Screening of bioactive compounds from Cucurbitaceae family edible plants of Bangladesh – Cucurbita pepo Linn.: A case study
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Textbook
91 Pages
Summary
Medicinal plants are rich sources of bioactive compounds and thus serve as important raw material for drug production. It has been established that those plants synthesize and accumulate some secondary metabolites like alkaloids, glycosides, tannins, volatile oils etc. that may possess a great potential for biological activity and can be a curative agent in therapeutic purposes.
Medicinal plants are also related to modern medicine. Some preparations may also contain substances other than the drug for the purpose of increasing therapeutic effect and decreasing adverse effects. Therapeutic use of plants continued with the progress of civilization and development of human knowledge. Scientists endeavored to isolate different constituents from plants and put them into the biological system and perform pharmacological tests to identify and isolate therapeutically active compounds, which have been used to prepare modern medicine. Most companies are involved in producing synthetic drugs, but the synthetic drugs’ overlong use has started serious side effects to manifest themselves, so even in the developed countries the interest for natural drugs and natural foods is reviving and we may pay attention to our herbs and medicinal plants.
Medicinal plants are also related to modern medicine. Some preparations may also contain substances other than the drug for the purpose of increasing therapeutic effect and decreasing adverse effects. Therapeutic use of plants continued with the progress of civilization and development of human knowledge. Scientists endeavored to isolate different constituents from plants and put them into the biological system and perform pharmacological tests to identify and isolate therapeutically active compounds, which have been used to prepare modern medicine. Most companies are involved in producing synthetic drugs, but the synthetic drugs’ overlong use has started serious side effects to manifest themselves, so even in the developed countries the interest for natural drugs and natural foods is reviving and we may pay attention to our herbs and medicinal plants.
Excerpt
Table Of Contents
Mondal, Shibam: Screening of bioactive compounds from Cucurbitaceae family edible
plants of Bangladesh Cucurbita pepo Linn.: A case study, Hamburg, Anchor Academic
Publishing 2017
PDF-eBook-ISBN: 978-3-96067-697-3
Druck/Herstellung: Anchor Academic Publishing, Hamburg, 2017
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Dedicated
to my parents
Acknowledgements
From the very beginning of my carrier as a student of pharmacy I always wished to do
something new and noble in the drug sector. I always tried to do something innovative which
can lead to discovery of compound.
I started my thesis work to investigate pharmacological activities with HPLC which could be
the first step to a long journey. I hope this preliminary work would lead to further step to
isolate compound through chromatographic technique and to determine structure.
I wish to express my deepest gratitude to my respected teacher and supervisor, Dr. Md.
Golam Hossain, Professor, Pharmacy Discipline, Khulna University, for his inspiration,
constant guidance, valuable suggestions, and unparalleled encouragement made throughout
the course of study, without which this piece of work would not have taken its present shape.
I also like to convey my gratitude to my co supervisor Gazi Md. Monjur Murshid, Associate
professor, Khulna University, Khulna as well as to all respected teachers of pharmacy
discipline whose knowledge and assistance helps this work to be successful.
I wish to give my cordial thanks to Md. Ashikul Islam, Lab. Assistant, Pharmacy Discipline,
Khulna University for his friendly cooperation. I am grateful to the authority of Pharmacy
Discipline for providing all kind of facilities.
I also extend my gratitude towards those who helped me in one way or other during tenure of
my study.
December 2015
The Author
Shibam Mondal
5
CONTENTS
Particulars
Page No.
Abstract
10
Chapter One
Introduction
11-27
1.1
Plant and medicine
11
1.2
Medicinal plants indirect contribution to modern synthetic drugs
12
1.3
Plant kingdom--storehouse of innumerable drugs
12
1.4
Herbal drug research: past and present perspective
13-15
1.5
Herbal drug development: The never ending chronicle
15-16
1.6
Traditional approaches to synthesize drugs from plants
17
1.7
Antioxidant
18
1.8
History of antioxidants
19
1.9
Classification of antioxidants
20-21
1.10 Commonly used antioxidants in foods
21-24
1.11 Plant Preview
25-27
Common Names
25
Botanical Name
25
Scientific classification
25
Morphology
26
Traditional uses
26
Distribution
27
Collection and identification
27
Chapter Two
Literature reviews
28-29
Chapter Three Aims & Objectives 30
Chapter Four Materials & Method 31-60
4.1
Extraction of Cucurbita Pepo Linn.
31-32
Drying and grinding
31
Cold extraction (ethanol extraction)
31
Extraction at a glance
32
Evaporation of the solvent
32
4.2
Phytochemical screening
32-35
4.3
Polyphenol determination by HPLC method
35-38
4.4
Antioxidant activity
39-45
Qualitative Anti-oxidant Activity
39-41
6
Particulars
Page No.
Quantitative Antioxidant Activity
42-43
Hydrogen Peroxide Scavenging Activity
43-44
Nitric Oxide (NO) Scavenging Assay
44-45
4.5
Antioxidant components
45-50
Total Phenolic Content Assay
45-47
Total Flavonoid Content Assay
47-48
Tannin Content Assay
48-50
4.6
Anthelmintic Activity
50-51
4.7
Cytotoxic Activity
51-54
4.8
Antibacterial Activity
54-59
4.9
Analgesic Activity
59-60
Chapter Five
Results
61-80
5.1
Phytochemical screening
61
5.2
Polyphenol determination by HPLC method
61-63
5.3
Antioxidant activity
63-70
Qualitative Anti-oxidant Activity
63-64
Quantitative Antioxidant Activity
64-66
Hydrogen Peroxide Scavenging Activity
67-68
Nitric Oxide (NO) Scavenging Assay
69-70
5.4
Antioxidant components
71-73
Total Phenolic Content Assay
71-72
Total Flavonoids Content Assay
72
Tannin Content Assay
73
5.5
Anthelmintic Activity
74-75
5.6
Cytotoxic Activity
75-77
5.7
Antibacterial Activity
77-78
5.8
Analgesic activity
79-80
Chapter Six
Discussion 81-83
Chapter seven Conclusion 84
Chapter Eight
References 85-89
Appendix 90
7
LIST OF TABLES
No.
Name
Page No.
1.1
Primary antioxidants that are commonly used in foods
20
1.2
Compounds that exhibit secondary antioxidant activity
21
4.1
Parameters of calibration graphs for the eleven phenolic standards.
38
4.2
Solvent system used for TLC
40
4.3
List of bacteria used for screening of antimicrobial activity
56
5.1
Different Phytochemical group tests performed and the results
61
5.2
Contents of polyphenolic compounds in the ethanol extract of
Cucurbita pepo Linn.
63
5.3
DPPH scavenging assay of standard (ascorbic acid).
64
5.4
DPPH scavenging assay of Cucurbita pepo Linn.
65
5.5
UV absorbance for the standard Ascorbic Acid
67
5.6
UV absorbance for the extract of Cucurbita pepo Linn.
67
5.7
Nitric oxide (NO) scavenging assay of ascorbic acid
69
5.8
Nitric oxide (NO) scavenging assay of Cucurbita pepo Linn.
70
5.9
UV Absorbance of gallic acid (standard) at 750 nm
71
5.10 UV absorbance of sample at 750 nm.
71
5.11 UV Absorbance of Quercetin (standard) at 430 nm.
72
5.12 UV absorbance of sample at 510 nm.
73
5.13 Absorbance of gallic acid at 725 nm
73
5.14 Absorbance of ethanol extract of Cucurbita pepo Linn. at 725 nm
73
5.15 Anthelmintic activity of extract of Cucurbita pepo Linn. on the basis
of paralysis & death time.
74
5.16 Brine shrimp lethality bioassay of Cucurbita pepo L.
75
5.17 Brine shrimp lethality bioassay of standard (Vincristine sulphate)
76
5.18 LC50 values
77
5.19 Determination of zone of inhibition
77
5.20 Effects of the crude extract of Cucurbita pepo Linn. at the doses of
250mg/kg and 500 mg/kg-body weight on acetic acid induced
writhing of mice.
79
5.21 Statistical evaluation of the writhing effect
79
8
LIST OF FIGURES
No.
Name
Page No.
1.1
Different drugs isolated from natural source
14
1.2
Chemical structures of -carotene lycopene (carotene)
23
1.3
Chemical structures of Tocopherols and Tocotrienols
24
1.4
Leaves & stems of Cucurbita pepo Linn.
25
1.5
Cucurbita pepo Linn.
26
4.1
A typical chromatogram by ascending technique
41
4.2
Hatchery (bottle method)
53
5.1
HPLC chromatogram of a standard mixture of polyphenolic compounds.
62
5.2
HPLC chromatogram of Cucurbita pepo Linn. extract.
62
5.3
Comparison of TLC plate for Cucurbita pepo Linn. with Standard
(Ascorbic acid) after applying DPPH.
63
5.4
TLC plates for Cucurbita pepo Linn. after applying 10% H
2
SO
4
64
5.5
IC
50
Value of standard Ascorbic acid
65
5.6
IC50 Value of Cucurbita pepo L.
66
5.7
DPPH Scavenging Assay of Cucurbita pepo Linn. (Asorbance vs.
Concentration)
66
5.8
IC
50
determination of Ascorbic acid
67
5.9
IC
50
determination of Cucurbita pepo Linn.
68
5.10 IC
50
values of ascorbic acid and Cucurbita pepo Linn.
68
5.11 IC
50
determination of Ascorbic acid
69
5.12 IC
50
determination of Cucurbita pepo Linn.
70
5.13 Total phenolic content determination of Cucurbita pepo Linn. with the
help of gallic acid standard calibration curve.
71
5.14 Total flavonoid content determination of Cucurbita pepo Linn. with the
help of Quercetin standard calibration curve.
72
5.15 Standard Gallic acid calibration curve
73
5.16 Observation of anthelmintic activity of Cucurbita pepo Linn.
75
5.17 Graphical representation of Lethality Test of Cucurbita pepo L.
76
5.18 Graphical representation of Lethality test of Vincristine sulphate
77
5.19 Zone of inhibition vs. bacterial strain
78
5.20 Effects of ethanolic extract of Cucurbita pepo Linn. on acetic acid
induced writhing in mice.
80
5.21 Percent writhing inhibition of acetic acid induced writhing in mice by the
extract of Cucurbita pepo Linn.
80
9
LIST OF ABBREVIATIONS
µg = Microgram
Abs = Absorbance
Avg. = Average
Conc. = Concentration
DPPH = 2, 2-diphenyl-1 - picryl hydrazyl
EtOH = Ethanol
GAE = Gallic Acid Equivalent
gm = Gram
hr = Hour
Kg = Kilogram
R
2
= The coefficient of determination
MeOH = Methanol
Min = Minute
ml = Milliliter
mm = Millimeter
QE = Quercetin Equivalent
SD = Standard Deviation
UV = Ultra-violet
WHO = World Health Organization
Wt = Weight
SEM = Standard Error Mean
10
Abstract
The current study was conducted to evaluate the antioxidant, anthelmintic, cytotoxic,
antibacterial and analgesic activities of ethanol extracts of leaves & stems of edible herbs
Cucurbita pepo Linn. Phytochemical screening revealed the presence of reducing sugars,
alkaloids, flavonoids, tannins, gums, glycosides and terpenoids. HPLC chromatographic
analysis was performed to reveal the presence of antioxidant compounds. p-Coumaric acid,
rutin hydrate, ellagic acid, and quercetin were found in the HPLC chromatographic analysis.
Antioxidant activity was evaluated by DPPH, NO and H
2
O
2
radical scavenging assays. With
regard to IC
50
, scavenging of DPPH, H
2
O
2
and nitric oxide were 30, 110 and 65 g/ml
respectively. After qualitative indication of the presence of antioxidant components, the
content of the major antioxidant components as total phenolic contents was found 17.49 mg
GAE/g of dry extract, total flavonoid contents of was found 25.43 mg Quercetin equivalent/g
of dry extract and total tannin content was found 1.30
mg GAE/g of dried plant material. It
showed antibacterial activity against selected bacterial strains. In brine shrimp lethality
bioassay, vincristine sulphate showed LC
50
at 0.36
g/ml whereas the extract showed LC
50
at
80
g/ml comparable to that of standard. In analgesic activity test Cucurbita pepo Linn.
extract, showed a significant inhibition (47.22%, 57.39%)(p<0.001) was produced at the
dose of 250 mg/kg, 500 mg/kg extract compared to control. . In case anthelmintic test,
ethanol extract concentration of 100 and 200 mg/ml showed paralysis at 11.97, 7.77 min and
death time found at 17.53, 9.58 min respectively (p<0.001).
Key words: Cucurbita pepo Linn., phytochemical screening, antioxidant activity, analgesic
activity, antibacterial activity
.
11
CHAPTER ONE
Introduction
1.1 Plant and medicine
Disease is as old as life itself, and man has always been in search of agents to cure diseases.
Plants and herbs have been in use for eradication of diseases and human sufferings since
antiquity. Man has been experimenting with the plants; some, he has used for food, others for
his dress, and stills others for treatment of diseases and to keep himself in a state of health.
Therapeutic uses of plants had in effect stored at the very beginning of human life on earth
when the primitive man, out of necessity and by intuition, resorted to using plants to alleviate
his sufferings from injuries and diseases. The medicinal plants have been used in traditional
medicine for hundreds of years with reputation as efficacious remedies although these may
not sufficient scientific data to substantiate their efficacy. Of these, surprisingly large number
are still of importance in modern medicine.
Medicinal plants are rich sources of bioactive compounds and thus serve as important raw
material for drug production. It has now been established that the plants synthesize and
accumulate some secondary metabolites like alkaloids, glycosides, tannins, volatile oils etc
that may possess a great potential for biological activity and can be a curative agent in
therapeutic purposes.
Medicinal plants are also related to modern medicine. Some preparations may also contain
substances other than the drug for the purpose of increasing therapeutic effect and decreasing
adverse effect. Modern medicine is those preparations which are produced scientifically by
using modern technology and know how are, which are in current use in the modern
pharmacopoeias for the care of health and management of diseases. Therapeutic use of plants
continued with the progress of civilization and development of human knowledge. Scientists
endeavored to isolate different constituents from plants and put them into biological system
and perform pharmacological tests to identify and isolate therapeutically active compounds,
which have been used to prepare modern medicine. Some manufacturing company involved
in producing the synthetic drugs, but the synthetic drugs over long use have started
manifesting serious side effects, so even in the developed countries the interest of natural
drugs and natural foods is reviving, so we may pay an attention to our herbs and medicinal
plants.
12
1.2 Medicinal plants--indirect contribution to modern synthetic drugs
Plants have contributed and are still contributing to the development of modern synthetic
drugs and medicine in a number of ways as stated below:
Novel structures of biologically active chemical compounds, isolated from
plant sources, often prompt the chemist to synthesis similar or better semi-
synthetic compounds.
Synthetic drugs with similar or more potent therapeutic activity are often
prepared by structural modification of the plant-derived compounds with
known biological activity.
Various analogues and derivatives of plant constituents with similar or better
pharmacological actions and therapeutic properties are often prepared by
chemists for use as potent drugs.
1.3 Plant kingdom--storehouse of innumerable drugs
Plant kingdom, the storehouse of thousands of unexplored compounds, possesses a great
potentiality for drug search even in the day of synthetic chemistry. The following data shows
how the plant kingdom enriches the modern medicinal practice:
About 33% of the drugs produced in the developed countries are derived from
plants.
An analysis of over 300 million prescriptions for the year 1960 revealed that
47 percent were for drugs of natural origin, mostly antibiotics.
In the United States, in 1980 alone, the consumer paid 8 billion dollars for
prescription drugs in which the active ingredients are still derived from plants.
If microbes are added, 60% of the modern medicinal products are of natural
origin.
Surprisingly, this large quantity of modern drugs come from less than 15% of the plants,
which are known to have been investigated pharmacologically, out of 500,000 species of
higher plants growing on earth.
13
1.4 Herbal drug research: past and present perspective
It's really unimaginable how far the modern science has advanced the human civilization.
Revolutionary changes have been and are being brought about in day-to-day life. Apart from
information technology, most miraculous happenings and advancement have taken place in
the biomedical research sector. The complete human genome has been unveiled and this is
sure to inspire the exploration of newer facets of biomedical research in near future. More
and more insights and emphases are being paid to the molecular bases of diseases. A good
number of drugs, either rationally designed or genetically engineered, are coming out each
and every year. Gene based therapy along with recombinant DNA technology has come into
the focus of modern medical research. Progress in science is providing crucial insights into
the underlying mechanisms of disease, opening up a rich field of potential targets for
pharmacological investigation. Day to day creation of new innovative diagnostics and
exploration of exciting molecular targets for new drugs has revolutionized and is still
enriching the human arsenal against maladies.
When such is the scenario of present day research, it wouldn't be unnatural
for anyone if he
turns a bit skeptical about the utility and significance of natural product study. Actually by
dint of modern science our dependence on nature might have lessened to a considerable
extent but our concern about nature and its components has enhanced manifold. Surprisingly
enough a significant portion of the world population still relies largely on natural products
and day by day more interest and importance are being converged to naturopathy,
aromatherapy, alternative medicine etc. This is partly due to dissatisfaction with certain
aspects of orthodox medicine and partly due to a desire to decrease dependency on 'drugs',
seeking a more 'natural' alternative1.
There is no denying the fact that synthetic chemistry has given birth to a lot of present day
pharmaceuticals but a significant number of these synthetic compounds do have their lead
molecules from plant source. According to a study, it was found that of the 520 new drugs
approved by FDA from 1983 to
1994, 30 were natural products and 173 were either semi-
synthetic based on a natural product core or modeled on a natural pharmacophore2.
Structural modification and optimization through the QSAR study of many plant derived lead
molecules resulted in a good number of wonder drugs. For example the chemical structure of
14
cocaine isolated from coca leaves paves the way for synthesizing modern drugs like procaine,
lidocaine, tetracaine and in the laboratory. Also the synthesis of life saving antibiotics-
penicillin needs a natural precursor 6-aminopencillanic acid (6-APA) derived from
Penicillium notatum. Plants can be envisaged as natural chemical factory because numerous
chemicals and secondary metabolites are continually being synthesized within the plants in
ambient temperature and pressure of nature. The laboratory synthesis of the anti-malarial
drug quinine demands an intensive work extending over about half a century.
However cinchona plant does it every day without much difficulty. Till to date, extensive
phytochemical analysis of plants yielded multifarious chemical compounds of different
classes such as steroids, terpenoids, flavonoids, alkaloids, chalcones, glycosides etc.
Clinically used plant metabolites such as taxol isolated from Taxus brevifolia; vincristine and
vinblastine obtained from Vinca rosea Linn. ; Morphine other opoids from poppy plant
(Papaver somniferum) and digoxin derived from Digitalis purpurea are only a few of the
many striking examples of wonder drugs from plant sources.
So the value and importance of plants in curing diseases and developing new drugs has not
yet tarnished in this modern high-tech world. Obviously there has been no exhaustion of what
Fig. 1.1: Different drugs isolated from natural source (Cragg GM et. al., 1997)
15
the plants can offer us to combat ailments. Researchers have researched out that there are
about 375 total drugs of pharmaceutical significance in the rain forests of the world, of which
only one-eighth have been discovered3. Such findings optimistically lend us scope to surmise
that plethora of magic molecules are still there in the plant kingdom, awaiting to be explored
by mankind. That is why Darwin rightly said:
"We only see how little has been made out in comparison with what remains unexplained and
unknown."
1.5 Herbal drug development: The never ending chronicle
Medicine is a progressive science. In every department attempts are being made to replace
empiricism by rationalism and nowhere is this more evident than in the science of
pharmacology and therapeutics. Thus when it is said that a drug like Saraca indica (Asoka) is
useful in menorrhagia or Boerhavia diffusa (Punarnava) in dropsy, the profession will not
accept these assertions, as these are symptoms and signs but not diseases; what is wanted to
know is their particular value in these various conditions and how they help to restore the
homeostasis within. The scientific mind is not satisfied by mere statements, no matter from
what source they originate, unless corroborated by clinical and experimental evidences4.
Herbal medicines have certain indications and side effects. The correct use of herbs requires a
clear understanding of how they work. Although most herbs are safe but they can cause
problems. Ancient people knew that certain plants had indisputable healing power. But they
could not explain how the plant's medicinal power worked, how much of the leaves or roots
or the selected plant part should be used, how long after harvesting should they wait before
using the plants or how long should they boil the plant parts for making a decoction.
Selection of medicinal plants by early man, without any prior knowledge about them, was
largely based on intuition, guesswork or trial and error. Curiosity and search for food had
contributed considerably to his knowledge about the plants and their beneficial role.
Superficial resemblance between a specific plant part and the affected organ or some
symptoms of the ailment
had also played some guiding role in his plant selection for curing
medicinal purpose. Observation of the other lower animal's instinctive discrimination
between toxic and palatable, innocuous plants might have played important role in this
respect.
16
Paracellus was the pioneer to consider about the posology- the science about the dose of
medicine. Pointing to the plant's inherent chemical composition he argued that herbal drug, if
used in crude form might kill a lot for curing a little. Actually the scientific curiosity
awakened during Renaissance. But the pace of medicinal discovery had a quantum leap in the
19th century. With the invention of microscope, the germ theory of disease, the use of
technology brought a turn to the understanding of how the human body functions, what
disease is and how it is cured. The chemists also learned to isolate the active substances from
plants. Since then herbal medicine entered into a new era of massive experimentation. Plant
extracts, which were once used as traditional beliefs started to get a scientific basis.
Since plant extract may contain a number of ingredients, each of them may have some effects
on the living body. On the other hand, a plant extract may contain a chemical component in
such a low concentration that may not serve the intended therapeutic interest. Again the crude
extract may have a number of chemical constituents performing the same therapeutic role.
Ingestion of such an extract may cause severe side effects due to the synergistic action of the
constituents. So the use of herbal drugs in crude form may be ineffective or even it may cause
toxicity. A proper example is the traditional use of periwinkle plant which was intended for
treating diabetes. Phytochemical and biological investigation of the plant indicated an
insignificant yield of an alkaloid having antidiabetic property. Rather, the presence of
anticancer alkaloids such as Vincristine, Vinblastine, Vinrosedine and Vineleurosine made
this plant an important source for developing anticancer drug Vincristine in large scale1.
However the work of plant analysis and research is still on the run. The story of medicinal
use of plants is continued to unfold, though in a quiet pace but as an independent way of
scientific research.
17
1.6 Traditional approaches to synthesize drugs from plants
Identification of plant parts
Collection of plants at suitable time and session
Grinding and sieving
Grinded materials are collected and stored in a cool and dry place
Extraction with suitable solvent
Fractionation of extracts
Tests are performed (pharmacological, antimicrobial etc.)
Identification of the chemical structure of active principle
Identification of adverse or beneficial activity
Synthesis of active substance
Drying of plants/plant parts in a suitable size
18
1.7 Antioxidant
Antioxidants are molecules which inhibit the oxidation of organic molecules. They are very
important, not only for food preservation, but also for the defense of living systems against
oxidative stress. Antioxidants are nutrients as well as enzymes. They are believed to play a
role in preventing the development of such chronic diseases as cancer, heart disease, stroke,
Alzheimer's disease, Rheumatoid arthritis and cataracts. In biological systems, the
antioxidants are defined as "any substance that when present at low concentrations compared
to those of an oxidizable substrate significantly delays or prevents oxidation of that
substrate." This covers all oxidizable cellular substrates, i.e., above-mentioned lipids,
proteins, DNA, and carbohydrates. The concept of antioxidants relates to what are
collectively termed reactive oxygen species (ROS). Reactive oxygen species (free radicals)
are a natural by-product of human metabolism. Because oxygen bounds in each of our cells,
it can sometimes pick up electrons from the enzymes that are naturally present to break down
nutrients from our food, giving rise to the various types of ROS. These species are called
"reactive" because of the ease with which they react with and damage crucial cellular
molecules including DNA, proteins and fats. This is where antioxidants come in: they can
neutralize ROS. They are able to achieve this by interacting with ROS and becoming
oxidized them, generating a harmless oxygen compound in the process. The oxidized
antioxidant can then be recycled or simply disposed of. Free radicals contribute to more than
one hundred disorders in humans including atherosclerosis, arthritis, ischemia and
reperfusion injury of many tissues, central nervous system injury, gastritis, cancer and AIDS.
Free radicals can easily damage the structural and functional components of the cells such as
lipids, proteins, carbohydrates, DNA and RNA. Oxidation process is one of the most
important routs for producing free radicals in food, drugs and even living systems. They are
generally byproducts of various endogenous processes that can be stimulated by external
factors such as air pollution, irradiation, smoking, stress and toxins present in food and/or
drinking water.
19
1.8 History of antioxidants
Use of substances to enhance quality of food by means of delaying lipid oxidation has been in
practice for centuries, although it was not chemically defined or understood. The first
recorded scientific observation on oxidation inhibitors came from Berthollet in 1797. Their
theory was described as ``catalyst poisoning'' in oxidative reactors, and this was well before
the free radical theory of peroxidation had been proposed. The earliest reported work on the
use of antioxidants to retard lipid oxidation appeared in 1843, in which Deschamps showed
that an ointment made of fresh lard containing gum benzoin or populin did not become rancid
as did the one with pure lard. Interestingly, the first reports on antioxidants employed for
food lipids were about using natural sources; in 1852, Wright reported that elm bark was
effective in preserving butterfat and lard. Moureu and Dufraise first reported the possibility of
using synthetic chemicals, especially phenolic compounds, to retard oxidative decomposition
of food lipids. Their work provided the basic information leading to theories of lipid
oxidation and antioxidants, which they referred to as ``inverse catalysis.'' Systematic
investigation of antioxidant activity based on the chemistry of radical chain peroxidation of
``model'' chemicals was reported by Lowry and his collegues and Bolland and ten have of
the British Rubber Producers Research Association. Antioxidant synergism in food was first
reported by Olcott and Mattill and this was significant in achieving oxidative stability in food
by using a combination of antioxidants found in the unsaponifiable fraction of oils. They
described the antioxidants as inhibitors and grouped them into acid type, inhibitols, and
hydroquinone and phenolics. Since the early 1960s, the understandings of autoxidation of
unsaturated lipids and antioxidative mechanisms have advanced significantly as a result of
development of effective analytical tools. Around the world a revival is seen in studying the
natural antioxidants in foods and the potential health benefits of natural antioxidants in
relation to prevention and therapy of oxidative stress and related diseases. Enough scientific
evidences have already been accumulated in relation to these conditions with free radicals
and reactive oxygen species. The quest for understanding the oxidation of lipids and its
prevention and control has continued since historical times and is still on.
20
1.9 Classification of antioxidants
Antioxidants may be broadly grouped according to their mechanism of action:
a) Primary or chain breaking antioxidants
b) Secondary or preventive antioxidants
Primary antioxidants
Primary antioxidants are also referred to as type 1 or chain-breaking antioxidants. Because of
the chemical nature of these molecules, they can act as free radical acceptors/scavengers and
delay or inhibit the initiation step or interrupt the propagation step of autoxidation. Below the
figure illustrates possible events that primary antioxidants may interfere along the lipid
autoxidation pathway. Primary antioxidants cannot inhibit photosensitized oxidation or
scavenge singlet oxygen.
Table 1.1: Primary antioxidants that are commonly used in foods.
Natural
Synthetic
Carotenoids
Flavonoids
Phenolic acids
Tocopherols and tocotrienols
Butylatedhydroxyanisole (BHA)
Butylatedhydroxytoluene (BHT)
Ethoxyquin, Propyl gallate (PG)
Tertiary-butylhydroquinone (TBHQ)
Secondary antioxidants
Secondary antioxidants are also classified as preventive or class II antioxidants. They offer
their antioxidant activity through various mechanisms to slow the rate of oxidation reactions.
The main difference with primary antioxidants is that the secondary antioxidants do not
convert free radicals into stable molecules. They act as chelators for prooxidant or catalyst
metal ions, provide H to primary antioxidants, decompose hydroperoxide to nonradical
species, deactivate singlet oxygen, absorb ultraviolet radiation, or act as oxygen scavengers.
They often enhance the antioxidant activity of primary antioxidants. Table-1.2 provides
examples of some of these compounds that exhibit secondary antioxidant activity
Details
- Pages
- Type of Edition
- Erstausgabe
- Year
- 2017
- ISBN (PDF)
- 9783960676973
- File size
- 4.5 MB
- Language
- English
- Institution / College
- Khulna University – Life Science
- Publication date
- 2017 (September)
- Grade
- 3.75
- Keywords
- antioxidant anthelmintic cytotoxic antibacterial analgesic ethanol extract Pumpkin Phytochemical screening
