2.6. Protein carbonyl content measurement
The protein carbonyl content was measured according to the method described by Huang et al. (2006), which uses the reaction of 2,4-dinitrophenylhydrazine (DNPH) with the carbonyl groups of proteins.
Egg white protein solutions were diluted to 4 mg/mL.
In 2.0-mL capped polyethylene centrifuge tubes, 0.35 mL protein solution was mixed with 1 mL 10 mM DNPH in 2 N HCl and incubated at 20 C for 2 h.
Equivalent protein solution was mixed with 1 mL 2 N HCl (absence of DNPH) as a blank.
Then 0.45 mL 40% trichloroacetic acid was added and vortexed and stood for 20 min then centrifuged at 10,000g for 10 min.
The sediment was washed thrice with 1.5 mL ethyl acetate solution (1:1, v/v).
The protein was suspended in 1.0 mL 6 M guanidine hydrochloride solution by being incubated in a water bath (37 C) for 20 min and with vortexed every 5 min.
The absorbance at 367 nm was recorded and the results were expressed as nmoles carbonyl groups per milligram
of soluble protein with molar extinction coefficient of 22,000 M cm.
2.6. Protein carbonyl content measurementThe protein carbonyl content was measured according to the method described by Huang et al. (2006), which uses the reaction of 2,4-dinitrophenylhydrazine (DNPH) with the carbonyl groups of proteins. Egg white protein solutions were diluted to 4 mg/mL. In 2.0-mL capped polyethylene centrifuge tubes, 0.35 mL protein solution was mixed with 1 mL 10 mM DNPH in 2 N HCl and incubated at 20 C for 2 h. Equivalent protein solution was mixed with 1 mL 2 N HCl (absence of DNPH) as a blank.Then 0.45 mL 40% trichloroacetic acid was added and vortexed and stood for 20 min then centrifuged at 10,000g for 10 min. The sediment was washed thrice with 1.5 mL ethyl acetate solution (1:1, v/v). The protein was suspended in 1.0 mL 6 M guanidine hydrochloride solution by being incubated in a water bath (37 C) for 20 min and with vortexed every 5 min. The absorbance at 367 nm was recorded and the results were expressed as nmoles carbonyl groups per milligramof soluble protein with molar extinction coefficient of 22,000 M cm.
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