A Carbene-generating Photoaffinity Probe for Beta-adrenergic Receptors

A new radioiodinated (2.2 Ci/mumol) iodocyanopindolol derivative carrying a 4-(3-trifluoromethyldiazirino)benzoyl residue has been synthesized. The long-wavelength absorption of the diazirine permits formation of the carbene by photolysis under very mild conditions. [125I]ICYP-diazirine binds with high affinity (Kd = 60 pM) to beta-receptors from turkey erythrocyte membranes. Upon irradiation, [125I]ICYP-diazirine is covalently incorporated in a Mr 40 000 protein. Stereoselective inhibition of photolabeling by the (-)enantiomers of alprenolol and isoproterenol indicated that the Mr 40 000 protein contains a beta-adrenergic binding site. The yield of specific labeling was up to 8.2% of total beta-receptor binding sites. The Mr 40 000 protein photolabeled in the membrane could be solubilized at comparable yield with either digitonin or Triton X-100. Irradiation of digitonin-solubilized turkey erythrocyte membranes with [125I]ICYP-diazirine resulted in specific labeling of two proteins with Mr 40 000 and 50 000. In guinea-pig lung membranes, at least five proteins were photolabeled, of which one (with approximately Mr 67 000) was labeled specifically.