Mayank Tandon, Thirumeignanam, D. and Dr. S.N. Rai. Ferric Reducing

Antioxidant Power (FRAP) Assay. Dairy Cattle Nutrition Division, N.D.R.I.,

Karnal, India.

ESTIMATION OF TOTAL ANTIOXIDANT ACTIVITY

Total antioxidant activity is measured by ferric reducing antioxidant power (FRAP)

assay of Benzie and Strain (1999). FRAP assay uses antioxidants as reductants in a

redox-linked colorimetric method, employing an easily reduced oxidant system present

in stoichiometric excess.

Principle

At low pH, reduction of ferric tripyridyl triazine (Fe III TPTZ) complex to ferrous form

(which has an intense blue colour) can be monitored by measuring the change in

absorption at 593nm. The reaction is non specific, in that any half reaction that has

lower redox potential, under reaction conditions, than that of ferric ferrous half reaction,

will drive the ferrous (Fe III to Fe II) ion formation. The change in absorbance is

therefore, directly related to the combined or “total” reducing power of the electron

donating antioxidants present in the reaction mixture.

Reagents

FRAP Reagent

a) Acetate buffer 300 mM pH 3.6: Weigh 3.1g sodium acetate trihydrate and add

16 ml of glacial acetic acid and make the volume to 1 L with distilled water.

b) TPTZ (2, 4, 6-tripyridyl-s- triazine) (M.W. 312.34) 10 mM in 40mM HCl

(M.W. 36.46)

c) FeCl3. 6H2O (M.W. 270.30) 20 mM

The working FRAP reagent was prepared by mixing a b & c in the ratio of 10:1:1 at the

time of use.

Standard: Ascorbic Acid (M.W. 176.13) 1000 µ M

Procedure

Sample (100 µl) (Plasma/ Milk/ Urine/ Feed Extract etc.) is mixed with 3 ml of working

FRAP reagent and absorbance (593 nm) is measured at 0 minute after vortexing.

Thereafter, samples are placed at 370C in water bath and absorption is again measured

FRAP Assay 1

after 4 minutes. Ascorbic acid standards (100µM-1000µM) were processed in the

same way.

Solutions Blank Standard Test

Sample (Plasma/

Milk/ Urine/ Feed

Extract etc.)

-- -- -- -- 100 μl

Standard

(Ascorbic acid)

-- -- 100 μl -- --

Working FRAP

Solution

3000 μl 3000 μl 3000 μl

- Mix well

- Blank the analyzer/ spectrophotometer with Blank

- Measure the OD of Standard and Test at zero minute and again after four

minutes at 593 nm.

Calculation: Results were calculated as follows.

FRAP value of Sample (µM)

= (Change in absorbance of sample from 0 to 4 minute / Change in absorbance of standard

from 0 to 4 minute) X FRAP value of standard (1000 µM)

Note: FRAP value of Ascobic acid is 2.

Reference: Benzie, F.F. and Strain, J.J. 1999. Ferric Reducing/ Antioxidant Power Assay: Direct

Measure of Total antioxidant Activity of Biological Fluids and Modified Version for Simultaneous Measurement of Total Antioxidant Power and Ascorbic Acid Concentration. Methods in enzymology. vol. 299:15-23.

FRAP Assay 2