Sterigmatocystin

Sterigmatocystin is a poison of the type dermatoxin, from the fungi genus Aspergillus. It appears on crusts of cheese with mold.

Introduction

Sterigmatocystin is a toxic metabolite structurally closely related to the aflatoxins (compare general fact sheet number 2), and consists of a xanthone nucleus attached to a bifuran structure. Sterigmatocystin is mainly produced by the fungi Aspergillus nidulans and A. versicolor. It has been reported in mouldy grain, green coffee beans and cheese although information on its occurrence in foods is limited. It appears to occur much less frequently than the aflatoxins, although analytical methods for its determination have not been as sensitive until recently, and so it is possible that small concentrations in food commodities may not always have been detected. Although it is a potent liver carcinogen similar to aflatoxin B1, current knowledge suggests that it is nowhere near as widespread in its occurrence. If this is the true situation it would be justified to consider sterigmatocystin as no more than a risk to consumers in special or unusual circumstances. A number of closely related compounds such as o-methyl sterigmatocystin are known, and some may also occur naturally.

Chemical and Physical Properties

Sterigmatocystin crystallises as pale yellow needles and is readily soluble in methanol, ethanol, acetonitrile, benzene, and chloroform. It reacts with hot ethanolic KOH and is methylated by methyl sulphate and methyl iodide. Methanol or ethanol in acid produces dihydroethoxysterigmatocystin.

Toxicity and Importance

The toxic effects of sterigmatocystin are much the same as those of aflatoxin B1. It is thus considered as a potent carcinogen, mutagen, and teratogen. It is less acutely toxic to rodents and monkeys but appears to be slightly more toxic to zebra fish. The LD50 in mice is in excess of 800 mg/kg. The 10 day LD50 in Wistar rats is 166 mg/kg in males, 120 mg/kg in females, and 60-65 mg/kg for ip. administration in males. The ip. 10 day LD50 for vervet monkeys is 32 mg/kg.

Chronic symptoms include induction of hepatomas in rats, pulmonary tumours in mice, renal lesions and alterations in the liver and kidneys of African Green monkeys. Rats fed 5-10 mg/kg of sterigmatocystin for two years showed a 90% incidence of liver tumours. It has been suggested that sterigmatocystin is about 1/10 as potent a carcinogen as aflatoxin B1.

Toxic effects of sterigmatocystin-fed laboratory animals have included kidney and liver damage and diarrhoea. Skin and hepatic tumours are induced in rats by dermal application. Cattle exhibiting bloody diarrhoea, loss of milk production and in some cases death were found to have ingested feed containing Aspergillus versicolor and high levels of sterigmatocystin of about 8 mg/kg. The acute toxicity, carcinogenicity, and metabolism of sterigmatocystin has been compared with those for aflatoxin and several other hepatotoxic mycotoxins.

The IARC-classification of sterigmatocystin is group 2B, which means it is likely carcinogenic to humans. In practice, the risk is quite low however, as this substance only appears on cheese crusts with mold, and because of that the chance of daily exposure is very low. A molded crust is best not to be consumed in whole, but after removing the crust, the cheese can still be consumed; Sterigmatocystin is a different kind of mold than that which appears on cheese itself, which can simply be removed before further consumption.

Products affected and Natural Occurrence

The occurrence of sterigmatocystin in raw materials and foods has not been reported often. The instances reported have usually been on mouldy, or poor quality materials such as wheat, maize, animal feed, hard cheese, pecan nuts and green coffee beans. While this lack of information may be due to deficiencies in the analytical methods, where surveys of good quality products have been carried out with reliable methodology, sterigmatocystin has rarely if ever been found. However, further assurance is required before sterigmatocystin can finally be dismissed as a risk because A. versicolor has been isolated frequently from cereals, grain products, fruits and marmalade, dried meat products and grapefruit juice. Relatively high levels of sterigmatocystin have been formed in bread, cured ham and salami after inoculation with A. versicolor.

Sampling and Analysis

Methods for extraction of sterigmatocystin have been commonly based on a mixture of acetonitrile and 4% aqueous potassium chloride. Methods for detection using TLC are not very sensitive having a limit of detection in the range 20-50 microgrammes/kg. TLC plates must be sprayed with aluminium chloride and heated. Analytical methods for the determination of sterigmatocystin have been reported using HPLC but again these are not very sensitive because of lack of UV absorbance or fluorescence, although a post column reaction with aluminium chloride has been used to increase sensitivity. More recently, methods using HPLC linked with atmospheric pressure ionisation mass spectrometric detection have been developed for foods such as cheese, bread and corn products.

Stability and Persistence

There appear to be no reports about the stability of sterigmatocystin, other than in solution, where it is similar to the aflatoxins. There is one report that phosphine gas significantly depresses the formation of sterigmatocystin when cereals are inoculated with A. versicolor.

Legislation and Control

No country has legislation for sterigmatocystin. Natural occurrence appears to be infrequent although only a limited number of surveys have been carried out. Soon after it was recognised as a highly toxic compound, the California Department of Health Services used TD50 values from the Cancer Potency Database to produce ‘no significant risk’ intake levels for humans. The level resulting was 8 microgrammes/kg body weight/day for a 70 kg adult

See also

• Aspergillus
• Dermatoxin
• Mycotoxin

External links

• Chemical explanation by European Mycotoxin Awareness Network
• PubChem Entry