4.4. DNA damage inhibitory effect of unfermented and
fermented red brown rice
The results of the nicking assay on unfermented rice show that it did
not possess sufficient antioxidants, which could scavenge the hydroxyl
radical and thus protect the DNA from oxidative damage. The result
was in line with previous tests where unfermented rice had low TPC,
total flavonoid content and low antioxidant activity in DPPH radical
scavenging assay.
Unfermented red brown rice showed weak DNA damage inhibitory
effect whereas fermented red brown showed distinctive results. The
ability of fermented red brown rice extract to protect the DNA from
hydroxyl radical damage was likely due to the presence of rich
polyphenol compounds in fermented red brown rice after microbial
fermentation. Polyphenol compounds are active antioxidants, which
could scavenge the hydroxyl radicals and thus protect the DNA
from oxidative damage[22]. However, once the extract concentration
increased to a point beyond 60 mg/mL, it lost its DNA damage inhibitory effect. This was in contrary to popular belief that increase
in antioxidant concentration could prevent the DNA from oxidative
damage. The reason of this could due to the nature of antioxidants.
Antioxidants contained many loose electrons around their structure.
This feature allows them to neutralize radical species by transferring
the electron to radical species and stabilizing them. This mechanism
is known as dissociative electron transfer. In Lu et al. study[23], it
was found that excessive antioxidants might cause reductive damage
to DNA. When the amount of radical species was scarce, excessive
antioxidants might transfer their electron to nucleotides bases
especially guanine base and induce it to radical guanine species
which led to chemical bond breakage. Ultimately, this would induce
irreversible nucleotide breakage and DNA damage.
This study demonstrated that the polyphenol content, radical
scavenging ability and DNA damage inhibitory effect of Sarawak local
red brown could be enhanced by microbial fermentation. The study
results suggested that fermented red brown rice could be consumed as
a source of natural antioxidants. However, further investigations are
required to validate the detailed effects of fermented red brown rice for
human consumption.
Fermented rice had higher polyphenol compounds (phenolic content
and flavonoid content) than unfermented rice. TPC in fermented rice
was enhanced 5 times by fermentation process. Total flavonoid content
in fermented rice was enhanced 2.5 times. In addition, DPPH radical
scavenging activity in fermented rice was higher than in unfermented
rice. Fermented rice extract showed DNA damage inhibitory effect
to a certain extent. It might protect DNA from ROS; however, at high
concentration it might induce reductive damage to DNA.