All-trans Retinal

Mizuno, M., Hamaguchi, H. and Tahara, T. (2002) Observation of resonance hyper-Raman scattering of all-trans-retinal. 

Park, S.-M. 1978. Electrochemical studies of P-carotene, all-trans-retinal and all-trans-retinol in tetrahydro-furan. J. Electrochem. Soc. 125 216-222. 

Interestingly, carotenoids more abundant in the blood plasma than zeaxanthin, such as lycopene, P-carotene, and P-cryptoxanthin, do not accumulate in the retina. RPE cells express p,p-carotene 15,15 -monooxygenase (BCO), formerly known as P-carotene 15,l5 -dioxygcnase, an enzyme that catalyzes the oxidative cleavage of P-carotene into two molecules of all-trans-retinal (Aleman et al., 2001 Bhatti et al., 2003 Chichili et al., 2005 Leuenberger et al., 2001 Lindqvist and Andersson, 2002). Therefore it may be suggested that p -carotene transported into RPE-cells is efficiently cleaved into retinal molecules. BCO cleaves also P-cryptoxanthin (Lindqvist and Andersson, 2002), and its absence in the retina may also be explained by its efficient cleavage to retinoids. However, lycopene, often the most abundant carotenoid in human plasma, cannot serve as a substrate for BCO, and yet it is not detectable in the neural retina (Khachik et al., 2002). 

Moreover, efficient rhodopsin regeneration may precede enzymatic reduction of all-fran.v-retinal to all-trans-retinol in the aged retina (Figure 15.2c) (Schadel et al., 2003). Upon rhodopsin regeneration, all-trans-retinal is released from the exit site of the protein into the lipid membrane (Figure 15.2c) (Schadel et al., 2003). From here the removal of all-tnms-retinal to the outer leaflet of the disc membrane is dependent on activity of ATP-binding cassette trasporter A4 (ABCA4) present in the rim of photoreceptor disc, known also as ABCR protein. 

Kim, S.R., Jang, Y.P., Jockusch, S., Fishkin, N.E., Turro, N.J., Sparrow, J.R., 2007b. The all-trans-retinal dimer series of lipofuscin pigments in retinal pigment epithelial cells in a recessive Stargardt disease model. Pmc Natl Acad Sci USA. 104, 19273-19278. 

Rhodopsin is a transmembrane protein linked to 1 l as-retinal, which, on photoabsorption, decomposes to opsin and all-trans-retinal 809... 

After absorbing a photon, the 11-cis-retinal undergoes photoisomerisation into its geometric isomer all-trans-retinal (2) (Figure 12.3). 

An isomerase [3] transfers all-trans -retinal to the ll-cis -form, in which it is available for the next cycle. A dehydrogenase [4] can also allow retinal to be supplied from vitamin A (retinol). 

In an endeavor to explain the anomolous spectral shifts when certain carotenoids such as astaxanthin and all-trans retinal are incorporated into their respective apoproteins, crustacyanin, and opsin, the perturbation of the absorption spectra of the chromophores adsorbed onto silica gel has been examined.37,38 Buckwald and Jencks 37 found that there was a small red shift... 

Figure 15 Hydrogenation of all-trans-retinal to all-trans-retinol.

Retinol can be oxidized to retinal (6.2) and further to retinoic acid (6.3). Cis-trans isomerization can also occur, e.g. the conversion of all trans-retinal to 11-cis-retinal (6.4), which is important for vision. 

---