Studies on the insect visual pigment sensitive to ultraviolet light: Retinal as the chromophoric group

1. The nature of the chromophoric group of the ultraviolet visual pigment of the neuropteran Ascalaphus macaronius Scop. was investigated. Analysis of the carotenoid content of the retina by the reaction with SbCl₃ indicated the presence of retinal.

2. Thin-layer chromatography of ethanol-ether extracts from retinae on silica gel demonstrated the presence of retinal isomers with R_F values corresponding to all-trans-retinal and II-cis- and/or 13-cis-retinal. Other retinal isomers and retinol were not detected.

3. Thermal denaturation released retinal from the ultraviolet visual pigment A and its photoproduct B. Analysis of the compounds resulting from thermal denaturation indicated that all-trans-retinal was predominately released. Formation of cattle rhodopsin from cattle opsin and ultraviolet visual pigment A denatured by Ag⁺ indicated that the stereoconfiguration of retinal in the native pigment is II-cis.

4. As a consequence of the retinal nature of the chromophore, the ultraviolet visual pigment of A. macaronius was designated “rhodopsin 345”, and photoproducts B and C correspond to acid and alkaline metarhodopsin, respectively. The stereoconfiguration of retinal in rhodopsin 345 and metarhodopsin, as well as the linkage of the chromophore in respect of the hypsochromic shift of rhodopsin 345, were discussed.