Mechanism of Inactivation of γ-Aminobutyrate Aminotransferase by 4-Amino-5-fluoropentanoic Acid. First Example of an Enamine Mechanism for a γ-Amino Acid with a Partition Ratio of 0

The mechanism of inactivation of pig brain γ-aminobutvric acid aminotransferase (GABA-T) by (S)-4-amino-5-fluoropentanoic acid (1, R = CH₂CH₂COOH, X = F) previously proposed [Silverman, R. B., & Levy, M. A. (1981) Biochemistry 20, 1197–1203] is revised. apo-GABA-T is reconstituted with [4-³H]pyridoxal 5'-phosphate and inactivated with 1 (R = CH₂CH₂COOH, X = F). Treatment of inactivated enzyme with base followed by acid denaturation leads to the complete release of radioactivity as 6-[2-hydroxy-3-methyl-6-(phosphonoxymethyl)-4-pyridinyl]-4-oxo-5-hexenoic acid (4, R = CH₂CH₂COOH). Alkaline phosphatase treatment of this compound produces dephosphorylated 4 (R = CH₂CH₂COOH). These results support a mechanism that was suggested by Metzler and co-workers [Likos, J. J., Ueno, H., Feldhaus, R. W., & Metzler, D. E. (1982) Biochemistry 27, 4377–4386] for the inactivation of glutamate decarboxylase by serine O-sulfate (Scheme I, pathway b, R = COOH, X = OSO₃ ^-).