Studies on a missing reaction in the visual cycle

Isomerization of all-trans-retinoids to 11-cis-retinoids in vitro

The key biochemical process of the vertebrate visual cycle required for rhodopsin regeneration, 11-cis-retinoid production from all-trans-retinoids, is shown to occur in vitro. A 600 X g supernatant from a frog retina/pigment epithelium homogenate transforms added all-trans-[3H]retinol, in a time-dependent fashion, to a mixture of 11-cis-retinol, 11-cis-retinal, and 11-cis-retinyl palmitate. 13-cis-Retinoids are formed in only minor amounts by nonspecific processes. Studies using washed particulate fractions of the 600 X g supernatant indicate that all-trans-[3H]retinol is isomerized to 11-cis-retinoids much more effectively than is all-trans-[3H]retinal or all-trans-[3H]retinyl palmitate. The 11-cis-retinoid biosynthetic activity is heat-labile, sedimentable by high-speed centrifugation, and largely found in the pigment epithelium rather than in the neural retina.

The specific inhibition of 11-cis-retinyl palmitate formation in the frog eye by diaminophenoxypentane, an inhibitor of rhodopsin regeneration

The antischistosomal drug 1,5-di-(p-aminophenoxy) pentane (DAPP), an inhibitor of rhodopsin regeneration in the vertebrate retina, is shown to completely block the production of 11-cis-retinyl palmitate in the frog eye. An untreated frog generates a large amount of 11-cis-retinyl palmitate during 1-2 days in the dark after a strong bleach. Also, it is demonstrated that DAPP can deplete the stores of 11-cis-retinyl palmitate in the dark-adapted frog eye. The specificity of DAPP's inhibition of dark-adaptation is explored, and the usefulness of employing retinotoxic drugs to investigate the physiology and biochemistry of rhodopsin regeneration is discussed.

Dark isomerization of retinals in the presence of phosphatidylethanolamine

1. Dark incubation of retinoids (retinyl ester, retinol, retinal, retinaloxime) in suspensions of rod outer segment membranes leads to substantial isomerization (and partial degradation) in the case of retinals only. 2. All-trans, 13-cis and 9-cis-retinal all isomerize at the delta 13 double bond leading to an equilibrium with approximately 75% trans and 25% cis isomer at this bond (all-trans in equilibrium 13-cis and 9-cis in equilibrium 9,13-dicis). 11-cis-Retinal isomerizes irreversibly to a mixture of all-trans and 13-cis-retinal. 3. The active compound appears to be phosphatidylethanolamine present in the membrane. The amino group and the phosphate, as well as the hydrophobic part of the phospholipid are essential. 4. At least three factors are important for the phosphatidylethanolamine-catalyzed isomerization as studied with the 13-cis isomer: the concentration of phosphatidylethanolamine, the concentration of Schiff base between retinal and phosphatidylethanolamine and the presence of lipid aggregates. 5. Based on these observations a mechanism is proposed, which satisfactorily explains the specificity of the isomerization pattern. 6. It is suggested that reisomerization of all-trans to 11-cis retinal in vivo takes place by fixation of all-trans retinal on an adequate surface (e.g. opsin) and a localized nucleophilic attack on the C-11 atom, followed by trapping of the isomerized chromophore by opsin. 7. It is further concluded that retinal does not occur in vivo as a free intermediate. Direct transfer from one protein to another (opsin, retinol dehydrogenase, retinal binding proteins) seems to take place.

Kinetic study of photoregeneration process of digitonin-solubilized squid rhodopsin

In the photoregeneration process of squid rhodopsin, an intermediate has been found at neutral pH values (phosphate buffer) with a flash light (lambda greater than 540 nm). An intermediate R430, with the 11-cis retinal as chromophore, is produced from metarhodopsin in light and is converted to rhodopsin through the processes R430 leads to P380 and P380 leads to rhodopsin. The pH dependence of the velocity of the conversions suggests that processes R430 leads to P380 and P380 leads to rhodopsin involve a protolytic reaction and that the ionized group is a histidine residue of opsin. Kinetic parameters show that the largest conformational change in opsin occurs in the conversion of R430 leads to P380.

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.