English abstract
y-Aminobutyric acid transaminase (GABA-T, EC 2.6.1.19) is a pyridoxal-5 phosphate (PLP) dependent enzyme that catalyzes the degradation of y-aminobutyric acid. The kinetics of this reaction are studied in vitro, both in the absence, and in the presence of two inhibitors: y-vinyl GABA (4-aminohex-5-enoic acid), and taurine. The former is a prescription pharmaceutical used by patients suffering from epile and schizophrenia, whereas, the latter is a natural product.
A kinetic model that is based on a theoretically derived rate equation is proposed. This model describes the kinetics of the transamination process, accounting for the competitive inhibition of y-vinyl GABA and taurine with respect to GABA, and for the uncompetitive inhibition of taurine with respect to a ketoglutarate.
The steady-state kinetic mechanism of GABA-T from Pseudomonas fluorecens obeys a ping-pong bi bi reaction mechanism. In absence of inhibition, at pH 8.6 and a temperature of 25 °C, Vimax was found to be 1.11 x 10° Ms', whereas, Km was found to be 1.28 + 0.1mM, and 1.63 $ 0.1mM for GABA and a ketoglutarate respectively.
The consequences of the GABA-T inhibition on the primary and secondary plots of the initial velocity against the substrates concentrations are examined. The difference in the inhibition mechanism and power of taurine with respect to y-vinyl GABA is also assessed. Y-Vinyl GABA and taurine are competitive inhibitors with respect to GABA. The respective Ki values were found to equal 26.2 + 3mM and 56.9 † 7mM. Taurine, on the other hand, was found to be an uncompetitive inhibitor of GABA-T with respect to a-ketoglutarate, with a Kii value of 213 + 10mM", whereas y-vinyl GABA showed no effect on the transamination ofa ketoglutarate with GABA-T
