Fumonisins

Naturally Occurring Fumonisins   The fumonisins are a group of at least 15 closely related mycotoxins that frequently occur in maize, the most important being fumonisin B1. Fumonisins were only identified during the mid-1980’ s, although their effects on horses had been recognised for at least 150 years before.

Fumonisin B1

They are polar metabolites produced by several species of Fusarium, including F. moniliforme, F. proliferatum, F. nygamai, F. anthophilum, F. dlamini and F. napiforme. Their structures are based on a long hydroxylated hydrocarbon chain containing methyl and amino groups. Two hydroxyl groups are esterified to two propane-1, 2, 3-tricarboxylic acids. Fumonisin B₁ differs from fumonisin B₂ in that it has an extra hydroxyl group at the 10 position. Fumonisins often occur together with other mycotoxins, which can include, for example, aflatoxins, deoxynivalenol and zearalenone.

Chemical and Physical Properties

Fumonisin B₁ (FB₁) has the empirical formula C₃₄H₅₉NO₁₅ (relative molecular mass: 721). The pure substance is a white hydroscopic powder that is soluble in water, acetonitrile-water or methanol. Fumonisins are soluble in polar solvents because of the 4 free carboxyl groups, the hydroxyl groups and the amino group. Their insolubility in many organic solvents such as chloroform and hexane commonly used in mycotoxin analysis partly explains the difficulty in their original identification. Fumonisins B₁ and B₂ are stable in methanol if stored at –18 șC but steadily degrade at 25 șC and above. However, they are reported to be stable over a 6- month period at 25 șC in acetonitrile-water (1:1).

Toxicity and Importance

The effects of fumonisins have been observed in a sporadic fatal disease in horses and related species called equine leucoencephalomalacia (ELEM) for many years. On a weight for weight basis, fumonisins are far less acutely toxic than the aflatoxins. However, fumonisins commonly occur in concentrations of parts per million in maize; up to 300 mg/kg has been reported. This contrasts with aflatoxins, which are usually measured in the low parts per billion concentrations in foods.

Fumonisins are considered to be toxic principally because oftheir effects on sphingolipid synthesis. Alteration in sphingolipid base ratios occurs almost immediately after exposure because fumonisin inhibits ceramide sythetase. This property is indicative of fumonisin exposure in a number of species, including horses and pigs. Animal studies with ¹⁴C-labelled fumonisin B₁ generally show the uptake to be poor and elimination rapid. The range of effects that fumonisins cause in mammals appears to be species-related. ELEM was firstly linked to the presence of Fusarium moniliforme in feed and more recently to the presence of fumonisins. In equines, affected animals commonly lose appetite, become lethargic and develop neurotoxic effects after a period of ingesting contaminated feed. Autopsy shows oedema in the brain and liquefaction of areas within the cerebral hemispheres. The liver is also generally affected and, in severe cases, gross liver lesions may be seen with fibrosis of the centrilobular areas. In pigs, fumonisins induce pulmonary oedema and hydrothorax, with thoracic cavities filled with a yellow liquid. There may also be respiratory problems and foetal mortality.

Rats fed culture material from F. moniliforme develop primary hepatocellular carcinomas. Similar effects were reproduced using purified fumonisins B₁, B₂ and B₃ although experimental carcinogenicity studies have been hampered by lack of pure standards. Fumonisin B₁ has also been shown to affect the foetus in pregnant rats, causing low litter weights and foetal bone development when compared with controls.

In humans, there appears to be a link between the high maize consumption in some areas of the world and the occurrence of oesophageal cancer. Further epidemiological studies are required to more precisely define the role of F. moniliforme and its metabolites in oesophageal cancer in the Transkei, China and Northern Italy, where incidence is high. Many studies of the toxicology of fumonisins are in progress. The full significance of fumonisins in maize for humans and animal health still remains to be determined.

Effects have been demonstrated on other organisms and fumonisin B₁ has shown to inhibit cell growth and cause accumulation of free sphingoid bases and alteration of lipid metabolism in Saccharomyces cerevisiae. It is phytotoxic, damages cell membranes and reduces chlorophyll synthesis. It also disrupts the biosynthesis of sphingolipids in plants and may play a role in the pathogenicity of maize by fumonisin-producing Fusarium species.

Products Affected and Natural Occurrence

When the fumonisins were first identified, it was considered that their occurrence was confined to maize. Subsequently, their presence is being noted in a range of products, which include rice, sorghum and navy beans, but so far in much lower concentrations than are common in maize. Significant accumulation of FB₁ in maize occurs when weather conditions favour Fusarium kernel rot.

Surveillance has shown that fumonisins may be present in a number of finished foods, such as polenta, maize-based breakfast cereals and beer and snack products. They have not been detected in milk, meat or eggs.

Sampling and Analysis

Aqueous methanol or acetonitrile is used for extraction from maize and maize products. Several analytical methods have been reported, including thin-layer chromatographic, liquid chromatographic, mass spectroscopic, post-hydrolysis gas chromatographic and immunochemical methods, although the majority of studies have been performed using HPLC analysis of a fluorescent derivative. Lack of a suitable chromophore in the molecule means that it must be derivatised with reagents such as p-anisaldehyde, fluorescamine or o-phthaldialdehyde to allow detection by HPLC.

Stability and Persistence

When maize is processed, maize screenings contain relatively high levels of fumonisins, and their removal can reduce the overall concentrations significantly. Dry milling results in distribution of fumonisins into different milled fractions so that concentrations in the bran, for example, may be higher than in the original whole maize grains, while concentrations in other fractions will be reduced. In experimental wet milling, fumonisin has been detected in steep water, gluten, fibre and germ, but not in the starch. The migration of fumonisins into aqueous solutions during steeping is thus an effective way of reducing concentrations of fumonisins in wet-milled products.

Fumonisins are quite stable and are not destroyed by moderate heat. However, no fumonisins are detected in tortilla flour made by treatment with calcium hydroxide (nixtamilisation), and it has been suggested that this process degrades fumonisins. An 80% reduction by heating at higher temperatures has been reported. However, caution is required in assessing risk as it has been reported that breakdown products such as ‘hydrolysed fumonisin’ may be formed and these may be almost as toxic as the parent compound. Relatively little is known about how food processing used in developing nations affects levels of fumonisins.

Legislation and Control

Human exposure to fumonisins is common worldwide, but there are considerable differences in the extent of human exposure between different maize-growing regions. This is most evident when comparing fully developed and developing countries. For example, although fumonisins can occur in maize products in the USA, Canada and Western Europe, human consumption of those products is modest. In parts of Africa, South-Central America and Asia, some populations consume a high percentage of their calories as maize meal and this often coincides with the growing areas where contamination may be the highest.

To date there has been no wide-scale introduction of limits for fumonisins. Limits have been set in some countries to protect susceptible animals such as horses (5 mg/kg) and pigs (50 mg/kg), and temporary guidelines have been introduced in a few countries to limit the exposure for humans; for example, Switzerland has set a limit for the sum of fumonisin B₁ and B₂ at 1,000 ”g/kg in maize intended for human consumption. Fumonisins were considered by the Joint FAO/WHO Committee on Food Additives at a meeting in February 2001 and this body allocated a group provisional maximum tolerable intake (PMTDI) for fumonisins B₁, B₂ and B₃ of 2 ”g/kg of body weight per day on the basis of the no observed effect level (NOEL) and a safety factor of 100. All estimates of fumonisins B₁ based on data available from national consumption were well below the group PMTDI, even if an allowance was included for fumonisins B₂ and B₃.