A novel photopigment candidate in membranes of cultured chick pineal cells

The photopigment of avian pineal which mediates light sensitivity was sought via its chromophore. Chick pineal cells in primary cultures were incubated overnight in the dark with 3H]retinol. Reduction of Schiff's bases with cyanoborohydride prior to SDS-PAGE revealed radioactivity bound to a 30 kDa component in pinealocyte membranes. All-trans-retinal, but not retinol or retinoic acid, incubated with pinealocyte homogenates prior to reduction, resulted in a loss of radioactivity from the 30 kDa region of the gel. The radioactivity was also displaced by NH2OH in the dark. Incubation of cultured cells or homogenates with retinoyl fluoride, an acylating agent specific for the retinal binding site of opsins, also displaced radioactivity from the protein. Furthermore, retinoyl fluoride, added to chick pineal cells in culture, blocked the suppressive effect of light on melatonin output by these cells. Taken together these results raise the possibility that the 30 kDa protein mediates photosensitivity in the chick pineal.