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At least 217 records · Page 12Linked to original sources
PubMed1978

Mechanism of the stereospectific irreversible inhibition of bacterial glutamic acid decarboxylase by (R)-(--)-4-aminohex-5-ynoic acid, an analogue of 4-aminobutyric acid.

4-Aminohex-5-ynoic acid inhibits bacterial glutamic acid decarboxylase in a time-dependent irreversible manner. The inhibition is stereospecific and requires the abstraction of the propargylic hydrogen from 4(R)-(--)-4-aminohex-5-ynoic acid. This leads to the generation of a reactive alkylating agent in the active site which can react with a nucleophilic residue. At complete inhibition, there is incorporation of one molecule of inhibitor per pyridoxal binding site. If the decarboxylation of glutamate occurs with retention of configuration, the irreversible inhibition of this enzyme by the 4-(R) isomer can be rationalized on the basis of reversibility of the protonation step in the normal catalytic mechanism.

Aminocaproates↗
PubMed1992

Evidence for a modulation of human chorionic gonadotropin (hCG) subunit messenger ribonucleic acid levels and hCG secretion by gamma-aminobutyric acid in human first trimester placenta in vitro.

The cytotrophoblasts are the site of production of liberins and statins in human placenta, whereas the syncytiotrophoblasts synthesize tropic hormones. These placental cell layers seem to interact like the hypothalamus and pituitary. In the central nervous system, gamma-aminobutyric acid (GABA)-ergic neurons represent one important control mechanism that seems to influence the lutropin biosynthesis indirectly (via gonadoliberin) and directly. It was the objective of the present study to find out whether GABA also may influence the biosynthesis and secretion of hCG by human first trimester placenta. Already one single pulse of GABA (1 h; 0.01-100 microM) stimulated hCG secretion significantly (P less than 0.0001). GABA also induced a marked increase in the mRNA levels of both subunits, with an optimum at 10 microM. The effect on hCG secretion was mimicked by the GABA-A receptor agonist muscimol (P less than 0.002), but under the experimental conditions used (multiple pulses; 1 microM), only the beta mRNA was increased. The GABA-A receptor antagonist bicuculline (two pulses; 10 microM) suppressed basal hCG secretion (P less than 0.001) and abolished the episodic secretion pattern observed in the control cultures. Applying a combination of equimolar amounts of GABA and bicuculline, hCG secretion and the episodic secretion pattern were similar as in control cultures. The data seem to suggest a regulation of hCG biosynthesis in human first trimester placenta in which GABA is involved, probably acting via GABA-A-like receptor sites.

Bicuculline↗
PubMed1995

Imidazoleacetic acid, a gamma-aminobutyric acid receptor agonist, can be formed in rat brain by oxidation of histamine.

It is generally accepted that in mammalian brain histamine is metabolized solely by histamine methyltransferase (HMT), to form tele-methylhistamine, then oxidized to tele-methylimidazoleacetic acid. However, histamine's oxidative metabolite in the periphery, imidazoleacetic acid (IAA), is also present in brain and CSF, and its levels in brain increase after inhibition of HMT. To reinvestigate if brain has the capacity to oxidize histamine and form IAA, conscious rats were injected with [3H]histamine (10 ng), either into the lateral ventricles or cisterna magna, and decapitated 30 min later. In brains of saline-treated rats, most radioactivity recovered was due to tele-methylhistamine and tele-methylimidazoleacetic acid. However, significant amounts of tritiated IAA and its metabolites, IAA-ribotide and IAA-riboside, were consistently recovered. In rats pretreated with metoprine, an inhibitor of HMT, labeled IAA and its metabolites usually comprised the majority of histamine's tritiated metabolites. [3H]Histamine given intracisternally produced only trace amounts of oxidative metabolites. Formation of IAA, a potent GABA-A agonist with numerous neurochemical and behavioral effects, from minute quantities of histamine in brain indicates a need for reevaluation of histamine's metabolic pathway or pathways in brain and suggests a novel mechanism for interactions between histamine and the GABAergic system.

Animals↗
PubMed1993

Divergent alterations in gamma-aminobutyric acid responses of male and ovariectomized rats after chronic benzodiazepine agonist exposure: analysis of gamma-aminobutyric acid-activated chloride influx.

Gonadal status in rats modulates the development of tolerance to the anticonvulsant effects of the benzodiazepines and the concomitant changes in cortical gamma-aminobutyric acid (GABA)A receptors after chronic benzodiazepine agonist exposure. The present study analyzed physiological GABA responsiveness after chronic benzodiazepine exposure by measuring GABA-activation of 36chloride influx into cortical and cerebellar microsacs. GABA-stimulated 36chloride influx was compared in groups of male and ovariectomized female rats after acute (2-3 day) or chronic (3 week) exposure to diazepam-filled or empty silastic implants. Chronic diazepam exposure increased cortical GABA-activated 36chloride influx in ovariectomized rats, but did not influence GABA responses in males. Acute exposure to diazepam did not alter cortical 36chloride influx in either hormone group. Vehicle-treated ovariectomized rats also had lower levels of cortical GABA-activated 36chloride influx than vehicle-treated males. In cerebellar microsacs, diazepam exposure enhanced GABA-induced 36chloride influx. This effect was observed after both acute and chronic treatments and hormone groups did not differ in their response to chronic benzodiazepine exposure. Enhancement of GABA-stimulated 36chloride influx by the benzodiazepine agonist midazolam was not altered in male or ovariectomized rats after chronic diazepam treatment. Thus, gonad-related factors influence the changes in GABAA receptors and the resulting GABA responses in the cortex, but not the cerebellum, observed after chronic benzodiazepine agonist exposure in rats. These regionally specific divergent alterations in GABAergic systems may be related to the differential development of tolerance to the anticonvulsant effects of the benzodiazepines observed in these hormone groups.

Animals↗
PubMed1999

Syntheses of (Z)-and (E)-4-amino-2-(trifluoromethyl)-2-butenoic acid and their inactivation of gamma-aminobutyric acid aminotransferase.

(Z)- and (E)-4-amino-2-(trifluoromethyl)-2-butenoic acid (4 and 5, respectively) were synthesized and investigated as potential mechanism-based inactivators of gamma-aminobutyric acid aminotransferase (GABA-AT) in a continuing effort to map the active site of this enzyme. The core alpha-trifluoromethyl-alpha,beta-unsaturated ester moiety was prepared via a Reformatsky/reductive elimination coupling of the key intermediates tert-butyl 2,2-dichloro-3,3,3-trifluoropropionate and N,N-bis(tert-butoxy-carbonyl)glycinal. Both 4 and 5 inhibited GABA-AT in a time-dependent manner, but displayed non-pseudo-first-order inactivation kinetics; initially, the inactivation rate increased with time. Further investigation demonstrated that the actual inactivator is generated enzymatically from 4 or 5. This inactivating species is released from the active site prior to inactivation, and as a result, 4 and 5 cannot be defined as mechanism-based inactivators. Furthermore, 4 and 5 are alternate substrates for GABA-AT, transaminated by the enzyme with Km values of 0.74 and 20.5 mM, respectively. Transamination occurs approximately 276 and 305 times per inactivation event for 4 and 5, respectively. The enzyme also catalyzes the elimination of the fluoride ion from 4 and 5. A mechanism to account for these observations is proposed.

4-Aminobutyrate Transaminase↗
PubMed1982

Properties of gamma-aminobutyric acid synthesis by rat renal cortex.

Substantial synthesis of gamma-aminobutyric acid occurs in rat renal cortex. Renal glutamate decarboxylase activity (24.3 +/- 2.9 (S.E.) nmols/mg protein per h) is 15% of that in brain; renal gamma-aminobutyric acid content (39.5 +/- 5.3 (S.E.) nmols/g wet wt.) is 5% of the whole brain concentration. Properties of glutamate decarboxylase were studied in homogenates of rat renal cortex and rat brain under conditions for which gamma-aminobutyric acid formation from [2,3-3H]glutamate and CO2 release from [1(-14)C]glutamate were equal. Several properties of renal glutamate decarboxylase distinguish it from the corresponding brain enzyme: (1) renal glutamate decarboxylase is selectively inhibited by cysteine sulfinic acid (Ki = 5X10(-5) M); (2) renal glutamate decarboxylase is less sensitive (Ki = 3-5X10(-5) M) to inhibition by aminooxyacetic acid than is the brain enzyme (Ki = 1X10(-6) M); (3) brain but not renal glutamate decarboxylase activity can be substantially stimulated in vitro by the addition of exogenous pyridoxal 5'-phosphate; (4) renal glutamate decarboxylase is significantly decreased in renal cortex from rats on a low-salt diet. Proximal tubules are enriched in glutamate decarboxylase compared to the activity in whole renal cortex or glomeruli (42, 22 and 14 nmols/mg protein per h, respectively). We speculate that renal gamma-aminobutyric acid synthesis does not reflect the presence of GABAergic renal nerves, but may serve a function in proximal tubular cells.

Animals↗
PubMed1986

4-Amino-2-(substituted methyl)-2-butenoic acids: substrates and potent inhibitors of gamma-aminobutyric acid aminotransferase.

4-Amino-2-(substituted methyl)-2-butenoic acids, where X (the substituted group) = F, Cl, OH, are synthesized from Cbz-protected tert-butyl 4-aminobutanoate. Successive substitutions at the alpha-carbon by phenylseleno and hydroxymethyl groups, followed by elimination of the selenoxide and halide substitution at the hydroxymethyl group, afford the compounds in good yields. An unexpected degree of stereoselectivity is observed in the selenoxide elimination step, which yields the desired E isomer as the sole product. These compounds complement two previously reported series of compounds (Silverman, R. B.; Levy, M. A. Biochem. Biophys. Res. Commun. 1980, 95, 250-255; J. Biol. Chem. 1981, 256, 11 565-11 568) and are used in an approach to map a section of the active site of gamma-aminobutyric acid aminotransferase (GABA-T). None of these compounds is a time-dependent inactivator of GABA-T, but all are potent competitive reversible inhibitors; the hydroxy compound has a Ki value of 5 microM. That these compounds are not inactivators suggests that either elimination of X does not occur or that there is no active site nucleophile in the appropriate position for reaction following elimination. With use of the fluoro analogue, enzyme-catalyzed fluoride ion release is demonstrated, indicating that elimination does occur. Unlike the previous two series of compounds (op. cit.) in which exclusive elimination occurs when the substituent is a halogen but exclusive transamination prevails for the hydroxyl-substituted analogues, in the series described here, the fluoro analogue gives a 4:1 ratio of elimination to transamination. This suggests that the 2,3-double bond stabilizes the product of azallylic isomerization of the Schiff base between the fluoro compound and pyridoxal phosphate. The results described here indicate that the design of a mechanism-based inactivator for GABA-T should not be based on electrophile generation near the 2-position of enzyme-bound GABA. Furthermore, substitution of an inhibitor with a 2-hydroxymethyl group (or other hydrogen-bonding substituent) and a 2,3-double bond may lend auspicious binding properties to the molecule for GABA-T.

4-Aminobutyrate Transaminase↗
PubMed1985

Antisera to gamma-aminobutyric acid. I. Production and characterization using a new model system.

Antisera to the amino acid gamma-aminobutyric acid (GABA) have been developed with the aim of immunohistochemical visualization of neurons that use it as a neurotransmitter. GABA bound to bovine serum albumin was the immunogen. The reactivities of the sera to GABA and a variety of structurally related compounds were tested by coupling these compounds to nitrocellulose paper activated with polylysine and glutaraldehyde and incubating the paper with the unlabeled antibody enzyme method, thus simulating immunohistochemistry of tissue sections. The antisera did not react with L-glutamate, L-aspartate, D-aspartate, glycine, taurine, L-glutamine, L-lysine, L-threonine, L-alanine, alpha-aminobutyrate, beta-aminobutyrate, putrescine, or delta-aminolevulinate. There was cross-reaction with gamma-amino-beta-hydroxybutyrate, 1-10%, and the homologues of GABA: beta-alanine, 1-10%, delta-aminovalerate, approximately 10%, and epsilon-amino-caproate, approximately 10%. The antisera reacted slightly with the dipeptide gamma-aminobutyrylleucine, but not carnosine or homocarnosine. Immunostaining of GABA was completely abolished by adsorption of the sera to GABA coupled to polyacrylamide beads by glutaraldehyde. The immunohistochemical model is simple, amino acids and peptides are bound in the same way as in aldehyde-fixed tissue and, in contrast to radioimmunoassay, it uses an immunohistochemical detection system. This method has enabled us to define the high specificity of anti-GABA sera and to use them in some novel ways. The model should prove useful in assessing the specificity of other antisera.

Adsorption↗