Chemical and Physical Properties of Lycopene its Food Sources and Enteral Absorption

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 ,3-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

All-trans lycopene

All-trans lycopene

5-cis lycopene

All-trans ^-carotene

All-trans ^-carotene

All-trans «-carotene

All-trans «-carotene

Lutein

Figure 1 Structures of lycopene and certain other carotenoids found in human blood and tissues.

Lutein

Figure 1 Structures of lycopene and certain other carotenoids found in human blood and tissues.

are less likely to aggregate into crystalline forms. However, these physicochemical differences and their biological consequences have yet to be adequately explored and described.

Of all the most common naturally occurring car-otenoids, lycopene is by far the most efficient in reacting with and quenching singlet oxygen, 1O2, which is a non-free-radical excited and reactive

Table 1 Lycopene content of selected foods

Food category Content as summarized by Clinton (1998) (mg per 100 g wet weight)

Food category Content as summarized by Clinton (1998) (mg per 100 g wet weight)

Fresh tomatoes

0.9-4.2

Canned tomatoes

Tomato sauce

6.2

Tomato paste

5-150

Tomato juice

5-12

Tomato ketchup

10-13

Tomato soup

Grapefruit

3.4

Guava

5.4

Papaya

2-5.3

Watermelon

2.3-7.2

Source: Clinton SK (1998) Lycopene: Chemistry, biology and implication for human health and disease. Nutrition Reviews 56: 35-51.

Source: Clinton SK (1998) Lycopene: Chemistry, biology and implication for human health and disease. Nutrition Reviews 56: 35-51.

form of oxygen. This form of oxygen reacts rapidly with lycopene to yield nonexcited triplet oxygen and excited triplet lycopene. The latter then dissipates its extra energy by solvent interactions, thus regenerating nonexcited lycopene and preserving its original structure by recycling. However, another of its chemical interactions with molecular oxygen appears to result in irreversible oxidation to yield one or more cyclic epoxides, which then probably undergo ring-opening. Nevertheless, there are many unresolved questions about the nature and importance of the many degradation and catabolic pathways that are believed to result in the irreversible destruction of lycopene both in vitro and in vivo.

Lycopene is an essential intermediate in the pathway for synthesis of the ,3-ionone ring-containing carotenoids such as ^-carotene in plant tissues, and in most plant tissues it is present in only minor amounts. However, in a few, including tomato fruit, watermelon, and red grapefruit, this conversion to the ,3-ionone ring products by the enzyme lycopene cyclase is hindered, so that the intermediate carotenoid forms, lycopene, phytoene and phy-tofluene, accumulate instead.

In the US, tomato products provide more than 85% of the total quantity of lycopene consumed by the human population. Mean lycopene intakes in the US are considerably greater than they are in the UK, where the mean daily intake is thought to be less than one-third that in the US, while lycopene intakes in Far Eastern countries such as China and Thailand appear to be much lower still. Wild tomatoes originated in Central America and were introduced into Europe following the opening up of the New World, and were later introduced back into North America from Europe. Because tomatoes are the major source of dietary lycopene in many human populations, some epidemiological studies have been designed on the simplistic assumption that tomato consumption can be used as a general proxy for lycopene consumption, and that any disease associations with tomato consumption can be attributed to the biological effects of lycopene. However, tomatoes also contain significant amounts of other carotenoids, vitamin C, bioflavonoids such as naringenin, and phenolic acids such as chlorogenic acid. Much of the existing epide-miological evidence for possible beneficial effects of lycopene (see below) cannot distinguish unequivocally between the biological effects of lycopene and those of the many other bioactive constituents present in tomatoes.

The bioavailability of lycopene from raw tomatoes is low, but it is greatly increased by cooking or by commercial processing such as conversion to soup, sauce, ketchup, etc., and its availability is also increased by increasing the fat content of the food. Interactions with other carotenoids are complex and have only partly been studied, for instance ,3-carotene in the same dish seems to increase the absorption of lycopene, but large doses of ,3-caro-tene given separately seem to decrease the lycopene content of serum lipoproteins. The contribution of several categories of tomato product to intakes in a recent survey of older people in Britain is shown in Table 2. The strength of the correlation between dietary lycopene intake and blood (serum or plasma) lycopene concentration varies greatly among studies

Table 2 Tomato products consumed by people aged 65years and over in Britain

Categories of tomatoes and tomato products

Percentage of each category consumed

Raw tomatoes

36.2

Processed tomatoes

Soups

8.8

Canned tomatoes

7.0

Grilled

5.4

Fried

3.2

Ketchup

0.4

Tomato-based products

Canned food

29.5

Pizza

2.3

Other

7.1

Total

99.9

Source: Re R, Mishra GD, Thane CW, and Bates CJ (2003) Tomato consumption and plasma lycopene concentration in people aged 65 years and over in a British National Survey. European Journal of Clinical Nutrition 57: 1545-1554. Reproduced with permission from Nature Publishing Group.

Source: Re R, Mishra GD, Thane CW, and Bates CJ (2003) Tomato consumption and plasma lycopene concentration in people aged 65 years and over in a British National Survey. European Journal of Clinical Nutrition 57: 1545-1554. Reproduced with permission from Nature Publishing Group.

and clearly depends on many factors, one of which is the degree of sophistication of the food table values, since subtle differences in food sources and meal composition affect its bioavailability very considerably.

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