receptor prior to synthesis of hbpa to address concerns about the possible loss of radioiodine from the new photophore during photolysis 125 i 3 iodo 4 hydroxybenzophenone was prepared from 4 hydroxybenzophenone by standard oxidative radioiodination the molecule was then subjected to photolysis under conditions identical to those used for receptor photolabeling with 125 i bpa sp li et al 1995a but using twice the photolysis time after photolysis greater than 98 of the radioiodine remained incorporated in the starting material using these same conditions which gave efficient crosslinking using bpa 8 sp only modest photolabeling of spr by 125 i hbpa 8 sp was observed thus it was desirable to modify the photolysis conditions to increase the efficiency of photolabeling spr in p388d 1 cells with 125 i hbpa 8 sp increasing the time of photolysis increased the incorporation of 125 i hbpa 8 sp only negligibly from lt 1 at 15 min to 2 8 incorporation at 2 h freezing the sample prior to and during photolysis resulted in a significant increase in incorporation to 2 9 for 15 min and 30 for 2 h photolyses a major broad band approximately 98 of total counts at approximately 50 75 kda was apparent on sds page after photoinsertion of 125 i hbpa 8 sp in p388d 1 cells with a minor band of the unincorporated 125 i hbpa 8 sp at the gel front the broad band is a result of the heterogeneous glycosylation of sp receptor li et al 1995a vigna s r bowden j j mcdonald d m fisher j okamoto a mcvey d c payan d g and bunnett n w j neurosci 14 834 845 1994 the photolabeling.