Transaminations catalysed by brain glutamate decarboxylase

Abstract

In addition to normal decarboxylation of glutamate to 4-aminobutyrate, glutamate decarboxylase from pig brain was shown to catalyse decarboxylation-dependent transamination of L-glutamate and direct transamination of 4-aminobutyrate with pyridoxal 5'-phosphate to yield succinic semialdehyde and pyridoxamine 5'-phosphate in a 1:1 stoichiometric ratio. Both reactions result in conversion of holoenzyme into apoenzyme. With glutamate as substrate the rates of transamination differed markedly among the three forms of the enzyme (0.008, 0.012 and 0.029% of the rate of 4-aminobutyrate production by the alpha-, beta- and gamma-forms at pH 7.2) and accounted for the differences among the forms in rates of inactivation by glutamate and 4-aminobutyrate. Rates of transamination were maximal at about pH 8 and varied in parallel with the rate constants for inactivation from pH 6.5 to 8.0. Rates of transamination of glutamate and 4-aminobutyrate were similar, suggesting that the decarboxylation step is not entirely rate-limiting in the normal mechanism. The transamination was reversible, and apoenzyme could be reconstituted to holoenzyme by reverse transamination with succinic semialdehyde and pyridoxamine 5'-phosphate. As a major route of apoenzyme formation, the transamination reaction appears to be physiologically significant and could account for the high proportion of apoenzyme in brain.

Retinal Ganglion Cell Depletion Alters the Phenotypic Expression of GABA and GAD in the Rat Retina

“…GAD-65, as apoGAD, is the main reservoir of inactive GAD and could be readily mobilized should additional GABA synthesis be needed. Regulation of GAD-67 is more complex and also depends on intracellular GABA levels (Porter and Martin, 1984;Porter et al, 1985;Lid& et al, 1987;Rimvall and Martin, 1992;Martin and Rimvall, 1993). This differential regulation could partially explain our findings.…”

Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis

“…Another well‐described mechanism by which GAD is regulated involves its co‐factor, pyridoxal‐5′‐phosphate (pyridoxal‐P; Porter et al . 1985; Spink et al .…”

“…The secondary reaction produces apoGAD which can be activated to holoGAD by reaction with pyridoxal‐P. The primary reaction is more than 1000 times more frequent than the secondary reaction (Porter et al . 1985).…”

“…Another well-described mechanism by which GAD is regulated involves its co-factor, pyridoxal-5¢-phosphate (pyridoxal-P; Porter et al 1985;Spink et al 1985;Martin 1987). A large fraction of GAD is present in brain as inactive apoenzyme (GAD without bound pyridoxal-P), a characteristic that is unusual, if not unique, among pyridoxal-P-dependent enzymes (Miller et al 1977;Martin et al 1991).…”

Identification of succinic semialdehyde reductases from <italic>Geobacter</italic>: expression, purification, crystallization, preliminary functional, and crystallographic analysis

“…GABA is metabolized through the pathway of 'GABA shunt' after being synthesized from glutamic acid by glutamic acid decarboxylase [5,6]. The GABA transaminase converts GABA to succinic semialdehyde (SSA), and the SSA is further processed to succinic acid by succinic semialdehyde dehydrogenase (SSADH) [7,8]. Succinic acid is then the final form of GABA to enter the tricarboxylic acid cycle.…”