Chemical modification of alpha 2-adrenoceptors. Possible role for tyrosine in the ligand binding site.

Tetranitromethane (TNM) is a reagent which reacts with the tyrosine and cysteine residues of proteins. Chemical modification of partially purified human platelet alpha 2-adrenoceptors with TNM resulted in an irreversible loss of binding activity. Typically, an 80-90% decrease in binding activity occurred with a 60-min exposure to 320 microM TNM. The loss of alpha 2-adrenoceptor activity caused by TNM could be prevented if alpha 2-adrenergic ligands were present during exposure of the receptor to TNM. The protection afforded by alpha 2-adrenergic ligands was dose-dependent and showed a positive correlation with the affinity of the ligand for the alpha 2-adrenoceptor. Prazosin, an alpha 1-specific antagonist, and propranolol, a beta-adrenergic antagonist, did not protect alpha 2-adrenoceptors against the inactivation caused by TNM. Saturation curve analysis revealed that the decrease in alpha 2-adrenoceptor activity caused by TNM was due to a decrease in Bmax with no change in Kd. alpha 2-Adrenoceptors were also inactivated with the sulfhydryl-specific reagent phenylmercuric chloride (PMC). The receptor inactivation caused by PMC could be reversed completely by subsequent treatment with dithiothreitol. Treatment of alpha 2-adrenoceptors with combinations of TNM and PMC showed that the receptor inactivation caused by TNM was most likely due to an interaction with tyrosine residues. These results indicate that tyrosine residues have a function in the conformational stability of alpha 2-adrenoceptors and may be directly involved with ligand binding to the receptor.