Isolation and characterization of endogenous modulators for GABA system

Neurotoxic effect of acamprosate, n-acetyl-homotaurine, in cultured neurons

Acamprosate (AC), N-acetyl-homotaurine, has recently been introduced for treating alcohol craving and reducing relapses in weaned alcoholics. AC may exert its action through the taurine system rather than the glutamatergic or GABAergic system. This conclusion is based on the observations that AC strongly inhibits the binding of taurine to taurine receptors while it has little effect on the binding of glutamate to glutamate receptors or muscimol to GABA(A) receptors. In addition, AC was found to be neurotoxic, at least in neuronal cultures, triggering neuronal damage at 1 mM. The underlying mechanism of AC-induced neuronal injury appears to be due to its action in increasing the intracellular calcium level, [Ca2+](i). Both AC-induced neurotoxicity and elevation of [Ca2+](i) can be prevented by taurine suggesting that AC may exert its effect through its antagonistic interaction with taurine receptors.

Identification of inosine as an endogenous modulator for the benzodiazepine binding site of the GABAA receptors

Previously we have reported the presence of endogenous ligands that are involved in the regulation of the binding of muscimol to the GABA binding site of the GABAA receptors. Here, we report the presence of multiple forms of endogenous ligands in the brain which modulate the binding of flunitrazepam (FNZP) to the benzodiazepine (BZ) binding site of the GABAA receptor. Furthermore, one of the endogenous ligands for the BZ receptors, referred to as EBZ, has been identified as inosine based on the following observations: (1) standard inosine and the EBZ have identical NMR and UV spectra; (2) the elution profile of inosine and the EBZ from a HPLC column are indistinguishable, and (3) inosine and the EBZ show identical activity in inhibiting [3H]FNZP binding.

Multiplicity of Brain Cysteine Sulfinic Acid Decarboxylase - Purification, Characterization and Subunit Structure

Cysteine sulfinic acid decarboxylase (CSAD), the rate-limiting enzyme in taurine biosynthesis, appears to be present in the brain in multiple isoforms. Two distinct forms of CSAD, referred to as CSAD I and CSAD II, were obtained on Sephadex G-100 column. CSAD I and CSAD II differ in: (1) the elution profile on Sephadex G-100 column; (2) the sensitivity towards Mn(2+), methione, and other sulfur-containing amino acids, and (3) their immunologic properties. CSAD II has been purified to about 2,500-fold by a combination of column chromatographies and polyacrylamide gel electrophoresis (PAGE). The purity of the enzyme preparation was established as judged from the following observations: (1) a single protein band was observed under various electrophoretic conditions, e.g., 5-20% nondenaturing PAGE, 7% nondenaturing PAGE and 10% SDS-PAGE, and (2) in nondenaturing PAGE, the protein band comigrated with CSAD activity. CSAD II has a molecular weight of 90 kDa and is a homodimer consisting of two 43 +/- 2 kDa subunits. CSAD appears to require Mn(2+) for its maximum activity. Other divalent cations fail to replace Mn(2+) in activation of CSAD activity. However, the precise role of Mn(2+) in the action of CSAD remains to be determined. Copyright 1996 S. Karger AG, Basel

Role of Protein Phosphorylation in Regulation of Brain L-Glutamate Decarboxylase Activity

In the brain, the gamma-aminobutyric acid (GABA) level is primarily controlled by the activity of its synthesizing enzyme, L-glutamate decarboxylase (GAD). At present, mechanisms responsible for regulation of GAD activity remain largely unknown. Here we report that GAD activity is inhibited by conditions favoring protein phosphorylation, and this inhibition can be reversed by phosphatase treatment. Furthermore, this inhibition appears to result from the suppression of a Ca(2+)-dependent phosphatase. Phosphorylation of GAD is demonstrated by direct incorporation of (32)P into the GAD protein. These results suggest that GAD activity in the brain is inhibited by phosphorylation and activated by dephosphorylation. A model for regulation of GABA synthesis related to neuronal excitation is discussed. Copyright 1994 S. Karger AG, Basel