Phytochemicals and Bioactive Compounds in Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 59

Carotenoids
Gap junctions are cell-to-cell channels which enable con-
nected cells to exchange low-molecular-weight compounds
like nutrients and signaling molecules. Induction of inter-
cellular communication via gap junctions can be achieved
with carotenoids and retinoids, and it is correlated with
inhibited cell growth of chemically transformed cells
(Bertram et al., 1991; Zhang et al., 1995). Evidence from
cell culture studies shows that central cleavage products of
carotenoids like retinoic acid derived from β -carotene or
4-oxo-retinoic acid from canthaxanthin are the ultimately
active components triggering gap junction communica-
tion (GJC) (Stahl et al., 2000). The relationship between
carotenoid intake and a reduced incidence of prostate can-
cer, as reported by Giovannucci et al. (1995), found that
prostate cancer risk was reduced by 35% among men with
serum lycopene concentrations in the highest quartile rela-
tive to those in the lowest quartile, with greater reductions
observed for aggressive prostate cancer for which risk was
reduced by over 50% (Giovannucci et al., 1995). Lycopene
was also shown to enhance the expression of connexin43,
a gene encoding a major gap junction protein, thereby up-
regulating GJC and acting as an anticarcinogen (Zhang
et al., 1992).
Animal studies have shown lutein and fucoxan-
thin to have a significant protection effect on 1,2-
dimethylhydrazine (DMH)-induced colon carcinogenesis
in mice (Kim et al., 1998). Many of the putative bio-
logical effects and health benefits by lycopene and other
carotenoids are hypothesized to occur via protection against
oxidative damage (Miller et al., 1996).
Velmurugan and coworkers suggested that lycopene
blocks experimental gastric carcinogenesis by upregulat-
ing GSH (glutathione)-dependent hepatic detoxification
systems, thereby protecting against carcinogen-induced
oxidative damage. They explained that lycopene-induced
elevation of GSH concentration and stimulation of hep-
atic detoxification systems, such as GPx (glutathione
peroxidase), GST (glutathione S-transferase), and GR
(glutathione reductase), offers protection against chemical
carcinogens by scavenging electrophilic moieties involved
in cancer initiation to less toxic products thereby altering
tumor development at extrahepatic sites (Velmurugan et al.,
2001).
Some prospective studies have observed that blood levels

Dietary fiber
The fermentation of oligosaccharides results in lower pH
levels, which are followed by production of nitrogen deriva-
tives like ammonia, which are accompanied by the elimi-
nation of carcinogenic substances.
There is strong evidence from epidemiologic studies and
experimental studies in animals that a diet high in fat and en-
ergy and low in fruits/vegetables and dietary fiber strongly
predisposes humans and animals to development of colon
cancer (Wynder et al., 1996). High dietary fiber intake is
associated with lower risk of colon and rectal cancer, but
fiber alone does not account for this association.

Antioxidant potential
The antioxidant potential of foods can be measured using
different methods, which include oxygen radical absorp-
tion capacity (ORAC), ferric reducing ability of plasma
(FRAP), and 2,2-diphenyl-1-picrylhydrazyl free radicals
(DPPH). Although all these methods are used to measure
antioxidant potential, they usually give differing results be-
cause in theory the measurement of each method is based
on different mechanisms of reaction (Prior and Cao, 1999).
It is therefore recommended that the antioxidant activity
of any given food should be investigated by more than
one method (Aruoma, 2003). The antioxidant potentials of
some fruits extracts are shown in Table 4.4.

Phenolics
In vitro studies have shown that flavonoids can induce a
large variety of mammalian enzyme systems involved in
important pathways that regulate cell division and prolifera-
tion, platelet aggregation, detoxification, and inflammatory
and immune response. Phenolics, particularly flavonoids,
have the ability to act as antioxidants, which affect oxygen-
free radicals and lipid peroxidation. Oxygen-free radicals
and lipid peroxidation are stipulated to be involved in
pathological conditions such as arteriosclerosis, cancer, and
chronic inflammation (Macheix et al., 1990). Thus the an-
tioxidant potential of phenolics renders them capable of
averting these pathological conditions.
Caffeic and ferulic acids, which are well-recognized
food phenolics with antioxidant activity, prevented photo-
oxidative stress in skins, as evaluated in two model ex-
periments, namely, UV radiation-induced peroxidation in
phosphatidylcholine in multilamellar vesicles and scaveng-
ing activity (Maga, 1978).
In a study of different free radical scavenging systems,
it was observed that the fruit skin of jambolao ( Syzy-
gium cumini ) had significant antioxidant activity, which
was attributed in part to antioxidant vitamins, phenolics or

of -carotene are consistently related to a lower risk of

cancer (van Poppel and Goldbohm, 1995). In a three-year
prospective study of 2,569 women, Negri et al. (1996) found
an inverse relationship between risk of breast cancer and
dietary levels of β -carotene, vitamin C, and calcium.

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