Zonas para cultivo en limpio con limitaciones por suelo

Natural products usually refer to secondary metabolites which has relatively complex structures. Secondary metabolites are usually characteristic of specific botanical sources in comparison to primary metabolites which occur in almost every plant. A general characteristic of natural products is that few of them have a clearly recognized function in the metabolic activities of the organisms in which they are found (Geissman et al., 1969).

Natural products can be classified into various families, such as alkaloids, terpenoids and phenolics including flavonoids. Natural products perform various functions, and many of them have interesting and useful biological activities (Harvey, 1999). The utility of natural products as a source of novel structures is still alive and well. The number of plants used as medicinal agents in commerce globally is unknown, but there are at least 1,000 species alone in China (Duke and Ayensu, 1985; Abas, 2005).

2.2 Cancer

Cancer is the general term for a series of neoplastic diseases that are characterized by changes in a cell leading to abnormal (unordered and uncontrolled) cellular proliferation (Pettit, 1997). The disorder occurs in the normal processes of cell division, which are controlled by the genetic material (DNA) of the cell. Cancers may be caused in one of three ways, namely incorrect diet, genetic predisposition, and via the environment (Reddy et al., 2003).

10 To date, mortality that results from the common forms of cancer is still unacceptably high. The Second Report of the National Cancer Registry of Malaysia suggested a total of 23,746 cancer cases were diagnosed among Malaysians in the year 2003, comprising 10,473 males and 13,273 females. The crude rate for males was 97.4 per 100,000 population and 127.6 per 100,000 populations for females. The age standardized incidence rate (ASR) for all cancers was 134.3 per 100,000 males and 154.2 per 100,000 females (The Second Report of National Cancer Registry, 2003).

2.2.1 Carcinogens

The majority of human cancers result from exposure to environmental carcinogens; these include both natural and manmade chemicals, radiation and viruses. Carcinogens may be divided into several classes, such as (i) genotoxic carcinogens, if they react with nucleic acids. These can be directly acting or primary carcinogens, if they are of such reactivity so as to directly affect cellular constituents; (ii) alternatively, they may be procarcinogens that require metabolic activation to induce carcinogenesis; (iii) epigenetic carcinogens are those that are not genotoxic (Reddy et al., 2003; Timbrell, 2000). It is also clear that genetic predisposition is one of the factors of human cancers apart from exposure to carcinogens. Thus, patients with the genetic xeroderma pigmentosum are more susceptible to skin cancer (Reddy et al., 2003).

Carcinogens in the diet that trigger the initial stage include moulds and aflatoxins (for example, in peanuts and maize), nitrosamines (in smoked meats and other cured products), rancid fats and cooking oils, alcohol, and additives and preservatives. A combination of foods may have a cumulative effect, and when incorrect diet is added to a polluted environment, smoking, UV radiation, free radicals,

11 lack of exercise, and stress, the stage is set for DNA damage and cancer progression (Reddy et al., 2003).

2.2.2 Cell cycle

In cells that are dividing, the nuclear DNA molecules must be duplicated and then distributed in a way that ensures the two new cells receive a complete set of genetic instructions. The cells pass through a series of discrete stages called G1 phase, S phase, G2 phase and M phase in order to accomplish these tasks (Kleinsmith, 2006). These four phases are collectively referred to as the cell cycle.

The cell cycle is commonly represented by a circular diagram (Figure 2.1). The G1, S and G2 phases are collectively referred as interphase. Besides providing the time needed for a cell to make copies of its DNA molecules, interphase is also a period of cell growth. Interphase occupies about 95 % of a typical cell cycle; whereas the actual process of cell division (M phase) only takes about 5 % (Kleinsmith, 2006). G1 is defined as the interval between M phase and S phase, and G2 is defined as the interval between S phase and M phase. S phase is defined as the time during the cell cycle when DNA synthesis is taking place, leading to a doubling of the amount of DNA per cell. M phase is the time when the amount of DNA per cell drops in half as cells divide. The restriction point is a control point near the end of G1 where the cell cycle can be halted until conditions are suitable for progression into S phase. Under normal conditions, the ability to pass through the restriction point is governed mainly by the presence of growth factors.

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Figure 2.1: The cell cycle

(Adapted from http://herb4cancer.files.wordpress.com/2007/11/cell-cycle2.jpg, 19 August 2009)

2.2.3 Carcinogenesis

The transformation of a normal cell into a cancerous cell is believed to proceed through many stages over a number of years or even decades. Carcinogenesis is a multistep process that involves initiation, promotion and tumour progression. Initiation involves a reaction between the cancer-producing substance (carcinogen) and the DNA of tissue cells. There may be a genetic susceptibility. This stage may remain dormant, and the subject may only be at risk for developing cancer at a later stage. Promotion involves a prolonged period of proliferation of the initiated cells, occurs very slowly over a period ranging from several months to years. During this stage, a change in diet and lifestyle can have a beneficial effect so that the person may not develop cancer during his or her lifetime. The third and final stage involves progression and spread of the cancer, at which point diet may have less of an impact. Preventing initiation is an important anticancer strategy, as are the opportunities to inhibit cancer throughout the latter stages of malignancy (Reddy et al., 2003; Kleinsmith, 2006).

Restriction point

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