Oxidation of /-carotene

The oxidation of carotenes results in the formation of a diverse array of xanthophylls (Fig. 13.7). Zeaxanthin is synthesised from P-carotene by the hydroxylation of C-3 and C-3 of the P-rings via the mono-hydroxylated intermediate P-cryptoxanthin, a process requiring molecular oxygen in a mixed-function oxidase reaction. The gene encoding P-carotene hydroxylase (crtZ) has been cloned from a number of non-photosynthetic prokaryotes (reviewed by Armstrong, 1994) and from Arabidopsis (Sun et al, 1996). Zeaxanthin is converted to violaxanthin by zeaxanthin epoxidase which epoxidises both P-rings of zeaxanthin at the 5,6 positions (Fig. 13.7). The... 

Grosch, W. and Laskawy, G., Co-oxidation of carotenes requires one soybean lipoxygenase isoenzyme, Biochim. Biophys. Acta, 575, 439, 1979. 

Coupled air oxidation of carotene and unsaturated fatty acids... 

In many intermediate-moisture foods, most chemical reactions will not be diffusion limited. In low-moisture foods (e.g., uw <0.2), however, the most important cause of chemical stability will generally be small diffusivity. This would apply to all examples in Figure 8.10, except for oxidation of carotene. A relation like that in Figure 8.10d is generally observed for oxidation reactions, where water is not a reactant, and even at awx 0 perceptible diffusion of 02 may occur see further point 6, below. 

Hydroxylation of polyolefins by Milas reagent can be so arranged that only one double bond is affected e.g., cyclopentadiene gives a mixture of 3-cyclo-pentene-l,2-diol and 4-cyclopentene-l,3-diol.139,143 Another application of this method was the selective oxidation of / -carotene to vitamin-A aldehyde.144... 

J.-P. Chauvet, R. Viovy, E.J. Land, R. Santus, and T.G. Truscott, One-Electron Oxidation of Carotene and Electron Transfer Involving Carotene Cations and Chlorophyll Pigments in Micelles, J. Phys. Chem., 87 (1983) 592. 

Woodall, A.A., Britton, G., Jackson, M.J., and Weesie, S.W.M. Oxidation of carotenes by free radicals-relationship between structure and reactivity. Biochemica et Biophysica Acta Subjects, 1336, 33 2. 1997. 

Ionones are metabolites of the corresponding carotenoids. As long ago as 1910, Richard WiUstatter [53] observed the oxidation of carotene. It could be shown experimentally that carotene reacted photochemically with oxygen in the absence of a sensitiser to produce ) -ionone and other oxidation products. [54] Also, thermolysis of -carotene produces ji-ionone in significant amounts. [55]... 

Lycopene is the central intermediate for aU the more highly functionalised carotenoids. The ends are cyclised under acid catalysis to a- or ) -carotene. Hy-droxylation of the former yields lutein. The xanthophyUs, (3J ,3 Jl)-zeaxanthin, canthaxanthin and astaxanthin, result, as in the example shown here, from oxidations of ) -carotene (Fig. 7.15). 

Egg PC/cholesterol /9-Carotene state 1527 cm-" resonance band 1400-1600 111527/I14401 reaches minimum at cholesterol/ lipid ratio of 0.9 indicating oxidation of carotene Verma and Wallach (1977)... 

Yamauchi R., Miyake N., Inoue H., Kato K. Products formed by peroxyl radical oxidation of -carotene. Journal of Agricultural and Food Chemistry, 41 708-713 (1993). 

Metabolism—Foods supply vitamin A in the form of vitamin A, vitamin A esters, and carotenes. Almost no absorption of vitamin A occurs in the stomach. In the small intestine, vitamin A and beta-carotene are emulsified with bile salts and products of fat digestion and absorbed in the intestinal mucosa. Here, much of the conversion of beta-carotene to vitamin A (retinol) takes place. There are wide differences in species and individuals as to how well they utilize the carotenoids. Their absorption is affected by several factors, including the presence in the small intestine of bile, dietary fat, and antioxidants. Bile aids emulsification fat must be absorbed simultaneously and antioxidants, such as alpha-tocopherol and lecithin, decrease the oxidation of carotene. Also, the presence of enough protein of good quality enhances the conversion of carotene to vitamin A—a matter of great importance in developing countries where protein is limited in both quantity and quality. 

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