.

Regulation of taurine biosynthesis and its physiological significance in the brain

Cysteine sulfinic acid decarboxylase (CSAD), the rate-limiting enzyme in taurine biosynthesis, was found to be activated under conditions that favor protein phosphorylation and inactivated under conditions favoring protein dephosphorylation. Direct incorporation of 32P into purified CSAD has been demonstrated with [gamma 32P]ATP and PKC, but not PKA. In addition, the 32P labeling of CSAD was inhibited by PKC inhibitors suggesting that PKC is responsible for phosphorylation of CSAD in the brain. Okadaic acid had no effect on CSAD activity at 10 microM suggesting that protein phosphatase-2C (PrP-2C) might be involved in the dephosphorylation of CSAD. Furthermore, it was found that either glutamate- or high K(+)-induced depolarization increased CSAD activity as well as 32P-incorporation into CSAD in neuronal cultures, supporting the notion that the CSAD activity is endogenously regulated by protein phosphorylation in the brain. A model to link neuronal excitation, phosphorylation of CSAD and increase in taurine biosynthesis is proposed.

Glutathione carbamoylation with S-methyl N,N-diethylthiolcarbamate sulfoxide and sulfone. Mitochondrial low Km aldehyde dehydrogenase inhibition and implications for its alcohol-deterrent action

S-Methyl N,N-diethylthiolcarbamate sulfoxide (DETC-MeSO) and sulfone (DETC-MeSO2) both inhibit rat liver low Km aldehyde dehydrogenase (ALDH2) in vitro and in vivo (Nagendra et al., Biochem Pharmacol 47: 1465-1467, 1994). DETC-MeSO has been shown to be a metabolite of disulfiram, but DETC-MeSO2 has not. Studies were carried out to further investigate the inhibition of ALDH2 by DETC-MeSO and DETC-MeSO2. In an in vitro system containing hydrogen peroxide and horseradish peroxidase, the rate of DETC-MeSO oxidation corresponded to the rate of DETC-MeSO2 formation. Carbamoylation of GSH by both DETC-MeSO and DETC-MeSO2 was observed in a rat liver S9 fraction. Carbamoylation of GSH was not observed in the presence of N-methylmaleimide. In in vitro studies, DETC-MeSO and DETC-MeSO2 were equipotent ALDH2 inhibitors when solubilized mitochondria were used, but DETC-MeSO was approximately four times more potent than DETC-MeSO2 in intact mitochondria. In studies with rats, the dose (i.p. or oral) required to inhibit 50% ALDH2 (ED50) was 3.5 mg/kg for DETC-MeSO and approximately 35 mg/kg for DETC-MeSO2, approximately a 10-fold difference. Furthermore, maximum ALDH2 inhibition occurred 1 hr after DET(-MeSO administration, whereas maximal ALDH2 inhibition occurred 8 hr after DETC-MeSO2 dosing. DETC-MeSO is, therefore, not only a more potent ALDH2 inhibitor than DETC-MeSO2 in vivo, but also in vitro when intact mitochondria are utilized. The in vitro results thus support the in vivo findings. Since oxidation of DETC-MeSO can occur both enzymatically and non-enzymatically, it is possible that DETC-MeSO2 is formed in vivo. DETC-MeSO2, however, is not as effective as DETC-MeSO in inhibiting ALDH2, probably because it has difficulty penetrating the mitochondrial membrane. Thus, even if DETC-MeSO2 is formed in vivo from DETC-MeSO, it is the metabolite DETC-MeSO that is most likely responsible for the inhibition of ALDH2 after disulfiram administration.

Carbamoylation of brain glutamate receptors by a disulfiram metabolite

S-Methyl-N,N-diethylthiolcarbamate sulfoxide (DETC-MeSO), a metabolite of the drug disulfiram, is a selective carbamoylating agent for sulfhydryl groups. Treatment of glutamate receptors isolated from mouse brain with DETC-MeSO blocks glutamate binding. In vivo, carbamoylated glutathione, administered directly to mice or formed by reaction of DETC-MeSO with glutathione in the blood, also blocks brain glutamate receptors. Carbamoyl groups appear to be delivered to brain glutamate receptors or to liver aldehyde dehydrogenase in vivo by a novel glutathione-mediated mechanism. Seizures caused by the glutamate analogs N-methyl-D-aspartate and methionine sulfoximine, or by hyperbaric oxygen, are prevented by DETC-MeSO, indicating that carbamoylation of glutamate receptors gives an antagonist effect. These observations offer an explanation for some of the previously reported neurological effects of disulfiram, such as its ability to prevent O2-induced seizures. Furthermore, some of the physiology of the disulfiram-ethanol reaction, that could not be accounted for based on the known inhibition of aldehyde dehydrogenase alone, may be explained by disulfiram's effect on glutamate receptors.

Protein phosphorylation and taurine biosynthesis in vivo and in vitro

Taurine is known to be involved in many important physiological functions. Here we report that both in vivo and in vitro the taurine-synthesizing enzyme in the brain, namely cysteine sulfinic acid decarboxylase (CSAD), is activated when phosphorylated and inhibited when dephosphorylated. Furthermore, protein kinase C and protein phosphatase 2C have been identified as the enzymes responsible for phosphorylation and dephosphorylation of CSAD, respectively. In addition, the effect of neuronal depolarization on CSAD activity and 32P incorporation into CSAD in neuronal cultures is also included. A model to link neuronal excitation and CSAD activation by a Ca2+-dependent protein kinase is proposed.

Homology modeling of the structure of bacterial acetohydroxy acid synthase and examination of the active site by site-directed mutagenesis

Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) catalyzes the thiamin pyrophosphate (TPP)-dependent decarboxylation of pyruvate and condensation of the resulting two-carbon moiety with a second alpha-keto acid. It belongs to a family of homologous, TPP-dependent enzymes which catalyze different reactions which start from decarboxylation of alpha-keto acids. A model for the structure of Escherichia coli AHAS isozyme II, based on its homology with pyruvate oxidase and experimental testing of the model by site-directed mutagenesis, has been used here to study how AHAS controls the chemical fate of a decarboxylated keto acid. Because of the potential conformational freedom of the reacting substrates, residues interacting with the substrate could not be identified directly from the model of AHAS. Three residues were considered as candidates for involvement in the recognition of alpha-ketobutyrate, as the amino acids at these sites in a unique low-specificity AHAS are different from those in typical AHASs, which are highly specific for reaction with alpha-ketobutyrate as second substrate, in preference to pyruvate. These residues were altered in AHAS II by site-directed mutagenesis. Replacement of Trp464 lowers the specificity by at least 1 order of magnitude, with minor effects on the activity or stability of the enzyme, suggesting that Trp464 contributes > or = 1.3 kcal mol-1 to interaction with the "extra" methyl of alpha-ketobutyrate. Mutations of Met460 or Thr70 have small effects on specificity and do affect other properties of the protein. A model for enzyme-substrate interactions can be proposed on the basis of these results. The model of AHAS also explains previously reported spontaneous mutants of AHAS resistant to sulfonylurea herbicides, which probably bind in the narrow depression which provides access to the bound TPP. A role for the C terminus of the enzyme polypeptide in determination on the reaction pathway is also possible.

Enzyme-coupled assays for proteases

We have developed a general strategy for assaying proteases that does not require the use of fluorogenic, chromogenic, or radiolabeled peptide substrates. The endo- or exoproteolytic hydrolysis of simple peptides can be followed spectrophotometrically by coupling the proteolytic event via enzyme-catalyzed reactions to a chromogenic redox dye. The couple can be used directly to follow the action of carboxy or amino peptidases on peptide substrates or can be coupled by use of carboxy or amino peptidases to follow the action of endoproteases on peptide substrates that are blocked at the amino or carboxy terminus, respectively. Liberated amino acids are detected by use of amino acid oxidase, oxygen, horseradish peroxidase, and the redox dye 2,2'-azino-bis-(3-ethyl-benzthiazoline-6-sulfonic acid (epsilon 414nm = 36,000 M-1 cm-1).

Quo vadis photorespiration: a tale of two aldolases

An O2-consuming side reaction of D-ribulose 1,5-bisphosphate carboxylase causes photorespiration in plants. This reaction may be an inevitable consequence of the enzyme's inability to protect its ene-diolate reaction intermediate from O2, a notion that is supported by the failure of persistent efforts to eliminate selectively its oxygenase activity by genetic manipulation. We have examined two a1dolases with similar ene-diolate intermediates, L-rhamnulose 1-phosphate aldolase and L-fuculose 1-phosphate aldolase. The former enzyme has an oxygenase activity, while the latter does not, suggesting that the reaction with O2 is not inevitable.

Studies of NMDA- and non-NMDA-mediated neurotoxicity in cultured neurons

The neurotoxic effects of various glutamate agonists were studied using whole fetal rat brain cultures. The results showed that L-glutamate (L-glu) and N-methyl-D-aspartate (NMDA) were the most potent agonists for inducing neurotoxicity, producing significant toxicity at 0.10 and 0.01 mM concentrations, respectively. Kainic acid (KA) and quisqualic acid (QA) also produced neurotoxicity, but only at a relatively high concentration (1.0 mM). No other glutamate agonist tested produced neurotoxicity in the cultures following brief incubations. The effects of each agonist were found to be Ca2+ dependent, and the selective NMDA Ca2+ channel agonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK-801), blocked the toxicity produced by all the glutamate agonists. Thus, the results of this study found little or no evidence for a direct non-NMDA receptor mediated neurotoxicity. These results suggest that the neurotoxicity produced by the non-NMDA agonists may be due to one of the following mechanisms: (i) non-specific binding of non-NMDA agonists to NMDA receptor; (ii) release of L-glu via non-NMDA agonists induced depolarization of cell membrane and subsequent activation of NMDA receptor by released L-glu; (iii) inhibition of L-glu uptake by non-NMDA agonists resulting in activation of L-glu receptors including NMDA receptors.

Herbicidal Activity of an Isopropylmalate Dehydrogenase Inhibitor

Isopropylmalate dehydrogenase (IPMDH) is the third enzyme specific to leucine biosynthesis. It catalyzes the oxidative decarboxylation of 3-isopropylmalate (3-IPM) to 2-ketoisocaproic acid. The partially purified enzyme from pea (Pisum sativum L.) shows a broad pH optimum of 7.8 to 9.1 and has Km values for 3-IPM and NAD of 18 and 40 [mu]M, respectively. O-Isobutenyl oxalylhydroxamate (O-IbOHA) has been discovered to be an excellent inhibitor of the pea IPMDH, with an apparent inhibitor constant of 5 nM. As an herbicide, O-IbOHA showed only moderate activity on a variety of broadleaf and grass species. We characterized the herbicidal activity of O-IbOHA on corn (Zea mays L.), a sensitive species; giant foxtail (Setaria faberi) and morning glory (Ipomoea purpurea [L.] Roth), moderately tolerant species; and soybean [Glycine max L. Merr.), a tolerant species. Differences in tolerance among the species were not due to differences in the sensitivity of IPMDH. Studies with [14C]O-IbOHA suggested that uptake and translocation were not major limitations for herbicidal activity, nor were they determinants of tolerance. Moreover, metabolism could not account for the difference in tolerance of corn, foxtail, and morning glory, although it might account for the tolerance of soybean. Herbicidal activity on all four species was correlated with the accumulation of 3-IPM in the plants.

The oxygenase reaction of acetolactate synthase

In addition to the physiological reactions catalyzed by acetolactate synthase, it supports an oxygen-consuming side reaction. Although the synthase and oxygenase activities are activated to somewhat different extents by various metals (Mn2+, Mg2+, Ca2+, Co2+, Zn2+, Ni2+, Cd2+, Cu2+, Ba2+, Al3+), the modest degree of these differences (at most 6-fold) and the high degree of promiscuity of the enzyme with respect to its metal requirement suggest that the metal is not intimately involved in the chemistry of either reaction. Saturation of the oxygenase reaction occurs at pyruvate concentrations below the limit of sensitivity for the oxygen electrode (< 10 microM), at higher concentrations pyruvate inhibits the rate of oxygen consumption. At a noninhibitory concentration of pyruvate (1 mM), inhibition of the reaction is also observed with alpha-ketobutyrate. Inhibition of the oxygenase reaction by high concentrations of pyruvate or alpha-ketobutyrate is presumably due to competition between these substrates and molecular oxygen for a common carbanionic reaction intermediate, the conjugate base of (hydroxyethyl)thiamin pyrophosphate. Inhibition of the reaction indicates that the lactylthiamin pyrophosphate intermediate can decarboxylate prior to binding of the second pyruvate or alpha-ketobutyrate. At high concentrations of pyruvate or alpha-ketobutyrate, only incomplete inhibition of the oxygenase reaction is achieved (65-89% or 89-93% maximal inhibition, respectively). This incomplete inhibition of the oxygenase reaction by alpha-keto acids indicates that the reaction is not Theorell-Chance with respect to addition of the second alpha-keto acid and that oxygen has more than one route of access to the carbanionic reaction intermediate.

Oxygenase side reactions of acetolactate synthase and other carbanion-forming enzymes

Enzymes that mediate carbanion chemistry must protect their reactants from solvent and undesirable electrophiles, such as molecular oxygen. A number of enzymes that utilize carbanionic intermediates were surveyed for O2-consuming side reactions. Several of these enzymes, acetolactate synthase, pyruvate decarboxylase, class II aldolase, and glutamate decarboxylase, catalyze previously undetected oxygen-consuming reactions, while others such as class I aldolase, [(phosphoribosyl)amino]imidazole carboxylase, 6-phosphogluconate dehydrogenase, isocitrate dehydrogenase, and triosephosphate isomerase do not. Prior to this work, only ribulosebisphosphate carboxylase was known to catalyze an oxygenase side reaction. These new example indicate that while O2-consuming side reactions are a more general feature of enzyme-mediated carbanion chemistry than has been previously appreciated, they are not necessarily an inevitable consequence of this chemistry. Expression of an oxygenase activity not only depends on the accessibility of the carbanionic intermediate to molecular oxygen but also may depend on the ability of the enzyme to stabilize the initially formed peroxide anion either through protonation with an appropriate enzymic group or through metal coordination.

Oxalyl hydroxamates as reaction-intermediate analogues for ketol-acid reductoisomerase

N-Hydroxy-N-isopropyloxamate (IpOHA) is an exceptionally potent inhibitor of the Escherichia coli ketol-acid reductoisomerase. In the presence of Mg2+ or Mn2+, IpOHA inhibits the enzyme in a time-dependent manner, forming a nearly irreversible complex. Nucleotide, which is essential for catalysis, greatly enhances the binding of IpOHA by the reductoisomerase, with NADPH (normally present during the enzyme's rearrangement step, i.e., conversion of a beta-keto acid into an alpha-keto acid, in either the forward or reverse physiological reactions) being more effective than NADP. In the presence of Mg2+ and NADPH, IpOHA appears to bind to the enzyme in a two-step mechanism, with an initial inhibition constant of 160 nM and a maximum rate of formation of the tight, slowly reversible complex of 0.57 min-1 (values that give an association rate of IpOHA, at low concentration, of 5.9 X 10(4) M-1 s-1). The rate of exchange of [14C]IpOHA from an enzyme-[14C]IpOHA-Mg2(+)-NADPH complex with exogenous, unlabeled IpOHA has a half-time of 6 days (150 h). This dissociation rate (1.3 X 10(-6) s-1) and the association rate determined by inactivation kinetics define an overall dissociation constant of 22 pM. By contrast, in the presence of Mn2+ and NADPH, the corresponding association and dissociation rates for IpOHA are 8.2 X 10(4) M-1 s-1 and 3.2 X 10(-6) s-1 (half-time = 2.5 days), respectively, which define an overall dissociation constant of 38 pM. In the presence of NADP or in the absence of nucleotide (both in the presence of Mg2+), the enzyme-IpOHA complex is far more labile, with dissociation half-times of 28 and 2 h, respectively. In the absence of Mg2+ or Mn2+, IpOHA does not exhibit time-dependent inhibition of the reductoisomerase.(ABSTRACT TRUNCATED AT 250 WORDS)

High-level synthesis of recombinant HIV-1 protease and the recovery of active enzyme from inclusion bodies

A complete chemical synthesis and assembly of genes for the production of human immunodeficiency virus type-I protease (HIV-PR) and its precursors are described. The T7 expression system was used to produce high levels of active HIV-PR and its precursors in Escherichia coli inclusion bodies. The gene encoding the open reading frames of HIV-PR was expressed in E. coli as a 10-kDa protein, while the genes encoding HIV-PR precursors were expressed as larger proteins, which were partially processed in E. coli to the 10-kDa form. These processing events are autoproteolytic, since a single-base mutation, changing the active-site aspartic acid to glycine, completely abolished the conversion. HIV-PR can be released with 8 M urea from washed cellular inclusion bodies, resulting in a preparation with few bacterial host proteins. After refolding, this preparation contains no nonspecific protease or peptidase activities. The recombinant HIV-PR isolated from inclusion bodies cleaves HIV-PR substrates specifically with a specific activity comparable to column-purified HIV-PR.

Comparative affinities of the epimeric reaction-intermediate analogs 2- and 4-carboxy-D-arabinitol 1,5-bisphosphate for spinach ribulose 1,5-bisphosphate carboxylase

2-Carboxy-3-keto-D-arabinitol 1,5-bisphosphate is a tightly bound intermediate of the carboxylase reaction of ribulosebisphosphate carboxylase/oxygenase. Two stereoisomers of an analog of this intermediate, 2-carboxy-D-arabinitol 1,5-bisphosphate (2CABP) and 4-carboxy-D-arabinitol 1,5-bisphosphate (4CABP), are exceptionally potent, virtually irreversible inhibitors of the spinach carboxylase, presumably due to their structural similarity to the gem-diol (hydrated carbonyl at C-3) form of the intermediate. Incubation of the enzyme with either leads to time-dependent loss of activity. Inhibition of the enzyme is biphasic, with initial dissociation constants of 0.47 and 0.19 microM and maximal rates for tight complex formation of 2.2 and 1.8 min-1 for 2CABP and 4CABP, respectively. These values give second-order rate constants for tight complex formation of 7.8 x 10(4) and 1.6 x 10(5) M-1 s-1. To determine the overall affinity of the spinach enzyme for 2CABP and 4CABP, the release rates were determined by dual isotope exchange (3H-inhibitor complex with free 14C-inhibitor). Exchange half-times of 1.82 and 530 days were observed for 4CABP and 2CABP, respectively. Overall dissociation constants of 28 pM (2.8 x 10(-11) M) and 190 fM (1.9 x 10(-13) M) were calculated from these dissociation rates together with the rates of association determined by inactivation kinetics. The difference in affinity of 2CABP and 4CABP corresponds to 2.9 kcal/mol, presumably reflecting the difference in interaction of the enzyme with the two hydroxyls of the intermediate's gem-diol. The kinetic behavior of these two inhibitors, in particular the rather slow maximal rates of association, are consistent with the expected behavior of analogs of a labile intermediate of an enzymic reaction that is far more stable than a transition state.

Purification and properties of Salmonella typhimurium acetolactate synthase isozyme II from Escherichia coli HB101/pDU9

A facile purification has been devised for recombinantly produced Salmonella typhimurium acetolactate synthase isozyme II. Purification of the enzyme was made possible by determining the complex set of factors that lead to loss of enzymic activity with this rather labile enzyme. When complexed with thiamin pyrophosphate, FAD, and magnesium, acetolactate synthase is subject to oxygen-dependent inactivation, a property not shared by the enzyme-FAD complex. When divorced from all of its tightly bound cofactors, losses of the enzymic activity are encountered at low ionic strength, especially at low protein concentrations. If purified and stored as the enzyme-FAD complex, acetolactate synthase is quite stable. The enzyme is composed of two types of subunits, a result that was not anticipated from previous studies of ilvG (the gene that codes for the large subunit of acetolactate synthase). These subunits were determined to be in equal molar ratio in the purified enzyme from the distribution of radioactivity between the two subunits after carboxymethylation with iodo[14C]acetate and their respective amino acid compositions. Besides the expected ilvG gene product (59.3 kDa), purified acetolactate synthase contained a smaller subunit (9.7 kDa; designated here as the ilvM gene product). On the basis of sequence homology of the small subunit with that coded for by the corresponding Escherichia coli gene sequence [Lawther, R. P., Calhoun, D. H., Adams, C. W., Hauser, C. A., Gray, J., & Hatfield, G. W. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 922-925], it is encoded by the region between ilvG and ilvE, beginning at base-pair (bp) 1914 (relative to the point of transcription initiation).(ABSTRACT TRUNCATED AT 250 WORDS)

pH profiles and isotope effects for aconitases from Saccharomycopsis lipolytica, beef heart, and beef liver. alpha-Methyl-cis-aconitate and threo-Ds-alpha-methylisocitrate as substrates

alpha-Methyl-cis-aconitate (cis-2-butene-1,2,3-tricarboxylate) was converted only to alpha-methylisocitrate (3-hydroxybutane-1,2,3-tricarboxylate) by aconitases from beef liver or S. lipolytica. While the kinetic parameters of beef liver (cytoplasmic) or heart (mitochondrial) aconitases did not vary over the pH range 4.9-9 with the natural substrates, and only slightly with the alpha-methyl substrates, the yeast aconitase exhibited a bell-shaped pH profile with all substrates and for binding of the competitive inhibitor, tricarballylate, with pK values around 7 and 9. The third pK of the substrates does not affect V/K, showing that these pK's are for catalytic groups on the enzyme. One of these catalytic groups presumably removes a proton to give the carbanion intermediate in the reaction, and the other protonates the hydroxyl group when it is eliminated to give water, possibly with the assistance of the Fe-S center. Beef liver aconitase showed a primary deuterium isotope effect of 1.12 (measured by equilibrium perturbation with deuterated alpha-methylisocitrate) which was pH independent and only slightly greater than the equilibrium isotope effect. Isotope effects with the yeast enzyme were also pH independent but about 1.22 on V/K (or when measured by equilibrium perturbation) and 1.7 on V. These data suggest a kinetic mechanism for beef aconitases in which product release occurs only by displacement by the substrate in a step independent of pH or of the protonation state of the substrate. With the yeast enzyme, product displacement either depends on the protonation state of the catalytic groups on the enzyme or can occur spontaneously at a finite rate.(ABSTRACT TRUNCATED AT 250 WORDS)

The sulfonylurea herbicide sulfometuron methyl is an extremely potent and selective inhibitor of acetolactate synthase in Salmonella typhimurium

The sulfonylurea herbicide sulfometuron methyl inhibits the growth of several bacterial species. In the presence of L-valine, sulfometuron methyl inhibits Salmonella typhimurium, this inhibition can be reversed by L-isoleucine. Reversal of growth retardation by L-isoleucine, accumulation of guanosine 5'-diphosphate 3'-diphosphate (magic spot), and relA mutant hypersensitivity suggest sulfometuron methyl interference with branched-chain amino acid biosynthesis. Growth inhibition of S. typhimurium is mediated by sulfometuron methyl's inhibition of acetolactate synthase, the first common enzyme in the branched-chain amino acid biosynthetic pathway. Sulfometuron methyl exhibits slow-binding inhibition of acetolactate synthase isozyme II from S. typhimurium with an initial Ki of 660 +/- 60 nM and a final, steady-state Ki of 65 +/- 25 nM. Inhibition of acetolactate synthase by sulfometuron methyl is substantially more rapid (10 times) in the presence of pyruvate with a maximal first-order rate constant for conversion from initial to final steady-state inhibition of 0.25 +/- 0.07 min-1 (minimal half-time of 2.8 min). Mutants of S. typhimurium able to grow in the presence of sulfometuron methyl were obtained. They have acetolactate synthase activity that is insensitive to sulfometuron methyl because of mutations in or near ilvG, the structural gene for acetolactate synthase isozyme II.

The stereochemical course of ribulosebisphosphate carboxylase. Reductive trapping of the 6-carbon reaction-intermediate

The carboxylation of ribulosebisphosphate involves a 6-carbon reaction-intermediate, 2-carboxy-3-keto-D-arabinitol 1,5-bisphosphate. This labile intermediate, radiolabeled with 14CO2, was reduced (and thereby stabilized) with NaB[3H4]. This yielded a doubly labeled mixture with the chromatographic properties of 2-carboxy- and 4-carboxy-D-arabinitol 1,5-bisphosphate. These products (as their respective gamma-lactones) were chromatographically resolved from the gamma-lactones of 2-carboxy-D-ribitol and 2-carboxy-D-xylitol 1,5-bisphosphate, the other possible reduction products to be considered. Reduction of the 6-carbon reaction-intermediate by NaBH4 required prior denaturation of the enzyme. Of various denaturants tested, only acid was rapid enough to give good yields of the carboxypentitol bisphosphates. The reduction products of the reaction-intermediate inhibited the carboxylase stoichiometrically. In contrast to 2-carboxyribitol and 2-carboxyxylitol bisphosphate, 2-carboxy- and 4-carboxyarabinitol bisphosphate are exceptionally potent, essentially irreversible, slow-binding inhibitors of ribulosebisphosphate carboxylase.

Relationship of the oxidation state of the iron-sulfur cluster of aconitase to activity and substrate binding

It is known that aconitase from mammalian mitochondria is only partially active as isolated but may be activated by incubation with iron, ascorbate, and a thiol, or with dithionite. It has been suggested that the added Fe in the activation mixture is essential for activation and that it is incorporated in the enzyme [Villafranca, J. J., & Mildvan, A. S. (1971) J. Biol. Chem. 246, 772-779; Gawron, O., Waheed, A., Glaid, A. J., & Jaklitsch, A. (1974) Biochem. J. 139, 709-714]. However, it is shown in this paper that, when the enzyme has a full complement of 3Fe and 3S, full activation is reached coulometrically, without iron or other chemical reducing agents. It is clear, therefore, that the role of activators is to reduce the iron--sulfur cluster of the enzyme. The appearance of catalytic activity on reduction of the cluster shows a pronounced lag, as does the decay of activity after reoxidizing the cluster. This suggests that catalytic activity requires a conformational change in the protein which is initiated by reduction of the cluster and that, following reoxidation, activity disappears only after the inactive conformation is assumed. Citrate and the competitive inhibitor trans-aconitate are bound to a comparable extent to the active and inactive forms, but only the active form can bind 1-hydroxy-2-nitro-1,3-propanedicarboxylic acid, a transition-state analogue. This is interpreted to show that in the inactive state aconitase cannot enter the conformation it assumes in the transition state during catalysis.

Isolation, characterization, and crystallization of ribulosebisphosphate carboxylase from autotrophically grown Rhodospirillum rubrum

Serial culture of Rhodospirillum rubrum with 2% CO2 in H2 as the exclusive carbon source resulted in a rather large fraction of the soluble protein (greater than 40%) being comprised of ribulosebisphosphate carboxylase (about sixfold higher than the highest value previously reported). Isolation of the enzyme from these cells revealed that it has physical and kinetic properties similar to those previously described for the enzyme derived from cells grown on butyrate. Notably, the small subunit (which is a constituent of the carboxylase from eucaryotes and most procaryotes) was absent in the enzyme from autotrophically grown R. rubrum. Edman degradation of the purified enzyme revealed that the NH2 terminus is free (in contrast to the catalytic subunit of the carboxylase from eucaryotes) and that the NH2-terminal sequence is Met-Asp-Gln-Ser-Ser-Arg-Tyr-Val-Asn-Leu-Ala-Leu-Lys-Glu-Glu-Asp-Leu-Ile-Ala-Gly-Gly-Glx-His-Val-Leu-. Crystals of the enzyme were readily obtained by dialysis against distilled water.

Inactivation of ribulosebisphosphate carboxylase by modification of arginyl residues with phenylglyoxal

Phenylglyoxal rapidly and completely inactivates spinach and Rhodospirillum rubrum ribulosebisphosphate carboxylases. Inactivation exhibits pseudo-first-order kinetics and a reaction order of approximately one for both enzymes, suggesting that modification of a single residue per protomeric unit suffices for inactivation. Loss of enzymic activity is directly proportional to incorporation of [14C]phenylglyoxal until only 30% of the initial activity remains. For both enzymes, extrapolation of incorporation to 100% inactivation yields 4-5 mol of [14C]phenylglyoxal per mol protomer. Amino acid analyses confirm the expected 2:1 stoichiometry between phenylglyoxal incorporation and arginyl modification and suggest that other kinds of amino acid residues are not modified. (Thus, inactivation correlates with modification of 2-3 arginyl residues per protomer). The substrate ribulose bis-phosphate and some competitive inhibitors reduce the rates of inactivation of the carboxylases and prevent modification of about 0.5-1.0 arginyl residue per protomer. Inactivation is therefore a consequence of modification of a small number of residues out of the 35 and 29 total arginyl residues per protomer in spinach and R. rubrum carboxylases, respectively.

Identification of essential lysyl and cysteinyl residues in spinach ribulosebisphosphate carboxylase/oxygenase modified by the affinity label N-bromoacetylethanolamine phosphate

We reported earlier (Schloss, J. V., and Hartman, F. C. (1977) Biochem. Biophys. Res. Commun. 77, 230-236) that N-bromoacetylethanolamine phosphate is an affinity label for spinach ribulosebisphosphate carboxylase/oxygenase. We now show inactivation to be correlated directly with the alkylation either of a single lysyl residue (in the presence of Mg2+) or of 2 different cysteinyl residues (in the absence of Mg2+), consistent with the likelihood that these residues are located in the active site region. This proposition is further supported by the demonstration that the residues are protected from alkylation by substrate, a competitive inhibitor, or the transition state analog 2-carboxyribitol bisphosphate. Tryptic peptides that contain the modified residues have been isolated and sequenced. One of the 2 cysteinyl residues that are subject to alkylation is only 3 residues distant in sequence from the lysyl residue modified by bromoacetylethanolamine phosphate. This lysyl residue is identical with 1 of the 2 lysyl residues alkylated by the previously described affinity label, 3-bromo-1,4-dihydroxy-2-butanone 1,4-bisphosphate (Stringer, C. D., and Hartman, F. C. (1978) Biochem. Biophys, Res. Commun. 80, 1043-1048).