Hydroxy carotenoids

More than 600 different carotenoids from natural sources have been isolated and characterized. Physical properties and natural functions and actions of carotenoids are determined by their chemical properties, and these properties are defined by their molecular structures. Carotenoids consist of 40 carbon atoms (tetraterpenes) with conjugated double bonds. They consist of eight isoprenoid units j oined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position and the remaining nonterminal methyl groups are in a 1,5-position relationship. They can be acyclic or cyclic (mono- or bi-, alicyclic or aryl). Whereas green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few... 

In some species of green algae keto-carotenoids were synthesized when growth was halted by environmental conditions and these compounds may be formed either by the catabolism of chlorophyll or by the result of de novo synthesis. The profile of incorporation of tracer from [2acetate by Haematococcus lacustris into carotenoid and keto-carotenoids indicated that the latter was the major pathway to the keto-compounds, but some breakdown of chlorophyll did occur and the relative contributions of these two pathways varied with the external conditions. In the marine isopod Idotea resecata incorporation of [ Cj-zS-carotene into hydroxy-carotenoids was very low and it was not possible to decide whether the keto-carotenoids present were formed by direct oxidation of jS-carotene to canthaxanthin via echinenone or via hydroxy-compounds. ... 

Jeevarajan AS, Khaled M and Kispert LD (1994b) Simultaneous electrochemical and electron paramagnetic resonance studies of keto and hydroxy carotenoids. Chem Phys Lett 225 340-345... 

There is much current debate about the relevance of such carotenoid repair processes to hydrocarbon carotenoids such as 8-carotene and lycopene in vivo where the parent carotenoid is unhkely to encounter the polar ascorbic acid. However, the cation radical, with a positive charge, maybe sufficiently polar and long-lived for such interactions to be possible. For the carotenoids found in the macula, where an efficient anti-oxidant process is crucial, the hydroxy carotenoids zeaxanthin, meso zeaxanthin and lutein are likely to be in a membrane orientation such that the corresponding cation radicals are efficiently repaired by the vitamin C (cf. vitamin E, below). 

Manuela Albrecht, Shinichi Takaichi, Sabine Steiger, Zheng-Yu Wang and Gerhard Sandmann, Novel hydroxy-carotenoids with improved antioxidative properties produced by gene combination in Escherichia coli. Nature Biotechnology, 18 (2000), 843-846. 

There are basically two types of carotenoids those that contain one or more oxygen atoms are known as xanthophylls those that contain hydrocarbons are known as carotenes. Common oxygen substituents are the hydroxy (as in p-cryptoxanthin), keto (as in canthaxanthin), epoxy (as in violaxanthin), and aldehyde (as in p-citraurin) groups. Both types of carotenoids may be acyclic (no ring, e.g., lycopene), monocyclic (one ring, e.g., y-carotene), or dicyclic (two rings, e.g., a- and p-carotene). In nature, carotenoids exist primarily in the more stable all-trans (or all-E) forms, but small amounts of cis (or Z) isomers do occur. - ... 

Carotenoids with ally lie hydroxy and keto groups such as the 3-hydroxy-4-keto group in astaxanthin which is widespread in marine animals, microorganisms, and algae undergo oxidation in the presence of alkali and air. Eor such samples, saponification is not recommended or must be carried out under anaerobic conditions. Eor this purpose, a special apparatus and procedure were developed by Schiedt et al. ... 

The oxidation of P-carotene with potassium permanganate was described in a dichloromethane/ water reaction mixture (Rodriguez and Rodriguez-Amaya 2007). After 12 h, 20% of the carotenoid was still present. The products of the reaction were identified as apocarotenals (apo-8 - to apo-15-carotenal = retinal), semi-P-carotenone, monoepoxides, and hydroxy-p-carotene-5,8-epoxide. 

Structurally, vitamin A (retinol) is essentially one half of the molecule of (3-carotene. Thus, (3-carotene is a potent provitamin A to which 100% activity is assigned. An unsubstituted (3 ring with a Cn polyene chain is the minimum requirement for vitamin A activity, y -Carotene, a-carotene, (3-cryptoxanthin, a-cryptoxanthin, and (3-carotene 5,6-epoxide, all having one unsubstituted ring, have about half the bioactivity of (3-carotene (Table7.4) On the other hand, the acyclic carotenoids, devoid of (3-rings, and the xanthophylls, in which the (3-rings have hydroxy, epoxy, and carbonyl substituents, are not provitamin A-active for humans. 

The Cso carotenoid decaprenoxanthin [2,2 -bis-(4-hydroxy-3-methylbut-2-enyl)-e,e-carotene (89)] has been prepared from the functionalized a-irone derivative (95), the synthesis of which is outlined in Scheme 1. In the mixture of (89) isomers formed, those with 2,6-trans end-groups predominated. 

The cyclization inhibitor nicotine prevents formation of the C50 carotenoid bacterioruberin [2,2 -bis-(3-hydroxy-3-methylbutyl)-3,4,3, 4 -tetradehydro-... 

The terpenes, carotenoids, steroids, and many other compounds arise in a direct way from the prenyl group of isopentenyl diphosphate (Fig. 22-1).16a Biosynthesis of this five-carbon branched unit from mevalonate has been discussed previously (Chapter 17, Fig. 17-19) and is briefly recapitulated in Fig. 22-1. Distinct isoenzymes of 3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase) in the liver produce HMG-CoA destined for formation of ketone bodies (Eq. 17-5) or mevalonate.7 8 A similar cytosolic enzyme is active in plants which, collectively, make more than 30,000 different isoprenoid compounds.910 However, many of these are formed by an alternative pathway that does not utilize mevalonate but starts with a thiamin diphosphate-dependent condensation of glyceraldehyde 3-phosphate with pyruvate (Figs. 22-1,22-2). 

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