We estimate antioxidant activity of a extract with different method like DPPH, FRAP, Molybdenum reducing method and NO scavenging activity etc. which type of compounds responsible for these particular method and what is the importance of these particular method. For the crude plant extract which method should be used for the estimation of antioxidant properties.
For most of the plant products, the compounds responsible for the antioxidant effects are phenolic. Thus a preliminary assessment would begin with the total phenol assay. You can also specifically measure total flavonoid contents as well. Oxidative stress which can be relieved by antioxidants are caused mainly by free radicals. The antioxidants generally scavenge these radicals thus it is important to measure the free radical scavenging activity using DPPH. There are also specific radicals that are involved in oxidative stress such as superoxide or hydrogen peroxide thus you can specifically measure for the scavenging properties of the crude extract on these radicals. Another measurement is the ability of the antioxidant to produce reduction of Ferric ions thus test such as FRAP or Reducing Power Assay are necessitated. Lastly, lipid peroxidation is also a mechanism of oxidative stress, thus the lipid peroxidative assay is important. All these are in vitro models and they do not point to a particular disease model. Some scientist claim that all plants have antioxidant activity so unless this activity plays a beneficial role in a diseased model of oxidative stress, it has no relevance, as such it will be important to measure the antoxidant activity in vivo most especially in a diseased model where excessive free radicals are produced.
For further information read my publication following this link: http://www.sciencedirect.com/science/article/pii/S2231253611130106
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Because each method has variable sublimation and utilization of free radicals. More over, solubility, reagent pH and light also effect free radical utilization and formation of product. Chemical reaction variability is equal to formation of product in reaction that indicate utilization of free radical mu moles/L/s
For most of the plant products, the compounds responsible for the antioxidant effects are phenolic. Thus a preliminary assessment would begin with the total phenol assay. You can also specifically measure total flavonoid contents as well. Oxidative stress which can be relieved by antioxidants are caused mainly by free radicals. The antioxidants generally scavenge these radicals thus it is important to measure the free radical scavenging activity using DPPH. There are also specific radicals that are involved in oxidative stress such as superoxide or hydrogen peroxide thus you can specifically measure for the scavenging properties of the crude extract on these radicals. Another measurement is the ability of the antioxidant to produce reduction of Ferric ions thus test such as FRAP or Reducing Power Assay are necessitated. Lastly, lipid peroxidation is also a mechanism of oxidative stress, thus the lipid peroxidative assay is important. All these are in vitro models and they do not point to a particular disease model. Some scientist claim that all plants have antioxidant activity so unless this activity plays a beneficial role in a diseased model of oxidative stress, it has no relevance, as such it will be important to measure the antoxidant activity in vivo most especially in a diseased model where excessive free radicals are produced.
For further information read my publication following this link: http://www.sciencedirect.com/science/article/pii/S2231253611130106
Thank you
Each method gives you different information about the antioxidant capacity of the tested extract. This includes the mechanism of antioxidant activity (H-transfer like ORAC or Single electron transfer like FRAP, CUPRAC; ABTS and DPPH are somewhere considered to have a mixed mechanism or rather H-transfer), the ability to quench ROS or metal ion reduction. Never the less influence on the activity has the temperature, pH, which could be different in each tested method. Thus you are able to determine the full antioxidant capacity of your extract.
http://www.orac-info-portal.de/_upload/presse/jf0502698-1.pdf
Because there are not universal methods providing universal answers but there are methods related to free radical specificity.
Assessing extract activity by ORAC and lipoperoxidation based methods you obtain a value ascribable to a certain antioxidant activity (meaning an activity related to a phisiological significant oxidisable substrate) while assessing extract efficacy by DPPH, TEAC (ABTS), FRAP, CUPRAC etc you obtain a radical scavenger activity (since you are determining the extract efficacy against quenching generated free radicals or the extract capability to modify the oxidative status of a certain metallic cation, as iron or copper).
Not necessarily a positive radical scavenger proved activity is related to an efficient in vivo antioxidant
This is due to different reaction chemistry involved in different in-vitro assays. The antioxidant activity is usually measured by quantifying the ability of antioxidant compounds (electron donors/reducing agents) to quench the free radicals. The free radicals are very labile species which often generated at uncontrolled rate in reaction medium due to fluctuation in reaction conditions.
To highlight the major one's, the antioxidant compound activity (say for polyphenols) is a function of pH, light and presence of pro-oxidants in reaction medium which often results in variations in the results obtained. Therefore, one should always test the different antioxidant activity assays and try to find correlation among them.
Because there are several mechanism through which antioxidants diminish the activity of reactive species.
Radical scavenging activity is one type of antioxidant potential of plant extract. Here, we are using different type of radicals like DPPH, NO, ABTS etc.. as source of radicals.
But these are not different methods. These are comes under free radical scavenging activity.
For someone to terme an extract has antioxidant activity/capacity, he must have done at least 2 tests.
This is because the antioxidant activity has two stages, the primary and secondary radicals.
If only one is done the it the radical scavenging activity that is done i.e antiradical activity (e.g DPPH; NO etc ).
If two tests are done then its both the primary and secondary then its termed antioxidant (ß -carotene, ferric chelating ion etc ).
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