Ultra-violet sensitizing pigment in blowfly photoreceptors R1-6; probable nature and binding sites
1992
Hamdorf, K. | Hochstrate, P. | Hoglund, G. | Moser, M. | Sperber, S. | Schlecht, P.
The spectral sensitivity of photoreceptors R1-6 in the blowfly Calliphora erythrocephala has a characteristic UV fine spectrum due to a sensitizing pigment complexed with the visual pigment, P490. The present study concerns the identity and steric configuration of the sensitizing pigment, and the number of its binding sites on P490. 1. Irradiation by "green" light demonstrated that "de novo" synthesis of visual pigment has priority over formation of UV sensitizing pigment, both using the same retinoid pool, and that in absence of sensitizing pigment an aberrant, more thermostable, visual pigment is formed. 2. The kinetics of increase in UV sensitivity after application of different retinoids to eyes with low content of P490, and the vibronic UV fine spectrum of the sensitizing pigment, make 13-cis-retinol the most probable candidate for the UV sensitizing pigment or its direct precursor. 3. beta-carotene, lutein or zeaxanthin cannot be directly used by the eye as precursors for the UV sensitizing pigment. Vitamin A2 probably is directly coupled to P490. At least two UV sensitizing pigment molecules can complex with P490. 4. Artificial UV sensitivity could be induced by the fluorescent thiol probe P-28, probably binding to cysteines 121 and 196 at the extracellular surface of P490.
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