Overestimation of serum concentrations of gamma-aminobutyric acid in patients with hepatic encephalopathy by the gamma-aminobutyric acid-radioreceptor assay.
Sera of patients with hepatic encephalopathy strongly inhibit the specific binding of gamma-aminobutyric acid to synaptic membranes. In a previous study, this inhibition of specific gamma-aminobutyric acid binding was attributed to gamma-aminobutyric acid itself, and it was assumed that serum gamma-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 gamma-aminobutyric acid-radioreceptor assay was tested. In view of the increased serum concentrations of several amino acids in hepatic encephalopathy, the effects of L-alpha-amino acids on the assay were studied. Five amino acids inhibited specific gamma-aminobutyric acid binding at a concentration of 0.5 mM or lower: glutamine; glutamate; taurine; proline, and OH-proline. Equimolar amounts of aminooxyacetate prevented the inhibition of specific gamma-aminobutyric acid binding by glutamine and glutamate but had no effect on that of gamma-aminobutyric acid, taurine, proline and OH-proline. Aminooxyacetate had no effect on specific gamma-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 gamma-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 gamma-aminobutyric acid. Accordingly, the gamma-aminobutyric acid binding inhibitory activity is not mediated by gamma-aminobutyric acid alone and is most likely due to glutamine.(ABSTRACT TRUNCATED AT 250 WORDS)
Ferenci
P
P
Department of Gastroenterology and Hepatology, University of Vienna, Austria.
Ebner
J
J
Zimmermann
C
C
Kikuta
C
C
Roth
E
E
Häussinger
D
D
D000596:Amino Acids / Q000097:blood; D002851:Chromatography, High Pressure Liquid; D002852:Chromatography, Ion Exchange; D006501:Hepatic Encephalopathy / Q000097:blood*; D006801:Humans; D011869:Radioligand Assay / Q000592:standards*; D011963:Receptors, GABA-A / Q000032:analysis*; D005680:gamma-Aminobutyric Acid / Q000097:blood*
D000596:Amino Acids; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid