Lycopene is the most commonly encountered of that subgroup of the naturally occurring carotenoids that have a straight-chain poly-isoprenoid molecule without any terminal ^-ionone ring structures (Figure 1). The chain length and number of conjugated double bonds determine the absorption spectrum, which peaks at 472 nm with a molar extinction coefficient, e1/o of 3450. It is one of the most nonpolar members of the carotenoids, and in organic solution it is also one of the most easily oxidized and thus is easily destroyed, which necessitates the use of rigorous precautions against its oxidative destruction during its extraction and analysis from plants, foods, animal tissues, and body fluids. Currently, such analytical determination is usually based on high-performance liquid chromatography (HPLC), using either its characteristic light absorption property, or its natural fluorescence, or its redox character, for detection and quantitation by absorbance or fluorometric or electrochemical detection. Another characteristic that greatly affects its stability and the problems of its storage and analysis is the phenomenon of cistrans isomerization. Naturally occurring lycopene in tomatoes, the major human food source of this carotenoid, is nearly 100% all-trans (Figure 1), but during the processing of food, and then during the processes of absorption and accumulation in animal tissues, there is a progressive increase in the proportion of a variety of cis-forms. Most of these cis-forms contain a single cis-bond (mono-cis-lycopene), and the 5-, 9-, 13- and 15- mono-cis-lycopenes account for more than 50% of the total lycopene in human serum. Smaller quantities of di-cis-lycopenes are normally also present. Curiously, another food source of lycopene, red palm oil, has a much higher natural proportion of the cis-forms of the pigment. Isomerization is catalyzed by low pH; therefore, stomach acid is believed to be a major factor in the conversion of the all-trans-lycopene ingested from tomatoes and their products to a mixture of cis-forms in the digestive tract. There is also evidence that further isomerization occurs between the digestive tract and the portal lymphatic lipid micelles. The cis-isomers differ from the all-trans form in their absorption and intertissue transportation properties, and also in their functional characteristics; for instance, they are more soluble in lipophilic solvents and structures and
Figure 1 Structures of lycopene and certain other carotenoids found in human blood and tissues.
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