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

Transaminases as suitable catalysts for the synthesis of enantiopure β,β-difluoroamines

Transaminases have shown the ability to catalyze the amination of a series of aliphatic and (hetero)aromatic α,α-difluorinated ketones with high stereoselectivity, thus providing the corresponding β,β-difluoroamines in high isolated yields (55-82%) and excellent enantiomeric excess (>99%). It was also observed that these activated substrates could be quantitatively transformed by employing a small molar excess of the amine donor since this amination process was thermodynamically favored. Selected transformations could be scaled up to 500 mg, showing the robustness of this methodology.

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.

Mechanism-based inactivation of gamma-aminobutyric acid aminotransferase by 3-amino-4-fluorobutanoic acid

The mechanism of inactivation of the pyridoxal 5'-phosphate (PLP)-dependent enzyme gamma-aminobutyric acid (GABA) aminotransferase by 3-amino-4-fluorobutanoic acid (2) has been investigated. As in the case of the homologue, 4-amino-5-fluoropentanoic acid (1), 2 equiv of radiolabeled inactivator become covalently attached to the enzyme, and no transamination, as determined by the lack of conversion of [1-14C] alpha-ketoglutarate into [1-14C] glutamate during inactivation, was observed. In the case of 1, the conclusion was that inactivation was completely the result of modification of the coenzyme and that there was no metabolic turnover; every enzyme molecule catalysed the conversion of one molecule of inactivator to the activated species, which inactivated the enzyme by an enamine mechanism. With 2, however, 6.7 +/- 0.7 equiv of fluoride ions were released during inactivation, and it took 7.6 +/- 0.7 inactivator molecules to inactivate each enzyme dimer. Since no transamination was occurring, another metabolic event besides inactivation must result from the PLP form of the enzyme. Inactivation of GABA amino-transferase with [1,2-14C]-2 produced [14C] acetoacetic acid (about 5.5 equiv) as the metabolite. The 1.93 +/- 0.25 equiv of radioactivity covalently bound to the enzyme after inactivation with [1,2-14C]-2 and gel filtration were completely released by base treatment. HPLC analysis showed that three radioactive compounds, identified as 2, the product of reaction of PLP with acetone (3), and the product of reaction of PLP with acetoacetate (4), were detected. The release of 3 and 4 and the prevention of release of radioactivity by treatment with sodium borohydride are consistent with the formation of covalent intermediates that have beta-carbonyl-like character, such as 6 and/or 7 (Scheme 2). Inactivation of [3H] PLP-reconstituted GABA aminotransferase with 2 followed by gel filtration then base denaturation released all of the radioactivity as a mixture of PLP, 3, and 4. Inactivation with [1,2-14C]-2 resulted in the release of 1.37 equiv of 14CO2, which was shown to be the result of decarboxylation of the acetoacetate/4 after release from the enzyme. These results are not consistent with a Michael addition mechanism (Scheme 3), but are consistent with inactivation by an enamine mechanism; release of the enamine five out of seven turnovers accounts for the formation of acetoacetate as the metabolite. To account for the detection of PLP and 2 after denaturation, it is suggested that a nonproductive formation of the Schiff base of PLP with 2 occurs in the second subunit of the enzyme; this complex is released and hydrolysed to PLP and 2 upon base denaturation.

The potential use of mechanism-based enzyme inactivators in medicine

Mechanism-based enzyme inactivator, alanine racemase, S-adenosylhomocysteine hydrolase, D-amino acid aminotransferase, gamma-aminobutyric acid aminotransferase, arginine decarboxylase, aromatase, L-aromatic amino acid decarboxylase, dihydrofolate reductase, dihydroorotate dehydrogenase DNA polymerase I, dopamine beta-hydroxylase, histidine decarboxylase, beta-lactamase, monoamine oxidase, ornithine decarboxylase, serine proteases, testosterone 5 alpha-reductase, thymidylate synthetase, xanthine oxidase.