**3.1.2 Absorption**

Studies using radiolabelled aflatoxin B1 in rats and monkeys have demonstrated little difference in the distribution and excretion of the toxin after either oral or intraperitoneal administration, therefore implying that absorption after oral exposure is complete (Dalezios et al., 1973; Wilson et al., 2002).

Aflatoxin B1 can also be absorbed rapidly, by passive diffusion, from the small intestines (especially the duodenum) into the mesenteric venous blood. Given the lipophilic nature of aflatoxin B1, the composition of the intestinal epithelium is an important criterion. Although the liver is regarded as the main site of aflatoxin transformation, gastrointestinal metabolism will reduce the exposure of the liver to aflatoxin B1 and, in terms of hepatic toxicity, is an important means of detoxification (Hsieh & Wong, 1994; Avantaggiato et al., 2004).

#### **3.1.3 Transformation**

The transformation of aflatoxin B1 results in both the activation and the detoxification of the toxin and may be considered as occurring in two phases (Ben-Ami et al., 2010). Firstly, the transformation of the toxin to a selection of metabolites and, secondly, the conversion of some of these metabolites to either water soluble conjugates or macromolecular adducts (Huffman and Gerber, 2010). The transformation process will be modulated by numerous factors including the genetic make-up of the species, nutritional and health status, and exposure to metabolic modifiers in foodstuffs (Meggs, 2009)

The major metabolites of aflatoxin B1 includes aflatoxin B1 -8,9-epoxide, -8,9-dihydro-8,9 diol;the aflatoxins-B2a, -P1 , M1 -Q1; aflatoxicol, aflatoxicol H1 and aflatoxicol M1 (Essigmann et al., 1982; Klich, 2009)(Figure 7). However, not all metabolites have been identified in all species.

#### **3.1.4 Activation**

In the liver, aflatoxin B1 may interact with both DNA and proteinto elicit the carcinogenic and acutely toxic effects of aflatoxin, respectively. Initially, aflatoxin B1 is converted, by cytochrome P450, to the highly reactive aflatoxin B1 -8,9-epoxide which in turn may be

Many animal studies in order to study the metabolic fate of aflatoxins, *in vivo* and *in vitro*  studies using animal tissues have been conducted mainly on aflatoxin B1. Limited studies have also been performed on humans involving the measurement of aflatoxin B1, and its metabolites, in blood, urine, milk and isolated tissues. Metabolism has been studied in many species and under many different conditions (Dalezios et al., 1973, Yunus et al.,

The metabolic fate of aflatoxins may be considered under the headings of administration, absorption, transformation (activation and detoxification), distribution and excretion.

Under natural conditions, exposure to the aflatoxins may occur orally (by food ingestion) and by tracheal and bronchial absorption (by the inhalation of contaminated dust particle). In addition to these natural routes, intraperitoneal (ip), intravenous (iv) and dermatitis

Studies using radiolabelled aflatoxin B1 in rats and monkeys have demonstrated little difference in the distribution and excretion of the toxin after either oral or intraperitoneal administration, therefore implying that absorption after oral exposure is complete (Dalezios

Aflatoxin B1 can also be absorbed rapidly, by passive diffusion, from the small intestines (especially the duodenum) into the mesenteric venous blood. Given the lipophilic nature of aflatoxin B1, the composition of the intestinal epithelium is an important criterion. Although the liver is regarded as the main site of aflatoxin transformation, gastrointestinal metabolism will reduce the exposure of the liver to aflatoxin B1 and, in terms of hepatic toxicity, is an important means of detoxification (Hsieh & Wong, 1994;

The transformation of aflatoxin B1 results in both the activation and the detoxification of the toxin and may be considered as occurring in two phases (Ben-Ami et al., 2010). Firstly, the transformation of the toxin to a selection of metabolites and, secondly, the conversion of some of these metabolites to either water soluble conjugates or macromolecular adducts (Huffman and Gerber, 2010). The transformation process will be modulated by numerous factors including the genetic make-up of the species, nutritional and health status, and

The major metabolites of aflatoxin B1 includes aflatoxin B1 -8,9-epoxide, -8,9-dihydro-8,9 diol;the aflatoxins-B2a, -P1 , M1 -Q1; aflatoxicol, aflatoxicol H1 and aflatoxicol M1 (Essigmann et al., 1982; Klich, 2009)(Figure 7). However, not all metabolites have been identified in all

In the liver, aflatoxin B1 may interact with both DNA and proteinto elicit the carcinogenic and acutely toxic effects of aflatoxin, respectively. Initially, aflatoxin B1 is converted, by cytochrome P450, to the highly reactive aflatoxin B1 -8,9-epoxide which in turn may be

administration have been used under experimental conditions.

exposure to metabolic modifiers in foodstuffs (Meggs, 2009)

**3.1 Metabolism of aflatoxin** 

**3.1.1 Administration** 

**3.1.2 Absorption** 

et al., 1973; Wilson et al., 2002).

Avantaggiato et al., 2004).

**3.1.3 Transformation** 

species.

**3.1.4 Activation** 

2010).

converted to aflatoxin B1 -dihydrodiol (Figure 8) (Baertschi et al., 1989; Coker, 1999; Kremer et al., 2007)

Fig. 7. The biotransformation of aflatoxin B1 (Baertschi et al., 1998)

Aflatoxin B1 is converted to at least seven metabolites, including a proposed unstable metabolite, the aflatoxin B1 -8,9-epoxide, which is the so called ultimate carcinogenic form (Hsieh and Wong, 1994; Magan et al., 2010). Aflatoxin M1 occurs in milk of cows fed on aflatoxin B1-containing feeds. This metabolite is found in the liver, kidneys and urine of sheep and in the livers of rats treated with aflatoxin B1 (Appleton et al., 1982; Micco et al., 1991). The carcinogenicity of aflatoxin B1 arises from interaction with guanine moiety of DNA, to produce the aflatoxin-N7-guanine adduct (Baerstchi et al., 1989), whereas the acute toxicity of aflatoxin B1 is believed to stem from interaction between the dihydrodiol and protein amino groups to produce Schiff base adduct (Autrup et al., 1987).

Mycotoxins: Quality Management, Prevention, Metabolism, Toxicity and Biomonitoring 131

aflatoxicol can be readily converted back to aflatoxin B1, it has been proposed that aflatoxicol may act as a reservoir for aflatoxin B1, *in vivo*, thereby prolonging the lifetime of the toxin in

Total conversion of aflatoxin B1 to M1 in cow's milk is estimated to be about 1%. In comparing carcinogenic activity in rats, aflatoxin M1 is less than one-tenth as active as aflatoxin B1. However, the acute toxicities of these substances are almost similar. The aflatoxin metabolites M1 and P1 can also form DNA adduct (Essigmann et al., 1983). Similarly, there is evidence that aflatoxin G1 can bind to DNA (Garner et al., 1979; O'brian et

Many methods have been used in an effort to detoxify contaminated feeds. Physical separation of obviously contaminated materials has proven successful in controlling aflatoxin contamination in peanuts. *Aspergillus flavus* and several other fungi emit a bright yellow-green fluorescence under ultraviolet light (Takayuki and Bjeldanes, 1993; Wilson et al., 2002). This telltale signal of fungal contamination has been useful in the physical

Heat treatment of contaminated crops has also been used to detoxify food or feed material. Generally, under dry conditions the aflatoxins are quite heat stable. Normal roasting conditions can reduce the aflatoxin B1 content in peanuts by 80% after an hour. Heating under conditions similar to the moist conditions used for autoclaving is much more effective in reducing aflatoxin content than dry heating (Park et al., 1988; Pitt & Miscamble, 1995). Several chemicals such as hydrogen peroxide, ozone, and chlorine have been used to destroy aflatoxins. These substances react readily with aflatoxins in food as well as with many desired substances, including vitamins. A more useful method of chemical

Ammonia was first used for the detoxification of aflatoxin-contaminated cottonseed meal, in the USA in the late 1960s (Park et al., 1988). The use of ammonia to detoxify corn meal and

The detoxified feed supports the growth of trout, cows, and other animals without apparent ill effects. An ammoniation process developed in Arizona involves placing a mixture of aqueous ammonia and cottonseed in large plastic bags used for silage (Cocker, 1999; Wu & Munkvold, 2008). The bags are sealed and allowed to stand in the sun for several weeks. The process has been shown to be effective in reducing the levels of aflatoxin in highly contaminated cottonseed (800ppb) to less than the 100 ppb action levels set by the FDA

Detoxication process involves numerous oxidising agents, aldehydes,acids and bases (inorganic and organic) that have been reported as potential chemical detoxification agents

The nature of the reaction products produced by the ammoniation of aflatoxin is still poorly defined. Most studies have focused on the reaction products of aflatoxin B1 produced under a variety of conditions including the treatment in vitro, of pure toxin or of pure toxin on an inert carrier. The ammoniazation process of aflatoxin B1 is illustrated in figure 8. Ammoniation, *in* 

separation of contaminated peanuts and corn as well as a few other crop samples.

detoxification of contaminated feed is treatment with ammonia.

cotton meal increases the nutritional value of the feed.

the body (Wong and Hsieh, 1978).

**3.2.1 Ammonia detoxification** 

(Pepplinski et al., 1983).

(Wild and Hall, 2000).

**3.2.2 Chemistry of ammoniazation** 

al., 2007).

**3.2 Detoxification** 

Fig. 8. Biotransformation process of aflatoxin B1 (Parker et al., 1998)

The major metabolites of aflatoxin B1 includes B1-8, 9-dihydro-8-9-diol; the aflatoxins –B2a,- P1, M1, -Q1; aflatoxicol, aflatoxicol H1 and aflatoxicol M1 (Essigmann et al., 1982; Smith et al., 2007). However, not all metabolites have been identified in all species. Aflatoxicol is a major aflatoxin B1 in rat plasma (Wong and Hsieh, 1978; Pestka & Bondy 1990). It is reported as having equivalent carcinogenic potency as aflatoxin B1 (Schoenhard et al., 1981; Pasquali et al 2010), and about 70% the mutagenicity (Coulombe et al., 1982Porbst et al., 2007). Since aflatoxicol can be readily converted back to aflatoxin B1, it has been proposed that aflatoxicol may act as a reservoir for aflatoxin B1, *in vivo*, thereby prolonging the lifetime of the toxin in the body (Wong and Hsieh, 1978).

Total conversion of aflatoxin B1 to M1 in cow's milk is estimated to be about 1%. In comparing carcinogenic activity in rats, aflatoxin M1 is less than one-tenth as active as aflatoxin B1. However, the acute toxicities of these substances are almost similar. The aflatoxin metabolites M1 and P1 can also form DNA adduct (Essigmann et al., 1983). Similarly, there is evidence that aflatoxin G1 can bind to DNA (Garner et al., 1979; O'brian et al., 2007).
