Retinal 3-carotenes, formation from

Nagao, A. and J. A. Olson. 1994. Enzymatic formation of 9-cis, 13-cis, and all-fraras retinals from isomers of beta-carotene. Faseb J 8(12) 968-973. 

The enzyme catalyzing the formation of retinal 2 by means of central cleavage of P-carotene 1 has been known to exist in many tissues for quite some time. Only recently, however, the active protein was identified in chicken intestinal mucosa (3) following an improvement of a novel isolation and purification protocol and was cloned in Escherichia coli and BHK cells (4,5). Iron was identified as the only metal ion associated with the (overexpressed) protein in a 1 1 stoichiometry and since a chromophore is absent in the protein heme coordination and/or iron complexation by tyrosine can be excluded. The structure of the catalytic center remains to be elucidated by X-ray crystallography but from the information available it was predicted that the active site contains a mononuclear iron complex presumably consisting of histidine residues. This suggestion has been confirmed by... 

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. 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

About 57 per cent of the photons that enter the eye reach the retina the rest are scattered or absorbed by the ocular fluid. Here the primary act of vision takes place, in which the chromophore of a rhodopsin molecule absorbs a photon in another n-to-n transition. A rhodopsin molecule consists of an opsin protein molecule to which is attached an 11-ds-retinal molecule (Atlas E3 and 3). The latter resembles half a carotene molecule, showing Nature s economy in its use of available materials. The attachment is by the formation of a proton-ated Schiff s base, utilizing the CHO group of the chromophore and the terminal NHj group of the side chain of a lysine residue from opsin (5). The free 11-ds-retinal molecule absorbs in the ultraviolet, but attachment to the opsin protein molecule shifts the absorption into the visible region. The rhodopsin molecules are situated in the membranes of special cells (the rods and the cones ) that cover the retina. The opsin molecule is anchored into the cell membrane by two hydrophobic groups and largely surrounds the chromophore (Fig. 12.52). 

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