10. Effect of aflatoxins on enzymes

In liver cells, cytoplasmic reductase and microsomal mixed-function oxidase system metabolize AFB1 to aflatoxicol and aflatoxins M1, Q1, P1 and B1-epoxide (the most toxic and carcinogenic derivative), which are less toxic than AFB1. These are further conjugate with other molecules and rapidly eliminated from the body [3]. The metabolites (AFQ1, AFM1 and AFP1) being formed from AFB1 and other naturally occurring AFs e.g. G1, B2 and G2 are weaker for epoxidation, thus possess less carcinogenic and toxic properties than AFB1. The AFM1, AFQ1 and AFP1 are secreted as metabolites of AFB1 in the urine and can be used as biomarkers [62].

Aflatoxins are liposoluble compounds that are readily absorbed at the site of exposure (usually gastrointestinal tract) into the blood stream to liver where they are metabolized in the microsomal system to active or detoxified metabolites [63]. AFB1 may occur as free or unconjugated forms of primary metabolites. Water soluble conjugate metabolites bound covalently with cellular macromolecules and degradation/metabolic products of AFB1 adducts. These conjugates of AFB1 metabolites are excreted in the bile and consequently eliminated through feces. Water soluble conjugates and degradation or metabolic products of AFB1 macromolecule adducts and unconjugated AFB1 metabolites are excreted into general circulatory blood system. This results the systemic distribution of AFB1 to eggs or milk

AFs are known to alter the synthesis, absorption, and transport of lipids to extra-hepatic tissues. Liver fatty acid composition is drastically altered among birds with aflatoxicosis [43]. AFB1-8, 9 epoxide (formed by action of cytochrome P450 on AFB1) may cause significant increase in hepatic lipid peroxide level. Lipid peroxidation initiates to affect membrane integrity negatively; membrane bound enzyme activities which lead to cell lysis. The oxidative damage of cell/tissue occurs when the concentration of reactive oxygen species (O2�, H2O2, and OH�) predominates the antioxidant capability of cells. This may be the consequence of significant decrease in nonenzymatic antioxidants (e.g. glutathione, vitamin E, and vitamin C) and enzymatic antioxidants (e.g. catalase, glutathione peroxidase, superoxide dismutase). Superoxide dismutase shields cells from oxidative damage by metabolizing free radical superoxide (O2�) to H2O2 and O2�. The metabolically produced H2O2 can then be decomposed enzymatically with glutathione peroxidase (GSH-Px) and catalase. Glutathione peroxidase not only decomposes H2O2 but also can interact with lipid peroxidation. Reduced protein biosynthesis may be responsible for the decline in enzyme activities. Significantly lower glutathione peroxidase levels further intensify the toxic effects of AFs [24]. AFs promote free radical formation thus causing liver peroxidation which in turn results in antioxidant depletion, oxidative stress and apoptosis. All of these contribute to the

The metabolites such as AFB1-N7-Gua, AFM1, AFB1-mercapturic acid and serum AFs-albumin are also considered as AF biomarkers [65]. AFs show specific selection for guanine bases with a guanine or cytosine at the 5<sup>0</sup> base causing G ! T transversion [66]. Puisieux et al. [67] showed that the guanine at the third position of codon 249 of the p53 gene (a known mutational hotspot in HCC (hepatocellular carcinoma) was the site of modification by AFB1 (in human

9. Absorption of aflatoxins in small intestine

132 Mycotoxins - Impact and Management Strategies

and body tissues [3].

development of malabsorption [64].

A marked decrease in digestive enzymes (pancreatic ribonuclease, amylase, trypsin and lipase), hypocarotenoidaemia, steatorrhea and bile salts can be observed during aflatoxicosis in poultry. Protein requirements for growth were increased during aflatoxicosis which can be alleviated by dietary methionine fortification [43]. Fernandez et al. [69] conducted trials to investigate the hematological and serological changes on broilers from 21 to 42 days of age with oral administration of 2500 μg/kg AFB1. It was found that hematological (red blood cell, hemoglobin, leucocytes, eosinophils and basophils) and serological (serum protein, aspartate aminotransferase, alanine aminotransferase, urea, creatinine) parameters remained unchanged but caused hepatic and renal lesions which matches the findings of Bianchi et al. [39]. AFs are known to reduce protein synthesis that may lead to decreased blood protein levels. The AFs intoxications have been reported to decrease total protein, cholesterol, triglyceride and glucose levels significantly [70].
