Aldehydes retinal

Vitamin Ai (retinol) is derived in mammals by oxidative metabolism of plant-derived dietary carotenoids in the liver, especially -carotene. Green vegetables and rich plant sources such as carrots help to provide us with adequate levels. Oxidative cleavage of the central double bond of -carotene provides two molecules of the aldehyde retinal, which is subsequently reduced to the alcohol retinol. Vitamin Ai is also found in a number of foodstuffs of animal origin, especially eggs and dairy products. Some structurally related compounds, including retinal, are also included in the A group of vitamins. 

A deficiency of vitamin A leads to vision defects, including a visual impairment at low light levels, termed night blindness. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde retinal, and then by enzymic isomerization to cw-retinal. c -Retinal is then bound to the protein opsin in the retina via an imine linkage (see Section 7.7.1) to give the red visual pigment rhodopsin. 

The cell illustrated opposite, a rod, has a structure divided by membrane discs into which the 7-helix receptor rhodopsin is integrated (see p. 224). In contrast to other receptors in the 7-helix class (see p. 384), rhodopsin is a light-sensitive chromoprotein. Its protein part, opsin, contains the aldehyde retinal (see p. 364)—an isoprenoid which is bound to the e-amino group of a lysine residue as an aldimine. 

Dietary carotenes and carotenoids are absorbed and transported in the plasma of humans and animals by lipoproteins.149 Tire conversion of carotenes to vitamin A (Box 22-A) provides the aldehyde retinal for synthesis of visual pigments (Chapter 23) and retinoic acid, an important regulator of gene transcription and development (Chapter 32).150 152c See also Section E,5. 

Ethers and esters derived from the alcohol also show activity in vivo. The ring structure of retinol ((i-ionone), or the more unsaturated ring in 3-dehydroretinol (dehydro-P-ionone), is essential for activity hydrogenation destroys biological activity. Of all known derivatives, all-/ran,v-retinol and its aldehyde, retinal, exhibit the greatest biological potency in vivo 3-dehydroretinol has about 40% of the potency of all-bms-retinol. 

For syntheses of vitamin A aldehyde (retinal) and higher terpene aldehydes, appropriate 2-methyl-2-butene-1,4-dials (32) with a protected aldehyde function were used as the C5 building blocks. They are accessible, in turn, via the C5 building block p-formylcrotyl acetate (8 b) in the form of its dimethyl acetal, or via P-formylcrotyl methyl ether (29)34 b) ... 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

The two methyl ketones, or- and jS-ionone, as well as the aldehyde retinal, have a... 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

Vitamin A - also called all-tra/75 -retinol, is an isoprenoid alcohol that plays a key role in vision and a role in controlling animal growth. Vitamin A must either be present in the diet, or derived from / -carotene, an isoprenoid compound prominent in carrots. See Figure 19.25 for the biosynthesis of dX -trans-xQtmo. Dehydrogenation of retinol yields the aldehyde, retinal, which has a role in vision. Another derivative of retinol is retinoic acid, which can be made by the oxidation of retinal. 

Retinol (RET-uh-nol) is the scientific name for vitamin A, a vitamin found only in animals. It occurs as a yellowish to orange powder with a slight brownish cast and is a relatively stable compound. Retinol is converted in the body from an alcohol to the corresponding aldehyde, retinal (C2oH280), one of the primary chemical compounds involved in the process by which light is converted to nerve impulses in the retina of the eye. Vitamin A is also required for a number of other biochemical reactions in the body, including growth and development of tissue and maintenance of the immune system. 

Chemical structure (Figure 1). Molecules formally composed of four isoprene units they naturally occur as an alcohol (retinol), an aldehyde (retinal), or as an acid (retinoic acid). From each of the three basic forms two variants exist vitamin A, with a jS-ionone ring and vitamin A2 with a dehydrated S-ionone ring. Native retinoids show cis-trans isomerism of the double bonds. Carotenoids are proforms of vitamin A (Figure 2). 

To give but one example of the importance of imines in biological systems, the active form of vitamin A aldehyde (retinal) is bound to the protein opsin in the human retina in the form of an imine called rhodopsin or visual purple (see Chemical Connections 4B). The amino acid lysine (see Table 18.1) provides the primary amino group for this reaction ... 

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