The irreversible incorporation upon ultraviolet illumination of the glycine receptor antagonist, [3H]strychnine, into synaptic membrane fractions of rat spinal cord has been investigated. The specificity of this photoaffinity-labelling reaction for the glycine receptor was demonstrated by the following results: (a) the Kd value (9.7 nM) of the glycine-displaceable irreversible incorporation of [3H]strychnine was similar to the previously reported Kd of [3H]strychnine binding to the glycine receptor; (b) pre-illumination of the membranes with unlabelled strychnine led to a corresponding reduction in the number, but not the affinity, of reversible glycine-displaceable [3H]strychnine binding sites; (c) the ultraviolet light-induced incorporation into the membranes of [3H]strychnine was inhibited by different glycine receptor agonists; other neurotransmitter substances had little or no effect. Also, [3H]strychnine alone was shown to be stable upon illumination with ultraviolet light; this suggests that photocrosslinking of [3H]strychnine may require energy transfer from specific groups of its high-affinity receptor binding site.
Upon sodium dodecyl sulphate/polyacrylamide gel electrophoresis a single labelled polypeptide with a relative molecular mass of 48 000 was revealed from spinal cord membranes photoaffinity-labelled with [3H]strychnine. Spinal cord membranes photoaffinity-labelled with the γ-aminobutyric acid receptor ligand [3H]flunitrazepam, however, gave a single polypeptide with a relative molecular mass of 50000.
Treatment of membranes, labelled with [3H]strychnine, by endoglycosidase H did not alter the relative molecular mass of the 48000-Mr labelled polypeptide. Trypsin treatment, on the other hand, successively produced major fragments of relative molecular masses of 42000 and 37000. Also, even after extensive treatment with trypsin or chymotrypsin, ≥ 90% of the radioactivity incorporated into the labelled membranes remained membrane-associated. It is concluded that the strychnine binding site of the glycine receptor is located on a protease-inaccessible, i.e. probably hydrophobic domain of the 48000-Mr subunit.
concentration producing half-maximal inhibition
sodium dodecyl sulphate
- buffer A
25 mM potassium phosphate buffer, pH 7.4, containing 200 mM KCl
Trypsin (EC 18.104.22.168)
endo-β-N-acetylglucosaminidase or endoglycosidase H (EC 22.214.171.124)
α-chymotrypsin (EC 126.96.36.199)