Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:2.6.1.19 (GABA transaminase)
 808 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

(3-Hydroxybenzyl)hydrazine and methylhydrazine have been found to be potent slow-binding inhibitors of the pyridoxal 5-phosphate (PLP)-dependent enzyme gamma-aminobutyric acid aminotransferase (GABA-AT). Both compounds follow mechanism A (Morrison, J.F.; Walsh, C. T. Adv. Enzymol. 1988, 61, 201-301) which does not involve formation of a rapidly reversible enzyme-inhibitor complex before the formation of the final tight complex. The rate constant for formation of the enzyme-inhibitor complex determined from the slow-binding kinetics was 2.08 x 10(3) and 1.98 x 10(4) M-1 min-1 for methylhydrazine and (3-hydroxybenzyl)hydrazine, respectively. The rate constant for dissociation of the enzyme--inhibitor complex determined for the slow-binding kinetics was 4.6 x 10(-3) and 5 x 10(-3) min-1, respectively. The inhibition constants calculated from the slow-binding inhibition kinetics are 2.2 microM for methylhydrazine and 0.3 microM for (3-hydroxybenzyl)hydrazine. Reactivation of the inhibited enzyme was not first order, perhaps due to a side reaction of the hydrazine, but was consistent with the results obtained from the slow-binding kinetics. Inhibition constants were calculated from the level of enzyme activity at equilibrium inhibition. These constants are 2.8 and 0.46 microM for methylhydrazine and (3-hydroxybenzyl)hydrazine, respectively, in good agreement with those calculated from the slow-binding inhibition kinetics. 3-Hydrazinopropionate also behaved as a slow-binding inhibitor. However, the dependence of its kinetics on the concentration of inhibitor could not be described by the slow-binding or slow, tight-binding inhibition models. These kinetics could not be described by the tight-binding character of the inhibition because the addition of the competitive inhibitor propionic acid at 100 times its Ki did not affect the shape of the curve for inhibitor concentration dependence. The slow-binding inhibition appeared to require 2-4 molecules of 3-hydrazinopropionate/enzyme. The reactivation of enzyme inhibited by 3-hydrazinopropionate was first order with a rate constant of 6.9 x 10(-3) min-1. Its equilibrium inhibition constant was calculated to be < 20 nM. However, the inhibition constant calculated was dependent on the concentration of inhibitor because of the unusual character discussed above and may be much lower. Only 1 PLP/enzyme dimer reacted with methylhydrazine or (3-hydroxybenzyl)hydrazine, as indicated by Scatchard plots, or with 3-hydrazinopropionate, as shown by a spectrophotometric titration. Slow-binding inhibition does not appear to be the result of a significant enzyme conformational change because there is no change in the tryptophan fluorescence of GABA-AT upon binding either methylhydrazine or 3-hydrazinopropionate. Implications for the design of hydrazine inhibitors of GABA-AT are discussed.
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PMID:Slow-binding inhibition of gamma-aminobutyric acid aminotransferase by hydrazine analogues. 857 11

1. The modulatory effects of L-glutamate and its structural analogues, and of gamma-aminobutyric acid (GABA), on sympathetic co-transmission were studied in the rat isolated vas deferens exposed to electrical field stimulation (EFS). 2. Application of exogenous L-glutamate caused a concentration-dependent (1 microM-3 mM) inhibition of the rapid twitch component of the biphasic EFS contraction. However, L-glutamate (1 microM-3 mM) had a minimal effect on the phasic contraction induced by exogenous adenosine 5'-triphosphate (ATP, 150 microM) and noradrenaline (50 microM). Unlike L-glutamate, D-glutamate had no effect on the EFS contraction. 3. The L-glutamate-induced inhibition of the EFS contractions was significantly attenuated by the glutamate decarboxylase (GAD) inhibitor 3-mercapto-propionic acid (150 microM) and was abolished in the presence of the GABA transaminase (GABA-T) inhibitor, 2-aminoethyl hydrogen sulphate (500 microM). 4. The L-glutamate-induced inhibition of the electrically evoked contraction was not affected by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)(30 nM), reactive blue 2 (30 microM) or the GABAA receptor antagonist bicuculline (50 microM). However, the GABAB receptor antagonist 2-hydroxysaclofen (50 microM) significantly inhibited the L-glutamate effect. 5. Similar to L-glutamate, GABA also caused a concentration-dependent (0.1-100 microM) inhibition of the EFS contractions. This GABA-induced inhibition was not affected by either the GABAA receptor antagonist bicuculline (50 microM) or reactive blue 2 (30 microM). However, a significant attenuation of the GABA-mediated effect was recorded with the GABAB receptor antagonist 2-hydroxysaclofen (50 microM). Contractions of the vas deferens induced by exogenous ATP and noradrenaline were not affected by GABA (0.1-100 microM). 6. The L-glutamate analogues, N-methyl-D-aspartate (NMDA) (1 microM-1 mM) and quisqualate (Quis 0.1 microM-0.3 mM) had no effect, whilst kainate (Kain, 1 microM-1 mM) caused an inhibition of the EFS-induced contractions. Effects of Kain could be abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX, 10 microM). NMDA, Quis and Kain had no effect on the exogenous ATP- or noradrenaline-induced contractions. 7. It is concluded that the excitatory amino acid L-glutamate modulates the electrically evoked vas deferens contraction through conversion to the inhibitory amino acid GABA by a specific GABA transaminase. The GABA formed may then act on GABAB receptors and cause inhibition of the contraction through a presynaptic mechanism.
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PMID:Presynaptic modulation by L-glutamate and GABA of sympathetic co-transmission in rat isolated vas deferens. 876 4