11-cis-Retinoids

As a result of the thermally induced rearrangement of 10,14-retroretinol (383), it was possible to synthesize 11-cis retinoids that are not otherwise readily obtainable. However, these hydrogen shifts take place without selectivity at the and double bonds. In the course of this work, the thermal instability of (IIZ, 13Z)-retinaldehyde (388) (Oroshnik et al, 1956) and (9Z, IIZ, 13Z)-retinaldehyde (385) was confirmed. These compounds undergo rapid isomerization to (13Z)-retinaldehyde (41) and (9Z, 13Z)-retinaldehyde (389). This pronounced tendency of the double bond in (IIZ, 13Z)-retinaldehydes to... 

All-trarti-retinoids are not able to support regeneration in frogs and mammalian retinas. The 11-cis retinoids that have been tested are retinaldehyde, retinol, and retinyl palmitate. 

Since the key to understanding visual pigment regeneration lies in obtaining an insight into how 11-cis-retinoid is formed from all-troiu-retinoid, this section is devoted to this difficult as yet unsolved problem. Under what circumstances do retinoids isomerize ... 

Saari has made the bold statement that Three derivatives of 11-cis-retinaldehyde serve as the chromophores of all known visual pigments. They are complexed with a protein component (an opsin), and the resulting protein-retinoid interactions determine the spectral sensitivity of the visual pigment. 85 No reference is given for this statement nor is any explanation of how these complexes exhibit an absorption spectrum in the visual region. However, except for the substitution of all-trans for 11 -cis in the above quotation and a slight modification to the retinoid involved, this work agrees completely with the statement and provides an explanation for how it is applied. 

Wang J, ChaiX, Eriksson U, and Napoli ]L (1999) Activity of human 11-cis-retinol dehydrogenase (Rdh5) with steroids and retinoids and expression of its mRNA in extraocular human tissue. Biochemical Journal 338, 23-7. 

Carlson, A., and Bok, D. (19%). Promotion of the release of 11-cis-retinal from cultured refinal pigment epithelium by interphotoreceptor retinoid-binding protein. Biochemistry 31, 9056-9062. 

In the case of CRBP and CRBPII, a number of retinoids have been tested (MacDonald and Ong, 1987 Cheng et al., 1991). No binding to CRBP or CRBPII occurred with 9-c - or 11-cis-retinol. However, binding was detected with 13-cis-retinol and 3-dehydroretinol. One unusual finding was that CRBPII but not CRBP had a detectable affinity for all-tram-retinaldehyde. Later studies (Li et al., 1991) showed that CRBP could bind retinol or retinal but that it had a 100-fold higher affinity for the alcohol. 

The retinal pool of 11 -cis-retinal is clearly critical for photon detection and visual signal transduction. There are two key aspects regarding this that are important to consider the precursor(s) from which 11 -cis-retinal derived, and, bioconversion of isomerized trans-retinal back to 11-cis-retinal that can recombine with opsin to re-form rhodopsin. The latter aspect is obligatory for restoration of the dark state, regenerating a photosensitive receptor capable of undergoing another cycle of photon detection and signal transduction. Considered together, these events represent the retinoid cycle in the visual process (22). The entire process of re-isomerization and formation of a new rhodopsin molecule occurs in two different retinal tissues and involves several enzymatic steps that are described below (see Fig. 7.14). 

Finally, an abundant protein in the RPE, RPE65, seems to play a role in the retinoid cycle, but the exact role remains a mystery. Mice lacking this protein accumulate all-trans-retinyl esters that concentrate within lipid droplets in RPE cells (84). These mice are completely devoid of 11-cis-retinal and other cis-retinoid metabolites and are severely com-... 

Retinoids 11-ci.y-retinaldehyde can be obtained through the National Eye Institute, Fundamental Retinal Processes Research Program, National Institute of Health, Bethesda, MD. 9-cw-retinaldehyde is obtainable from Sigma Chemical Company Both retinoids are stored dry, under argon at -70°C m the dark, and dissolved in ethanol for labeling purposes. 

Both 11-ci -retinaldehyde and 11-cw-retinol have been recovered from purified rCRALBP after labeling of the crude-bacterial lysate with only 11-cts-retinaldehyde This indicates that retinoid modification can occur during the labeling and/or protem-punfication steps. 

The retinoids in dark-adapted eyes occur predominantly in the photoreceptor cell outer segments, which contain visual pigment, and in the adjacent retinal pigment epithelium (RPE) cells, which contain variable amounts of retinyl esters. These esters consist mainly of palmitate (A4) and stearate and are sometimes sequestered in lipid droplets (Bridges, 1975 Young and Bok, 1979 Alvarez etal., 1981 Bridges etal., 1982). The only major isomers present aieall-trans and 11-cis. The 11-cis isomer is restricted to ocular tissues. 

In the dark-adapted eyes of most animals, the retinoid stores in the RPE represent between 1 and 6 mol Eq of the retinal visual pigment. It is not known whether the magnitude of these stores depends on the vitamin A status of the animal, although it is probable that they serve as a reserve that protects the visual system from retinoid depletion under conditions of dietary deficiency. Since these stores typically contain up to 75% 11-m-retinyl ester, the provision of 11-cis isomer for visual pigment synthesis may also be important under certain circumstances. However, the presence of this isomer in the RPE is not critical for regeneration, which can proceed efficiently in the absence of appreciable supplies of 11-cis-retinyl ester [see Bridges (1976b) for further discussion of this question]. 

A typical HPLC profile for the retinoid present in bovine RPE cytosol is shown in Fig. 6. After the initial breakthrough region at 8 min, only two major peaks are present. They correspond to 11-cw-retinaldehyde (peak 1) and all-rrani-retinol (peak 6). Therefore, most of the retinaldehyde in the cytosol is 11-cis, while nearly all of the retinol is all-trans. This has been found to be generally true for bovine and human RPE cytosols. 

Fig. 6. Visual cycle retinoids extracted from the cytosol of bovine RPE cells (A) and standard retinoids (B). Normal-phase-high-performance liquid chromatography as described by Bridges and Alvarez (1982b). Peak 1, 11-eis-retinaldehyde peak 2, 13-cu-retinaldehyde peak 3, M-trans-retinaldehyde peak 4, 11-cis-retinol peak 5, I3-cis-retinol peak 6, all-tranr-retinol.

Are the various isomers mobilized and transported in different ways in the RPE This appears to be the case. The retinyl esters stored in the RPE contain 11-cis as well as all-trans isomers. As discussed in Section III,D, 11-cw- and all-franj-retinoids are streamed separately in the RPE cytosol, the 11-cis isomer as retinaldehyde and the all-trans isomer as retinol, each bound to its own, separate transport protein. Whether both retinoids are transported through... 

All-tra/w-retinol, 11-cw-retinol (A2) and 11-m-retinaldehyde are found in the 1PM (Liou et al., 1982c). At present, it is not clear which retinoid, or which isomer, is in transit from the RPE to the ROS. With regard to carrier proteins, the evidence suggests that CRBP and CRAIBP are present in the 1PM (Liou et al., 1982c), but it is probable that they have leached from the RPE and retina cytosols where both proteins occur. On the other hand, interstitial retinol-binding protein is believed to be a true IPM protein because it is absent from the RPE and retina cytosols. This conclusion has been confirmed by immunocytochemical observations in the author s laboratory (Fong et al., 1984) and by Bunt-Milam and Saari (1983). Although IRBP binds the all-tranj-retinol that is released when rhodop-sin is bleached, it also carries some 11-cis isomer, and it has not been established whether it may be implicated in the two-way transport of retinoids. 

In no case has the formation of an 1 l-c -retinoid from all-/ra/is-retinoid been confirmed in vitro in darkness. However, numerous studies have shown that 11-cis isomers of retinaldehyde, retinol, and retinaldehyde oxime are easily converted to all-trans in a variety of solvents (Hubbard, 1956, 1966 Futterman and Rollins, 1973 Futterman and Futterman, 1974 Rotmans et al., 1972), particularly under conditions favoring Schiff base formation or interaction with phospholipids such as phosphatidylethanolamine (Groenendijk et al., 1980). Various studies of this process have been carried out with the rationale that important clues concerning the mechanism of the reverse reaction would be obtained. 

---