Lycopene isomeric forms

Isomeric Forms of Lycopene and Factors Influencing Their Presence and Distribution... 

All-trans lycopene is rapidly isomerized to an equilibrium mixture with its cis isomers both in cell culture medium (Liu et al. 2006) and in vivo in prostate tissue (Clinton et al. 1996, van Breemen et al. 2002). The cis isomers of lycopene are absorbed better than the all -trans isomers when fed to humans (Unlu et al. 2007). The 5 -cis isomer predominates in plasma (Gustin et al. 2004). Since lycopene absorption by prostate cells might be due to facilitated diffusion (Liu et al. 2006), it is likely that the cis isomers of lycopene form a significant proportion of intracellular lycopene in the experiments that are reviewed later. 

Phytoene can be converted to the carotenes by pathways indicated in part in Fig. 22-5 and Eq. 22-10. One of the first products is lycopene, the red pigment of tomatoes and watermelons, which is an all-trans compound. If 15-Z phytoene is formed, it must, at some point, be isomerized to an all-E isomer, and four additional double bonds must be introduced. The isomerization may be nonenzymatic. The double bonds are created by an oxygen-dependent desaturation, which occurs through the trans loss of hydrogen atoms. 

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

With very few exceptions, the natural configuration of lycopene in plants is all-trans. Thermodynamically, this corresponds to the most stable configuration (Emen-hiser et al., 1995 Wilberg and Rodriguez-Amaya, 1995). Upon exposure to high temperatures, light, catalysts, or active surfaces, seven of the double bonds of lycopene can isomerize to the less stable mono- or poly-civ- conformations. Stereoisomeric forms of lycopene have also been described with special emphasis on the light absorption properties of different molecular structures (Zechmeister, 1962). 

Lycopene is known to exist in a variety of geometric isomers, including all-trans, mono-di, and poly-d,v forms. The basis of our understanding of lycopene susceptibility to isomerization stems from early research by Zechmeister and his collaborators (1962). 

Upon exposure to thermal energy, absorption of light, or involvement in specific chemical reactions, interconversion of lycopene isomers is known to take place. The cis isomers of lycopene, formed by rotation around any of its conjugated double bonds, have chemical and physical characteristics distinctly different from their all-frans counterparts. Some of the differences resulting from trans-to-cis isomerization include lower melting point, decreased color intensity, a shift in the lambda max, smaller extinction coefficients, and the appearance of a new maximum in the ultraviolet spectrum. ... 

The desaturase (and following cyclase) enzymes are integral membrane proteins that can be solubilized by the detergent CHAPS. Enzymatic activity is lost when the enzymes are solubilized, but it is restored when reconstituted in proteoliposomes (Britton, 1993). In the first phase of desaturation, the conversion of (15Z)-phytoene into (15Z)- -carotene occurred in the dark, with oxygen as an essential requirement. The reaction was independent of cofactors such as NADP and NADPH. (15Z)- -Carotene could not be desaturated further, but when this compound was isomerized by light, the isomeric mixture formed underwent further desaturation to produce lycopene, not... 

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