Overestimation of serum concentrations of γ-aminobutyric acid in patients with hepatic encephalopathy by the γ-aminobutyric acid-radioreceptor assay

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Abstract
Sera of patients with hepatic encephalopathy strongly inhibit the specific binding of γ-aminobutyric acid to synaptic membranes. In a previous study, this inhibition of specific γ-aminobutyric acid binding was attributed to γ-aminobutyric acid itself, and it was assumed that serum γ-aminobutyric acid is increased 5- to 30-fold in patients with hepatic encephalopathy. The findings of that study, however, were not confirmed by other analytical methods. Therefore, the validity of the γ-aminobutyric acid-radioreceptor assay was tested. In view of the increased serum concentrations of several amino acids in hepatic encephalopathy, the effects of L-α-amino acids on the assay were studied. Five amino acids inhibited specific γ-aminobutyric acid binding at a concentration of 0.5 mM or lower: glutamine; glutamate; taurine; proline, and OH-proline. Equimolar amounts of amino-oxyacetate prevented the inhibition of specific γ-aminobutyric acid binding by glutamine and glutamate but had no effect on that of γ-aminobutyric acid, taurine, proline and OH-proline. Aminooxyacetate had no effect on specific γ-aminobutyric acid binding itself. The inhibitory activity of a serum sample from a patient with hepatic encephalopathy was inhibited by 0.5 mM aminooxyacetate. The γ-aminobutyric acid binding inhibitory activity of a serum sample of a patient with hepatic encephalopathy was purified by gel chromatography and contained several amino acids at concentrations of about 0.1 mM, 3.5 mM glutamine but no detectable γ-aminobutyric acid. Accordingly, the γ-aminobutyric acid binding inhibitory activity is not mediated by γ-aminobutyric acid alone and is most likely due to glutamine. Thus, using the γ-aminobutyric acid-radioreceptor assay serum, “true γ-aminobutyric acid” concentrations in patients with hepatic encephalopathy were overestimated.