On the mechanism of isomerization of ocular retinoids by the crayfish Procambarus clarkii.

The eyes of some crustaceans store substantial amounts of retinyl esters, with most of the retinol in the 11-cis configuration. Earlier work in this laboratory suggested that in lobster and crayfish the mechanism of isomerization of retinol to the 11-cis form involves the hydrolysis of all-trans retinyl esters. Although this is the same process as that occurring in the vertebrate eye, it is different from the retinal photoisomerase reaction known in other arthropods, specifically diurnal insects (Hymenoptera and probably Diptera). Using homogenates of crayfish, we have tested this proposed mechanism by inhibiting retinyl ester synthetase activity in the presence of exogenous all-trans retinol. Inhibition of lecithin:retinol acyl transferase with 5 mumol l-1 retinyl bromoacetate or 2 mmol l-1 phenylmethylsulfonyl fluoride blocks the formation of both all-trans and 11-cis retinyl esters as well as 11-cis retinol, as shown by direct assay and by the decrease in counts derived from tritiated all-trans retinol. The similarity of this isomerization to the mechanism in vertebrate pigment epithelium is thus an interesting example of convergent evolution in the biochemistry of visual pigments, in which the pigments themselves (the opsins) are largely conserved across phyla.