Carotenoid conversion

Scott KJ and Rodriguez-Amaya D. 2000. Pro-vitamin A carotenoids conversion factors retinol equivalents-fact or fiction Food Chem 69 125-127. 

Scott, J.K. and Rodriquez-Amaya, D. (2000) Pro-vitamin A carotenoid conversion factors retinol equivalents - fact or fiction. Food Chem., 69, 125-127. 

An important function of certain carotenoids is their provitamin A activity. Vitamin A may be considered as having the stmcture of half of the P-carotene molecule with a molecule of water added at the end position. In general, all carotenoids containing a single unsubstituted P carotene half have provitamin A activity, but only about half the activity of P carotene. Provitamin A compounds are converted to Vitamin A by an oxidative enzyme system present in the intestinal mucosa of animals and humans. This conversion apparendy does not occur in plants (see Vitamins, VITAMIN a). 

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. 

Many carotenoids function in humans as vitamin A precursors however, not all carotenoids have provitamin A activity (Table 3). Of the biologically active carotenoids, -carotene has the greatest activity. Despite the fact that theoretically one molecule of -carotene is a biological source of two molecules of vitamin A, this relationship is not observed and 6 p.g -carotene is equivalent to 1 p. vitamin A. Although -carotene and vitamin A have complementary activities, they caimot totally replace each other. Because the conversion of -carotene to vitamin A is highly regulated, toxic quantities of vitamin A cannot accumulate and -carotene can be considered as a safe form of vitamin A (8). 

The carotenoid isomerase (CRTISO) was the first isomerase associated with the desaturation steps and named at a time when Z-ISO was unknown to exist ise.ws.ieo.iei (and reviewed in references ). In vitro analysis of substrate conversion " and transcript profiling in planta associated CRTISO with the desaturation steps. Isaacson demonstrated that CRTISO is specific for the 7,9 or 7,9- cis bond configuration and is not involved in the isomerization of the l5-l5-cis double bond to the trans conformation. As recently shown, Z-ISO is required for isomerization of the 15-15 cis double bond of phytoene produced in dark-grown tissues as well as in stressed photosynthetic tissues. Therefore, desaturation of phytoene to lycopene involves a two-step desaturation by PDS, followed l5-cis isomerization by Z-ISO, and then each pair of double bonds introduced by ZDS is followed by CRT-ISO-mediated isomerization of the resulting conjugated double bond pair. 

Little is known of how the biosynthetic metabolon is assembled, what mechanisms control the membrane-specific targeting, and how the conversions to apocarotenoids occur. Yet the current approach to drive import of bacterial or plant genes is to use transit sequences of a stromal protein that may limit the effectiveness of the transgene. In addition, for specific applications of controlling carotenoid composition, we need to better understand the interactions of the various enzymes,... 

More severe conditions, 35 ml of 35% methanolic KOH added to 10 mL extract in EtOAc and shaken for 20 min at 50°C, are necessary for the total conversion of bixin, an ester of a carotenoid acid, to norbixin in snacks. Since saponification yields the norbixin salt (K or Na, depending on the alkali) that is soluble in the aqueous phase, the pH should be decreased to 3.5 or even lower to allow extraction of the protonated norbixin by EtOAc and diethyl ether. ... 

Stewart, L, and Wheaton, T.A., Conversion of 3-citraurin to reticulaxanthin and 3-apo-8 -carotenal to citranaxanthin during isolation of carotenoids from citrus. Phytochemistry, 12, 2947, 1973. 

The work by Hill et al. also noted differences for ASTA compared with the other carotenoids studied. Its radical cation was not formed initially from CC1302 but was formed solely through the proposed addition radical. Unfortunately, LYC could not be studied due to its insolubility in TX 100 micelles. However, since LYC appears, from its quenching of 02 and its protection against N02 to be the most efficient natural carotenoid antioxidant, we repeated this work using 4% TX 405 TX 100 (4 1) mixed micelles for both 0-CAR and LYC (unpublished) and have observed LYC behaving in a different manner to the other carotenoids as there appears to be no conversion of the adduct to the radical cation. 

Nagao, A, 2004. Oxidative conversion of carotenoids to retinoids and other products. J Nutr 134, 237S-240. 

Schmidt R. 2004. Deactivation of singlet oxygen by carotenoids internal conversion of excited encounter complexes. J Phys Chem 108 5509-5513. 

Tanumihardjo SA. 2002. Factors influencing the conversion of carotenoids to retinol bioavailability to bioconversion to bioefficacy. Int J Vitam Nutr Res 72 40-5. 

Ethylene coordinates the expression of genes responsible for enhanced respiratory metabolism, chlorophyll degradation, carotenoid synthesis, conversion of starch to sugars, increased activity of cell wall-degrading enzymes, aroma volatile production, and so on. All these events stimulate a series of biochemical, physiological, and structural changes making fruits mature and attractive to the consumer. 

Sin< recognition that antenna carotenoids have a low lying Sj electronic state it has l en generally assumed that EET to chi proceeds from this state. This is based largely on two considerations. 1. Spectral overlap between the carotenoid Si state and the Qy chi absorption transition is more favourable than for the S2 state. This assumption is however not supported by the recent study of Mimuro et al. [204] in which spectral overlap can be shown to be rather similar for a number of mainly Si and mainly Sj emitters. This point may therefore need further clarification. 2. Extremely fast Sj-Si internal conversion. Recent studies based on fluorescence yield measurements and also on femtosecond absorption measurements [200,205,207] indicate in vitro Sj-Si relaxation times of... 

---