Active-site-directed irreversible inhibition of rat brain 4-aminobutyrate aminotransferase by ethanolamine O-sulphate in vitro and in vivo

Abstract

1. Partially purified preparations of rat brain 4-aminobutyrate aminotransferase were inhibited in a time-dependent manner by ethanolamine O-sulphate. The inhibition was not reversed by dialysis. 2. The inhibitor formed an initial reversible complex with the enzyme (K(i)=4.4x10(-4)m) and the rate of inactivation followed pseudo-first-order kinetics (k=7.15x10(-4)s(-1)). The inclusion of 4-aminobutyrate markedly slowed the rate of inactivation. 3. Ethanolamine O-sulphate did not inhibit glutamate decarboxylase, alanine aminotransferase or aspartate aminotransferase. 4. Intracisternal injection of ethanolamine O-sulphate into rats led to rapid inactivation of 4-aminobutyrate aminotransferase in vivo.

Structures of γ-Aminobutyric Acid (GABA) Aminotransferase, a Pyridoxal 5′-Phosphate, and [2Fe-2S] Cluster-containing Enzyme, Complexed with γ-Ethynyl-GABA and with the Antiepilepsy Drug Vigabatrin

“…The drug is sold as a racemic mixture, and it is not known how many of the side effects arise from the administration of the inactive enantiomer (R-isomer). Other mechanism-based inactivators of GABA-AT, such as gabaculine (30,31) and ethanolamine-O-sulfate (32), are generally toxic, nonspecific, and/or cross the blood-brain barrier poorly.…”

Suicide Reaction of Bacterial ω-Amino Acid Transaminaseswith EthanolamineO-Sulfate

“…In the case of mammalian GABA-T an active p-aminoethylene was suggested for the possible intermediate, which was formed by a p-elimination of the inhibitor catalyzed by the enzyme. 3 ) The results presented here, together· with that obtained with mammalian GABA-T, also suggest a common structure of the active site or a catalytic mechanism of w-amino acid transaminases.…”

A Regional Study of 4‐Aminobutyrate Metabolism and Amino Acid Levels in Rat Brain Following Chronic Oral Administration of Ethanolamine O‐Sulphate

“…The formation of aspartate from oxaloacetate would therefore be expected to decline. Direct inhibition of aspartate aminotransferase seems unlikely in view of the lack of effect of high concentrations of EOS on the purified enzyme (Fowler and John, 1972). The decrease in GAD activity in EOS-treated animals may also contribute to changes in amino acid metabolism.…”

“…The possible clinical utility of GABA-T inhibitors in several neurological disorders has frequently been proposed (for review see Palfreyman e t al., 1981). The first specific GABA-T inhibitor to be described was ethanolamine O-sulphate (EOS; Fowler and John, 1972), an active-site-directed irreversible inhibitor of GABA-T. Other inhibitors employing a similar mechanism of action have since been developed: y-acetylenic GABA (Jung and Metcalf, 1975; Jung et al, 1977a), y-vinyl GABA ; Jung et al, 1977h), and gabaculine (Matsui and Deguchi, 1977; Rando and Bangerter, 1977). However, not all of these inhibitors appear to possess the required degree of biochemical specificity: yacetylenic GABA also inhibits L-glutamate decarboxylase (GAD, EC4.1.1.15; Junget al, 1977~) and transaminases other than GABA-T (John et al, 1979); a n d in v i v o c a n produce convulsions.…”