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CHAPTER 2 Literature Review
2.11 FUMONISINS
2. 11. 1 Occurrence and production of fumonisins
Fusarium moniliforme is a soil and seed-borne fungal pathogen of maize (Zea mays) (Figure 2.3) (Nelson et aI., 1991; Bacon et ai., 1992; Munkvold, 1996).
Figure 2.3: Typical Fusarium ear rot (Munkvold, 1996).
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Infection of the maize plant and kernel varies depending on the point(s) of entry by the fungus into the developing maize plant (Bacon and Nelson, 1994; Munkvold et aI., 1997).
In addition, the extent of fumonisin contamination varies with geographical location, agricultural practices, and the maize genotype, which determines the susceptibility of the plants to fungal invasion. Fumonisin production is further influenced by temperature, humidity, drought stress, rainfall during the pre-harvest and harvesting periods, as well as by the storage of the maize kernels under improper moisture conditions (Bacon and Nelson, 1994).
The fumonisins are a class of polar metabolites, originally isolated from Fusarium moniliforme [now known as F. verticil/ioides} (Bezuidenhout et aI., 1988) and later, from F. proliferatum, F nygamai, and F. napiforme. The worldwide occurrence of fumonisins in maize and maize-based products has been well documented and reviewed (Doko and Visconti, 1994; Pohland, 1994; Marasas, 1996; Bullerman, 1996;
Shephard et al., 1996a; Patel et al., 1997; Castelo et al., 1998; de NUs, 1998;
de Nijs et aI., 1998a; 1998b). Also reported are the sporadic natural occurrence of fumonisins in sorghum, rice and navy beans (Tseng et aI., 1995; Patel et aI., 1996;
Munibazi and Bullerman, 1996; Bhat et aI., 1997).
The polarity of the fumonisin molecule (Figure 2.4) determines its level of carcinogenicity (Gelderblom et aI., 1993), i.e., the more polar a molecule, the greater the cytotoxic response. In addition to polarity, the presence of a free amino group and the location of the hydroxyl group could also affect the biological activity of these compounds. Thus, both the amino group and the intact molecule play an important role in the toxic and cancer promoting activity of fumonisins. This would explain the associations of FBI with soluble and insoluble portions of cells (Cawood et aI., 1994).
Feed-derived fumonisins are poorly absorbed by farm animals therefore fumonisin residues in milk (Scott et aI., 1994; Maragos and Richard, 1994; Becker et aI., 1995; Richard et al., 1996), eggs (Vudathala et aI., 1994), and meat (Prelusky et al., 1994; Prelusky et aI., 1996;
Smith and Thakur, 1996) are either undetectable or detected at extremely low levels. Due to the heat stability (Dupuy et aI., 1993; Howard et aI., 1998), light stability (I ARC , 1993b), water solubility (United States National Toxicology Program, 1999), poor absorption and metabolism, as well as rapid excretion by animals, most fumonisin will
eventually be recycled into the environment in a manner that will concentrate its spatial distribution.
2.11.2 Structure, physical and chemical properties of fumonisins
Fumonisin BI (FBI) has the empirical fonnula C34HS9NOJS and is the diester of propane- 1 ,2,3-tricarboxy lic acid and 2-amino-12, 16-dimethy 1-3,5,10,14, 15-pentahydroxyeicosane (relative molecular mass: 721) (Figure 2.4). It is the most prevalent of fumonisins, a family of toxins with at least 15 identified members. The pure substance is a white hygroscopic powder, which is soluble in water, acetonitrile-water or methanol, is stable in acetonitrile- water (1 : 1), at food processing temperature and in light, but is unstable in methanol. The 15 fumonisins reported have been grouped into four main categories (Plattner, 1995;
Musser and Plattner, 1997; Abbas et
at.,
1998):• fumonisin Al (F AI), fumonisin A2 (F A2), fumonisin A3 (F A3), fumonisin AKI (FAKI);
• FBI, FB2, fumonisin B3 (FB3), fumonisin B4 (FB4);
• fumonisin C I (FC I), fumonisin C2 (FC2), fumonisin C3 (FC3), fumonisin C4 (FC4);
and
• fumonisin PI (FPJ), fumonisin P2 (FP2) and fumonisin P3 (FP3).
Fumonisin B2, FB3 and FB4 differ from FBI in that they lack hydroxyl groups present in FBI; FAI, FA2 and FA3 are like FBI, FB2and FB3, but are N-acetylated; FAKI is like FAI but is IS-keto functionalised; the fumonisin 'C's are like the fumonisin 'B's but lack the methyl group adjacent to the amino group; and the fumonisin 'P's have a 3-hydroxypyridium group instead of the amine group in the fumonisin 'B's (Gelderblom et aI., 1992; Plattner eta!., 1992; Branham and Plattner, 1993).
Figure 2.4:
CooH
Fumoni in 51 min p ntol PI)
The Chemical structure of Fumonisin B\ and aminopentol (Merrill, 2002).
The "B" series are believed to be the most abundant and toxic naturally occurring analogues (Sydenham et aI., 1992; Thiel et aI., 1992). Because of the four free carboxyl
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groups and the amine group, the compounds are water soluble, but insoluble in non-polar organic solvents. Three dimensional minimum energy confonnation for FBI, FBz, FB3 and FB4 were calculated (Beier et al., 1995), and it was found that the amine and carboxylic acid groups are spatially related, suggesting that they have chelating properties, and hence, could cause membrane ion leakage (Abbas et at., 1992).
2.11.3 Fumonisins and sphingolipids
Fumonisins bear structural resemblance to Sa (Figure 2.5), an intermediate in the biosynthesis of complex sphingolipids (Sweeley, 1991; Bell et al., 1993). This structural similarity led to the hypotheses that the mechanism of action of these mycotoxins might be via disruption of sphingolipid metabolism; via interaction with enzyme(s) of sphingolipid metabolism, or a combination of both these factors (Merrill et al., 1996a; 1996b). The fumonisins have been shown to inhibit ceramide (CER) synthase, the key enzyme in sphingolipid biosynthesis. Since sphingolipids play critical roles in the regulation of cell growth, cell differentiation and transformation (Merrill, 1991), disruption of the metabolism of these bioactive molecules may be the mechanism for toxicity and carcinogenicity of these mycotoxins (Y 00 et at., 1992; Merrill et at., 1993a; 1993b; 1993c).
The implication that several human and animal diseases arise from defects in sphingolipid hydrolysis increased interest in the study of sphingolipid metabolism.
OH
Sphinganine
Phytosphingosine
Psychosine
CH20
galactose
I
Figure 2.5: Chemical structures of select sphingolipids.
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2.12 2.12.1
SPHINGOLIPIDS
Structure and occurrence of sphingolipids
Sphingolipids are components of the plasma membrane of eukaryotic cells. These compounds are the most structurally diverse class of membrane lipids, being composed of over 70 long-chain (sphingoid) bases, dozens of amide-linked fatty acids, and more than 300 head groups (Merrill and Sweeley, 1996).
Sphingolipids are composed ofa long chain sphingoid base, an amide linked fatty acid and a polar head group at the I-position (Figure 2.5). Sphingolipids contain a CER backbone, which anchors them in the outer leaflet of the lipid bilayer. The CER backbone can be modified by attachment of phosphorylcholine to form sphingomyelin (SM), or by attachment of one or more sugar residues to form glycosphingolipids (GSLs). Except for CER, which has a hydroxyl at the I-position, and for SM, which has a phosphorylcholine head group; all other sphingolipids contain carbohydrate head groups and hence are designated GSLs (Rannun and Bell, 1989).
The first classes of sphingolipids were named according to the tissue from which they were isolated (e.g., SM, cerebrosides, psychosine) (Figure 2.5) leaving the false impression that these compounds are unique to neuronal tissues. Sphingolipids are present in all eukaryotic cells where they primarily occur in cell membranes and related intracellular membranes, such as Golgi and lysosomal membranes, as well as in some prokaryotic organisms (Merrill et aI., 1997). In addition to biosynthesis, catabolism of sphingolipids also occurs.
In the intestines, SM and other complex GSLs are digested, and the gut cells absorb CER and So.
There have been a few analyses of the amounts and types of sphingolipids in foods (Ohnishi and Fujino, 1982; Blank et a/., 1992; Jensen and Newburg, 1995); and as a result relatively little is known about the role that these components of the diet may contribute to, or protect against disease.
2.12.2 Fate and significance of sphingolipids
Sphingolipids serve as structural molecules in membranes (Rannun and Bell, 1989;
Rakomori, 1990; Merrill, 1991) and as regulators of cell growth, differentiation and
neoplastic transformation through participation in cell-cell communication, cell receptors and signalling systems, and endothelial cell permeability (Merrill, 1991;
Wang et aI., 1991). Ceramides derived from sphingolipid metabolism affect DNA synthesis. The role of CER as an intracellular second messenger for tumour necrosis factor -a (TNF-a), interleukin P (IL-P) and other cytokines; as well as So, So-I-phosphate among other sphingolipid metabolites, have been demonstrated to modulate cellular calcium homeostasis, cell cycle progression and apoptosis. Carcinogens and toxins may act via unscheduled initiation or inhibition of these pathways leading to disruption of metabolism of these molecules, and leading, ultimately, to the toxicity and carcinogenicity of these mycotoxins (Yoo et aI., 1992; Merrill et aI., 1993b).