Ultra-violet sensitizing pigment in blowfly photoreceptors R1-6; probable nature and binding sites

Abstract

Summary. 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. [3-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.

Retinal perception and ecological significance of color vision in insects

“…This “sensitizing pigment”, 3-hydroxy-retinol, is attached to and transfers energy obtained by the absorption of UV light to the visual pigment, thus adding a characteristic UV peak to the absorption spectrum and creating broadband-sensitivity (300-600nm) (Fig. 2C) [37,38,39]. In some vertebrates, the visual pigment’s chromophore can also tune the spectral sensitivity.…”

Insect retinal pigments: Spectral characteristics and physiological functions

“…This process is only reversed when 11-cis-3-hydroxy-retinal is supplied, either exogenously and/or endogenously via illumination with violet/violet light (Schwemer, 1989;Hamdorf et al, 1992;Fig. 6).…”

“…The sensitization of rhodopsin by the ultraviolet-absorbing antenna pigment is the reason for the strongly enhanced spectral sensitivity in the ultraviolet, measured in physiological responses (Hardie, 1986). Hamdorf et al (1992) have investigated this observation in a detailed in vivo electrophysiological study on the speed of incorporation of various retinoids in the visual pigment molecules. The retinoids (e.g.…”

Light-dependent pigment migration in blowfly photoreceptors studied by in vivo CLSM

“…the decrease in the metarhodopsin fraction occurring when the pupil closes, can now be easily calculated by using the template curves for visual pigments [ 29], taking into account the contribution of the ultraviolet-absorbing sensitizing pigment (Fig. 6(a); [ 30,31 ] ), and the pupil absorbance spectrum [ 32]. Fig.…”