Carotenoids metabolic transformations

FIGURE 15.3 Hypothetical pathways responsible for carotenoid uptake, metabolic transformations, tran-scytosis to the neural retin, or secretion to the blood. 

Metabolism. A survey of the metabolic transformation of carotenoids by animals is included in the review by Fox. 4-Hydroxyretinoic acid (187) and 4-oxoretinoic acid (188) have been identified as metabolites of retinoic acid a l-trans and 3-cis) by hamsters, and hamster liver microsomes. ... 

Parry JM, Kadhim M, Barnes W, Danford N (1981) Assays of marine organisms for the presence of mutagenic and/or carcinogenic chemicals. In Dawe CJ, Harshbarger JC, Kondo S, Sugimura T, Takayama S (eds) Phyletic approaches to cancer. Jpn Sci Soc, Tokyo, pp 141-166 Partali V, Tangen K, Liaaen-Jensen S (1989) Carotenoids in food chain studies-III. Resorption and metabolic transformation of carotenoids in Mytilus edulis (edible mussel). Comp Biochem Physiol 92B 239-246... 

It is generally assumed that animals do not have the capability to carry out de novo carotenoid synthesis. However, they occassionally have the ability to modify structurally carotenoids obtained through their diet. The absolute configuration of the carotenoids involved may throw light on such metabolic transformations. 

Schiedt, K., Leuenberger, F.J., Vecchi, M. and Glinz, E. (1985) Absorption, retention and metabolic transformations of carotenoids in rainbow trout, salmon and chicken. Pure and Appl. Chem. 57, 685-692. 

Table 56.6 Important food crops transformed with biosynthetic and regulatory genes for carotenoid metabolism ...

Both intact carotenoids and their apolar metabolites (retinyl esters) are secreted into the lymphatic system associated with CMs. In the blood circulation, CM particles undergo lipolysis, catalyzed by a lipoprotein lipase, resulting in the formation of CM remnants that are quickly taken up by the liver. In the liver, the remnant-associated carotenoid can be either (1) metabolized into vitamin A and other metabolites, (2) stored, (3) secreted with the bile, or (4) repackaged and released with VLDL particles. In the bloodstream, VLDLs are transformed to LDLs, and then HDLs by delipidation and the carotenoids associated with the lipoprotein particles are finally distributed to extrahepatic tissues (Figure 3.2.2). Time-course studies focusing on carotenoid appearances in different lipoprotein fractions after ingestion showed that CM carotenoid levels peak early (4 to 8 hr) whereas LDL and HDL carotenoid levels reach peaks later (16 to 24 hr). 

Figure 9.2. The inherent metabolic flexibility of the isoprenoid pathway leading to the synthesis of some carotenoid pigments. Genes coding for two enzymes capable of acting on carotenoid structures were introduced into Escherichia coli which had already been transformed to give it the capacity to make p,p-carotene. Both of the two introduced new enzymes (one shown with red arrows and the other with blue arrows) acted on multiple substrates because of their lack of specificity. The resulting matrix of transformations means that nine different products can be made by just two tailoring enzymes. (Adapted from Umeno et al. ° who used data from Misawa et al. °)...

Less is known concerning the metabolism of carotenoids than is known about their biosynthesis. Much of the work that has been done in this area has been concerned with animal systems. These studies have dealt mainly with two areas the synthesis of vitamin A from -carotene and pigment transformations in avian species and echinoderms. The latter topic has been extensively reviewed (Thommen, 1971) and will not be discussed here. In addition to the synthesis of vitamin A, brief discussions of two other topics are included. These are the synthesis of trisporic acids in Mucorales and the metabolism of carotenoids in senescent tissues. 

Zhu, C., Naqvi, S., Breitenbach, J., Sandmann, G., Christou, P. Capell, T. (2008). Combinatorial genetic transformation generates a library of metabolic phenotypes for the carotenoid pathway in maize. Proc. Natl. Acad. Sci. USA, 105, 18232-18237. 

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