Testing for Natural Aflatoxin Inhibitors

In the United States, corn with more than 20 parts per billion (ppb) of aflatoxin--which is the equivalent of just 1 ounce in 3,125 tons--is not considered fit for feeding to animals that produce meat or milk for humans. 

A known carcinogen, aflatoxin is the metabolic byproduct of Aspergillus flavus fungi. Grain with more than 5 ppb gets thumbs down for making food-grade corn products. And in the South and in areas where occasional drought stresses corn and increases A. flavus levels, farmers may lose opportunities to produce corn valued for export markets.

Finding natural compounds in corn that affect the toxin-producing machinery of A. flavus is a first step toward identifying corn genes that might be modified to make the microbe less harmful. The strategy could be joined with efforts to breed corn that discourages growth of the fungus.

Now, a faster, cheaper test is helping researchers detect genetically regulated compounds in corn that inhibit or promote the ability of A. flavus fungi to produce aflatoxin. Agricultural Research Service chemist Robert A. Norton developed the new procedure at the National Center for Agricultural Utilization Research in Peoria, Illinois.

"We can now realistically test a much wider range of compounds for toxin-producing activity--including lipids--using 1 milligram [thousandth of a gram] or less of the test compound," he says.

Norton purchases the compounds for testing, some of which cost up to hundreds of dollars per milligram, though most cost less. Despite the expense, Norton says that it's cheaper to buy the compounds than to tediously extract them from corn.

"And with the new testing method, we don't have to use as much of them," he says.

His procedure involves placing the test compound, along with about 29 microliters [millionths of a liter] of a nutrient medium and A. flavus spores, on a small disk. The disk is hung by a pin from a Teflon cap inside a bottle containing a small amount of water. After 5 days, researchers measure fungal growth on the disk. They use a small amount of solvent to extract aflatoxin from the fungus; high-performance liquid chromatography measures the amount. The method saves time, nutrient medium, and solvent.

Norton currently tests up to 200 samples per week. So far, he has pinpointed several aflatoxin-synthesis inhibitors, including carotenoids that impart yellow color to modern corn hybrids and a colorless benzoxazolinone compound. He also plans to test colorless anthocyanin-related compounds that could be bred into yellow corn.--By Ben Hardin, Agricultural Research Service Information Staff.

Robert A. Norton is in the USDA-ARS Bioactive Agents Research Unit, National Center for Agricultural Utilization Research, 1815 N. University St., Peoria, IL 61604; phone (309) 681-6251, fax (309) 681-6693. 

"Testing for Natural Aflatoxin Inhibitors" was published in the July 1998 issue of Agricultural Research magazine. Click here to see this issue's table of contents.