Use of photolabels to probe the Na,K-ATPase and the beta-adrenergic receptor.

Radioactive photoaffinity labels have been used to probe the cardiac glycoside-binding site of Na,K-ATPase and the catecholamine-binding site of the beta-adrenergic receptor. For the Na,K-ATPase, a systematic positioning of the photoactive group on the first, second, and third digitoxoses of digitoxin showed that the specific radioactivity in the alpha subunit decreased 5- to 20-fold as the photoactive group was extended further away from the steroid nucleus, whereas the beta subunit is positioned very close to the alpha subunit in the region of the cardiac glycoside-binding site. For the beta-adrenergic receptor, a new class of orthoiodophenylazide derivatives of pindolol was prepared with carrier-free 125I. Photolysis of the beta-adrenergic receptor of duck, turkey, pigeon, and frog erythrocyte membrane with one of these compounds (iodoazidobenzylpindolol) allowed identification of the receptor polypeptides. It was found that the size of the polypeptides and the number of polypeptides varied.