(3R,4S),(3S,4R) and (3R,4R),(3S,4S)-4-amino-5-fluoro-3-phenyl pentanoic acids as mechanism-based inactivators of 4-aminobutyric acid aminotransferas

4-Aminobutyric acid (GABA) is a known inhibitory neurotransmiter of the mammalian central nervous system. Its metabolism is controlled by two pyridoxal phosphate dependent enzymes. GABA is biosynthesized from L-glutamate by means of the enzyme L-glutamate-1-caroboxylase (GAD), and is degraded to succinic semialdehyde by the enzyme 4-aminobutyric acid: 2-oxoglutarate aminotransferase (GABA-T). Decreased brain GABA levels have been observed in convulsive states. These convulsions cease upon GABA level increase to normal values. A deficiency of GABA has also been implicated in several disorders such as Huntington's disease, epilepsy, and Parkinsonism. Since GABA does not readily penetrate the blood brain barrier, it cannot be directly administered to raise the brain concentration of GABA. Inhibition of GABA-T has been employed as an alternative method of elevating GABA levels in the brain.;Several potent mechanism-based inactivators of GABA-T have been described which do, indeed, increase whole brain GABA levels in vivo. However, some of them are not specific for GABA-T and inhibit GAD too. (3R,4S),(3S,4R) and (3R,4R),(3S,4S)-4-Amino-5-fluoro-3-phenylpentanoic acids were designed and studied as potential specific inactivators of GABA-T. These compounds were synthesized starting from ethyl cinnamate and diethylacetamidomalonate in a total of seven steps each and were found to be mechanism-based, irreversible inactivators of purified pig-brain GABA-T. They were also found not to inhibit partially purified pig-brain GAD. In comparison with 4-amino-5-fluoropentanoic acid, a known irreversible activator of GABA-T, which also is a potent reversible inhibitor of GAD, the addition of a phenyl group at carbon 3 of the GABA backbone decreased the ability of the molecule to bind to the active site of GABA-T. This modification also led to a decrease in the rate of inactivation of GABA-T. However, these compounds had increased specificity for the target enzyme because of their lack of inhibition of GAD, the other PLP dependent enzyme involved in the GABA metabolic pathway.