Retinal, from carotenoids

A number of terpenoids appear to be derived from carotenoids by cleavage of the polyene chain. These include retinal (C20). trisporic acids (Cig), abscisic acid (C15), a-ionone (C13), and loliolide (Cu). 

The diet usually provides both preformed vitamin A from animal products and provitamin A carotenoids from vegetables and fruits. Approximately 50 of the 600 known carotenoids can be oxidatively converted into retinal in mammals, primarily by central cleavage (16), but also in part by eccentric cleavage (16). Retinyl esters, the major dietary form of vitamin A from animal products, are hydrolyzed in the intestinal lumen in the presence of pancreatic esterases and conjugated bile salts (13-15). Retinal produced from carotenoid cleavage is reduced to retinol, which is esterifled and incorporated into chylomicra. Chylomicron remnants are taken up primarily by parenchymal cells of the liver but also by other tissues. Retinyl esters are hydrolyzed to retinol, which can (1) combine... 

CRBP(II) sequesters atRCHO generated from carotenoids and allows its reduction into atROH, catalyzed by an ER retinal reductase (uncharacterized). In contrast to CRBP(I), CRBP(II) does not allow oxidation/dehydrogenation of its ligands. LRAT accesses the CRBP(II)-atROH complex and produces atRE for incorporation into chylomicrons. During conversion into remnants by lipoprotein lipase in adipose, chylomicrons retain most of their RE, as they do cholesterol esters. 

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

It has been shown in many studies that protective effects of carotenoids can be observed only at small carotenoid concentrations, whereas at high concentrations carotenoids exert pro-oxidant effects via propagation of free radical damage (Chucair et al., 2007 Lowe et al., 1999 Palozza, 1998, 2001 Young and Lowe, 2001). For example, supplementation of rat retinal photoreceptors with small concentrations of lutein and zeaxanthin reduces apoptosis in photoreceptors, preserves mitochondrial potential, and prevents cytochrome c release from mitochondria subjected to oxidative stress induced by paraquat or hydrogen peroxide (Chucair et al., 2007). However, this protective effect has been observed only at low concentrations of xanthophylls, of 0.14 and 0.17 pM for lutein and zeaxanthin, respectively. Higher concentrations of carotenoids have led to deleterious effects (Chucair et al., 2007). 

Prado-Cabrero, A., D. Scherzinger et al. (2007). Retinal biosynthesis in fungi Characterization of the carotenoid oxygenase CarX from Fusarium fujikuroi. Eukaryotic Cell 6(4) 650-657. 

Ruch, S., P. Beyer et al. (2005). Retinal biosynthesis in eubacteria In vitro characterization of a novel carotenoid oxygenase from Synechocystis sp. PCC 6803. Mol. Microbiol. 55(4) 1015-1024. 

Scherzinger, D., S. Ruch et al. (2006). Retinal is formed from apo-carotenoids in Nostoc sp PCC7120 in vitro characterization of an apo-carotenoid oxygenase. Biochem. J. 398 361-369. 

P-carotene is only one of many antioxidants, which can be detected in the skin. Other carotenoids, for example, lutein and zeaxanthine, are preferentially found in the macula lutea, the so-called yellow spot in the eye. Here, carotenoids are subject to a metabolism typical for that tissue, which cannot be found in other tissues (e.g., formation of meso-zeaxanthine). In addition, they can specifically be absorbed into the macula. In the macula, they protect the retinal pigment epithelial cells against oxidative damage from UV light. Indeed, these two carotenoids can be protective against age-dependent macula degeneration. 

Figure S2.4 shows the structures of 11 -c/.v-retinal and its more stable isomer all-frans-retinal. The reti-nals are related to the alcohol retinol, or vitamin A,. Mammals cannot synthesize these compounds de novo but can form them from dietary carotenoids such as /3-carotene. A deficiency of vitamin A causes night blindness, along with serious deterioration of the eyes and other tissues.

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