Chemical modifications of striatal A2A adenosine receptors: a possible role for tyrosine at the ligand binding sites.
A2A adenosine receptors were examined in bovine striatal membranes following exposure to tetranitromethane (TNM) which modifies tyrosine and cysteine residues. TNM (0.05-0.5 mM) treatment caused an irreversible, concentration-dependent decrease in the binding activity of the selective A2A agonist [3H]CGS 21680. Protection studies showed that TNM inactivation could be prevented by the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) and by the antagonist xanthine amine congener (XAC), suggesting that TNM modified residues at the ligand-binding sites. Scatchard analysis of the binding data showed that 0.15 mM TNM decreased the [3H]CGS 21680 Bmax value from 447 +/- 39 to 273 +/- 21 fmol/mg of proteins without any significant change in the Kd values (13.5 +/- 1.4 and 14.7 +/- 1.5 for control and treated membranes, respectively). We carried out a series of successive chemical modifications with the reducing agent dithiothreitol (DTT), which indicated that the residues modified by TNM, under our experimental conditions, are tyrosine residues and not cysteine residues.
Martini
C
C
Istituto Policattedra di Discipline Biologiche, Università di Pisa, Italy. cmartini@farm.unipi.it
Trincavelli
L
L
Lucacchini
A
A
D000241:Adenosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive; D002417:Cattle; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism*; D004229:Dithiothreitol; D004305:Dose-Response Relationship, Drug; D006863:Hydrogen-Ion Concentration; D010627:Phenethylamines / Q000494:pharmacology*; D043705:Receptor, Adenosine A2A; D018047:Receptors, Purinergic P1 / Q000737:chemistry / Q000187:drug effects*; D013774:Tetranitromethane / Q000494:pharmacology*; D014443:Tyrosine / Q000737:chemistry
D010627:Phenethylamines; D043705:Receptor, Adenosine A2A; D018047:Receptors, Purinergic P1; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D014443:Tyrosine; D013774:Tetranitromethane; D000241:Adenosine; D004229:Dithiothreitol