Studies on the regeneration of rhodopsin from all-trans-retinal in isolated rat retinae

Separation of geometric isomers of retinyl ester, retinal and retinol, pertaining to the visual cycle, by high-performance liquid chromatography

A method for the analytical and/or preparative separation of the 13-cis, 11-cis, 9-cis and all all-trans isomers of retinyl palmitate ester, retinal and retinol by high-performance liquid chromatography is described. A straight-phase adsorption system consisting of Si 60 silica gel as the stationary phase and mixtures of n-hexane and dioxane as mobile phases were employed, using photometric detection at either 320 or 360 nm, depending on the nature of the compounds being studied. With simple adaptation of the phase system, all geometric isomers within each group could be separated. The precision of the method was 0.5% at the 10-microgram (about 40-nmole) level and 4% at the 10-ng (about 40-pmole) level (n = 3). The detection limit was about 0.5 ng (about 2 pmole). The suitability of the phase system is demonstrated by chromatograms of test mixtures and of eye extracts. The fractionation of 0.5 mg of an isomeric sample could be achieved on a column of length 250 mm and I.D. 10 mm.

Synthesis of rhodopsin and opsin in vitro

Isolated bovine retinas have been used to study the synthesis of rhodopsin and the renewal of photoreceptor rod outer segments. Both leucine and glucosamine served as radioactive tracers to follow rhodopsin synthesis. In both cases the rod outer segment preparations contained large amounts of labeled macromolecules chromatographically distinct from rhodopsin, the latter representing only about 10% of the high molecular weight labeled material. However, electrophoresis on polyacrylamide gels with sodium dodecyl sulfate indicated that as much as 60-68% of the radioactivity coincided with opsin, the apoprotein of rhodopsin. Treatment of labeled rod outer segments with 9-cis-retinal caused much of the non-rhodopsin label to be converted to isorhodopsin. After such treatment the fraction of the label in visual pigment rose from about 10 to 51% with leucine as the radioactive tracer and to 78% with glucosamine. Similar treatment of bleached outer segments labeled with leucine gave identical results with complete regeneration of isorhodopsin (lambda max 487 nm) which then accounted for 56% of the labeled macromolecules. No such conversion occurred in controls lacking 9-cis-retinal. Both 9-cis- and 11-cis-retinal were effective but all-trans-retinal was ineffective in producing the conversion. Under in vitro conditions opsin appears to be accumulated in the outer segment prior to the addition of retinal.