diss

The TS diet had no effect on growth performance of broilers compared to the C group.
This was in agreement with that of Huang et al. (2014), where no differences were
seen for ADFI, ADG, and F/G in broilers fed diets supplemented with 4 g/kg taurine.
Lu et al. (2018) found that 5 g/kg taurine supplementation did not influence
the growth performance of chronic heat-stressed broilers.In the present study,
there were no significant differences in moisture, protein, or fat in breast muscle
between the two dietary groups, which suggested that TS had no influence on
the nutritional value of poultry meat. In addition to nutritive value, consumer
are concerned with chicken meat quality. Muscle pH directly affects the colour,
tenderness and water-holding capacity (Monin and Sellier, 1985). It has been reported
that dietary supplementation 5 g/kg taurine significantly alleviated any decrease in
pH 45min in breast meat from chronic heat-stressed broilers (Lu et al., 2019).
Similarly, in the current study, increased pH 24h was observed in meat from the TS birds.
A delay in pH decline can lead to a reduction in protein denaturation, which, in turn,
can increase the water-holding capacity of breast muscle (Li et al., 2018).
This explained the decreased drip and cooking losses seen in breast muscle
from the TS fed birds in this study. The present study indicated that TS had a
significant effect on the meat quality of broilers. However, Huang et al. (2014)
found that 4 g/kg taurine supplementation did not affect the meat quality of
breast muscle in male Cobb chickens, which included pH, meat colour and drip loss.
This difference in response may have been due to the higher dose of taurine and
the breed of broiler used. High levels of free radicals, particularly the reactive oxygen
and nitrogen species, can cause oxidative stress to the body and lead to the damage of
biomacromolecule, including proteins, lipids, and nucleic acids
(Young and Woodside, 2001). The OH - , O 2 - ions and DPPH and ABTS are the most
representative free radicals. Taurine exhibited radical scavenging activities against OH -
and O 2 - in previous trials (Cheong et al., 2013). Patel et al. (2016) found
that taurine enhanced the scavenging activities of OH - and DPPH in vitro. In this study
broilers in the TS group exhibited higher scavenging activities of O 2 - , OH - , and
DPPH which suggested that TS significantly enhanced the antioxidant capacity of
breast muscle in broilers.The over production of free radicals constitutes a main
cause of lipid peroxidation, which can produce a large
amount of products and damage tissues, ultimately causing exudation of
intracellular water and increased drip loss (Cong et al., 2017a).
It has been reported that taurine reduced the content of MDA and 4-HNE
in serum of diabetic rats (Haber et al., 2003). Consistently, the current data
showed that the contents of MDA and 4-HNE in breast muscle were decreased
significantly in samples from the TS group Protein oxidation plays an importan
t role in meat quality. Dawson et al. (2000) demonstrated that taurine
significantly reduced the formation of protein carbonyls induced by trivalent
iron and levodopa (a dopamine precursor). In this study, carbonyl was
decreased significantly in the taurine group, which was consistent with
previous findings (Li et al., 2010). The current results indicated that the
beneficial effects of taurine on meat quality might be related to an improvement
in redox status.Antioxidant enzymes are the first line that defend against
free radicals (Rodriguez et al., 2004). It has been found that taurine significantly
increased the activities of SOD, GPx, and CAT of liver in mercury intoxicated rats
(Jagadeesan and Sankarsami, 2007). Similarly, in the current research,
broilers in the TS group exhibited significantly higher activities of T-AOC and SOD t
han those in the control group. This increased SOD activity may be related t
o the fact that taurine can remove hypochlorous acid, through reacting with it t
o form chlorotaurine and protect SOD from oxidative damage (Higuchi et al., 2012).
Moreover, the increased activity of antioxidant enzymes may partly explain why
the scavenging activity of free radicals were enhanced.It is well-known that
the Nrf2 pathway plays an important role in the body from oxidative s
tress (Jaiswal, 2004). A variety of antioxidants have been found to have a
protective effect on cells through the Nrf2 signalling pathway (Zhang et al., 2018).
Sun et al. (2009) observed a significant increase in the mRNA expression of Nrf2
in cells treated with taurine. Similarly, in the current study, taurine supplementation
significantly upregulated the mRNA expression of Nrf2. Nrf2 has been shown to encode
the transcription of phase II detoxification enzymes, including HO-1, NQO1, SOD, CAT,
and GPx (Bai et al., 2017). HO-1 is the rate-limiting enzyme in the degradation of haem
and catalyses the production of carbon monoxide, biliverdin, and ferrous iron,
which protects tissues from oxidative damage (Chiu et al., 2002). NQO1 is an
inducible enzyme, which catalyses two-electron reductions and detoxification of
quinones and its derivatives (Zhang et al., 2008). It has been reported that taurine
can reduce oxidative damage in chronic heat-stressed broilers through activating Nrf2
pathway and upregulate mRNA expressions of HO-1, NQO1 (Lu et al., 2019).
Consistently, the results of the present study showed that broilers in the TS group
exhibited significantly higher mRNA expressions of HO-1, NQO1, SOD, CAT,
and GPx. These results may explain the beneficial effects of TS on antioxidative
capacity in broilers.In conclusion, TS had no effect on growth performance in broilers.
Feeding the TS diet increased pH 24h and water-holding capacity of breast
meat from broilers. This improvement could be attributed to the fact that
taurine enhanced the anntioxidative capacity of breast meat via activating
the Nrf2-ARE pathway.