ABSTRACT

The antioxidant activity of the seed extract and fractions of Monodora tenuifolia (Fam. Annonaceae) was evaluated. The Monodora tenuifolia seed was extracted with pet ether 40-60°C to produce the crude extract. Fractionation of the extract by column chromatography using pet ether 60-80°C and diethyl-ether produced 2 fractions (F1) and (F2). Phytochemical analysis of Monodora tenuifolia seed extract showed the presence of some plant secondary metabolites, viz: alkaloids, flavonoids, proteins, carbohydrates, saponins, glycosides, cyanogenic glycosides, cardiac glycosides, tannins, steroidal aglycon while, 0 and C glycosides, anthracene glycosides and reducing sugar were absent. The 3 fractions showed the presence of vitamin A and vitamin El The pet-ether extract and the fractions (F1 and F P ) reduced CCle-induced lipid peroxidation in rat liver homogenate. They also exhibited significant antioxidant activity in nitric oxide induced lipid peroxidation. The crude extract and diethylether fraction (F2) produced dose-dependent protective effect against lipid peroxidaiton and free radical generation in liver homogenate. The acute toxicity study with the crude extract showed no signs of obvious toxicity up to a dose level of 5000 mglkg. These results suggest that Monodora tenuifolia seed extract possessed significant antioxidant properties and could be used for the treatment of diseases associated with free radical generation.

CHAPTER ONE

1.I         Antioxidant: An overview

Antioxidants are a group of substances, which when present at low concentrations, in relation to oxidizable substrates, significantly inhibit or dctlay oxidative processes, while often being oxidized themselves (Kanner et al., 1999).

The application of antioxidants are widespread, in industries they are used in preventing polymer from oxidative degradation, rubber and plastic from losing strength, gasoline from autooxidation, synthetic and natural pigments from discolouration and as additives to cosmetics, food (especially food with high fat content) beverages and baking products (Kanner et a/, 1999).

In recent years there has been an increase in the application of antioxidant in medicine as information is constantly gathered linking the development of human diseases to oxidative stress (Halliwell et a/., 1999). The generally accepted hypothesis in any biological system is that, an important balance must be maintained between the formation of reactive oxygen and nitrogen species (ROS and RNS, respectively). The reactive species such as superoxide (02)) hydrogen peroxide (H202), hydroxyl radical (OH), nitrogen oxide (NO), and hypochlorous acid (HOCI), are all products of normal metabolic pathways of human organs, but under certain condition, when in excess they can exert harmful effects.

To maintain an oxido/redox balance, organs protect themselves from the toxicity of excess ROSlRNS in different ways, including the use of endogenous and exogenous antioxidants.

OXIDATIVE STRESS                                          DEFENSE SYSTEM

I                                                                                             I

- Mitochondria ' - Peroxisomes * -Inflammatory Cells

- Radiation

- Ozone

- Xenobiotics

- ~araoxonase'

- Glutathione-

peroxidase

- Vitamin C - Flavonoids - Glutathione

-Superoxide

dimutase

Fig. 1: Shows a balance between oxidative stress and defense system

.            11.1  Natural antioxidant of low and high molecular weight

Naturally occurring antioxidants of high or low molecular weight, can differ in their mechanism and site of action (Sahart, 2001). They can

be divided into the following categories: -

(a)         Enzymes

(b)         High molecular weight proteins

(c)         Low molecular weight antioxidants

a)               Enzymes: The best studied cellular antioxidants are the enzymes, superoxide dimutase (SOD), catalase and glutathione peroxidase (GPx). These attenuate the generation of reactive oxygen species by removing potential oxidants or by transforming ROSIRNS into relatively stable compounds. SOD, which was discovered in the late 60s, catalyzes the transformation of the superoxide radical into hydrogen peroxide, which can then be further transformed by the enzyme catalase into water and molecular oxygen (Sahart, 2001). Glutathione peroxidase (GPx) reduces lipid peroxides (ROOH), formed by the oxidation of poly-unsaturated fatty acid (PUFA) to a stable, non-toxic molecule hydroxyl fatty acid (ROH) (Sahart, 2001). Less well studied (but probably just as important) enzymatic antioxidants are the peroxiredoxins and the recently discovered sulfiredoxin. Other enzymes that have antioxidant properties (though this is not their primary role) include Paraxonase, Glutathione - strainsferases, and aldehyde, dehydrogenases (Current Medicinal Chemistry, 2005).

'            b) High molecular weight proteins: These preventive antioxidants hinder the formation of new ROS. These antioxidants are protein that binds ROS to protect essential proteins. The group includes albumin, metallothonine, transferring, ceruloplasmin, myoglobin, happtoglobin and ferritin (Current Medicinal Chemistry, 2005).

These are all present in plasma and bind to redox active metals and limit the production of metal - catalyzed free radicals (Current

Medicinal Chemistry, 2005). Metals such as iron, copper, chromium, vanadium and cobalt are capable of redox cycling in which a single electron may be accepted or donated by the metal (Current Medicinal Chemistry, 2005). Albumin and ceruloplasmin can bind copper, ions, and transferin binds free iron. Haptoglobin binds heme-containing protein and can thus clear them from the circulations (Current Medicinal Chemistry, 2005). Both free and heme associated protein have pro-"oxidant properties due to their reaction with H202to form ferry1 speci