Specific reaction of 9-cis-retinoyl fluoride with bovine opsin

Opsin readily undergoes Schiff base formation between an active site lysine and 9-cis- or 11-cis-retinaldehyde to form the visual pigments isorhodopsin (lambda max = 487 nm) and rhodopsin (lambda max = 500 nm), respectively (Dratz, 1977). It would be predicted that 9-cis-retinoyl fluoride (1), an isostere of 9-cis-retinal, should be an active site directed, mechanism-based labeling agent of opsin, since a stable peptide bond should be formed instead of a Schiff base. It is shown here that 9-cis-retinoyl fluoride (1) reacts with opsin in a time-dependent fashion (t1/2 = 9 min at 25 microM 1) to form a new, nonbleachable pigment with a lambda max of approximately 365 nm. beta-Ionone competitively slows down the rate of the reaction. The absorbance of the new pigment at approximately 365 nm is similar to that of model amide compounds. This result is consistent in a general and qualitative way with the Nakanishi-Honig point-charge model for visual pigments which requires that the chromophore be charged, a situation not possible when the retinoid is linked to opsin via a peptide bond rather than a protonated Schiff base [Honig, B., Dinur, U., Nakanishi, K., Balogh-Nair, V., Gawinowicz, M.A., Arnabaldi, M., & Motto, M.G. (1979) J. Am. Chem. Soc. 101, 7084-7086]. 9-cis-Retinoyl fluoride (1) is approximately 4-fold more potent than all-trans-retinoyl fluoride (2) as an inactivator of bovine opsin. Importantly, 13-cis-retinoyl fluoride (3) is inactive, and no new absorption band at 365 nm is observed.(ABSTRACT TRUNCATED AT 250 WORDS)