An improved method for the preparation of [14C]pyridoxal 5'-phosphate

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.

Pulsed electron paramagnetic resonance studies of the lysine 2,3-aminomutase substrate radical: evidence for participation of pyridoxal 5'-phosphate in a radical rearrangement

The role of pyridoxal 5'-phosphate (PLP) in the radical-mediated amino group migration catalyzed by lysine 2,3-aminomutase from Clostridia SB4 has been investigated by electron spin echo envelope modulation (ESEEM) spectroscopy. This pulsed electron paramagnetic resonance (EPR) method was used to estimate the distance between the unpaired electron in the alpha-radical of beta-lysine, a steady-state intermediate in the reaction, and deuterium at the C4' position of the cofactor, PLP. [4'-2H]PLP was synthesized and exchanged into the enzyme. The steady-state radical was generated in the labeled samples and in samples with unlabeled PLP by addition of L-lysine.H2SO4 to activated enzyme. ESEEM spectra of the samples prepared with [4'-2H]PLP exhibited distinctive low-frequency modulations that were not present in spectra of matched samples with unlabeled PLP. Fourier transformation of the modulations yielded a prominent doublet signal centered about the Larmor frequency of deuterium. The magnitude of the doublet splitting of the 2H ESEEM signal exhibited angle selection across the CW EPR powder pattern. The observed angle selection, as well as simulation of the time domain spectra, indicated that the doublet splitting was due to the combined effects of the 2H hyperfine and nuclear quadrupole interactions. The influences of the quadrupole interaction and of isotropic and dipolar hyperfine interactions were explored by simulations of the ESEEM spectra. The analysis indicates a distance of < 3.5 A between the 2H at C4' of PLP and the radical center at C alpha lysine. The data are most compatible with an aldimine linkage between PLP and the beta-nitrogen of beta-lysine.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of inactivation of gamma-aminobutyrate aminotransferase by 4-amino-5-fluoropentanoic acid. First example of an enamine mechanism for a gamma-amino acid with a partition ratio of 0

The mechanism of inactivation of pig brain gamma-aminobutyric 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 21, 4377-4386] for the inactivation of glutamate decarboxylase by serine O-sulfate (Scheme I, pathway b, R = COOH, X = OSO3-).

An enzymatic method for the synthesis of [14C]pyridoxal 5'-phosphate from [14C]pyridoxine

A new enzymatic method for the synthesis of [14C]pyridoxal 5'-phosphate is presented. [14C]Pyridoxal 5'-phosphate was synthesized from [14C]pyridoxine through the successive actions of pyridoxal kinase and pyridoxamine 5'-phosphate oxidase in a reaction mixture containing ATP, [14C]pyridoxine, and both enzymes. [14C]Pyridoxal 5'-phosphate was isolated by omega-aminohexyl-Sepharose 6B column chromatography. The overall yield of the product was more than 60%, starting from 550 nmol of [14C]pyridoxine. The radiochemical purity of the products, as determined by thin-layer and ion-exchange chromatography, was greater than 98%.