Regeneration of rhodopsin and bacteriorhodopsin. The role of retinal analogues as inhibitors.

The rate of regeneration of rhodopsin, from 11-cis-retinal and opsin, and bacteriorhodopsin from all-trans-retinal and bacterio-opsin, in the presence or absence of compounds whose structures partially resemble retinal were measured. Some of these compounds severely slowed down the regeneration process, but did not influence the extent of regeneration. In the case of compounds with a carbonyl functional group they were not joined to the active site of the apo-protein via a Schiff's base linkage since after treatment with NaBH4 an active apo-protein remained. The most effective inhibitors of rhodopsin regeneration were molecules whose structure could be superimposed on 9-cis or 11-cis retinal up to carbon atom 11. These C13 and C15 molecules were not distinguished between aldehyde, ketone or alcohol functional groups. The regeneration of bacteriorhodopsin was not inhibited by retinal analogues with short side chains. The most effective inhibitors were the all-trans C17-aldehyde (beta-ionylideneacetaldehyde) or C18-ketone (beta-ionylidenepent-3-ene-2-one) which, compared to retinal, lack two or three carbon atoms from the end of the poylene chain. The inhibition was very dependent upon the presence of the all-trans isomer and required aldehyde or ketone as functional group nitriles and alcohols were less effective. However, similarly to retinol, the all-trans C17 and C18 alcohols underwent a bathochromic shift and showed fine-structured spectra when mixed with bacterio-opsin.