Submitted to: Food and Chemical Toxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 4, 2001
Publication Date: September 1, 2001
Interpretive Summary: Fumonisins are a group of related toxins produced by certain molds that can contaminate corn. They can cause fatal diseases in horses and pigs, cause less severe toxicity in other animals, and may be a cause of esophageal cancer in people. We previously reported the likely mechanism of action of fumonisins at the sub cellular level. The toxins can interfere with the formation and function of a group of lipid compounds that are vital to every cell, the sphingolipids. In this study, we showed that a particular part of the fumonisin molecule, the primary amino group, must be available for fumonisin to block sphingolipid formation. The group can be blocked by other groups, such as an acetyl, and the fumonisin is inactivated. We also found that the acetyl group can spontaneously relocate to another part of the fumonisin molecule, freeing the amino group, and restoring the fumonisin activity. When this change occurs, it is difficult to detect, requiring sophisticated equipment and expertise. The results are important as ways to inactivate fumonisins are explored. They also are important to other scientists working trying to unravel how fumonisins work. This is because our findings alert the scientific community studying fumonisins that derivatives of fumonisin can change, even when carefully stored, and purity must be carefully verified.
Technical Abstract: Fumonisin B1 (FB1) is a mycotoxin produced by Fusarium verticillioides. It inhibits ceramide synthase, which is the proposed underlying mechanism responsible for the myriad of toxic endpoints observed. We previously reported that N-acetylation of FB1 prevents ceramide synthase inhibition, but cautioned that FA1 can contain a contaminant with the ability to inhibit ceramide synthase. We now report that FA1 spontaneously rearranges to O-acetylated analogs, and the products inhibit ceramide synthase. Liver slices exposed to 0.5 micronM contaminated FA1 containing O-acetylated FB1 had sphinganine:sphingosine (Sa:So) ratios of 1.15-1.64. Control slices had Sa:So ratios of 0.07-0.24. Clean-up to remove the O-acetylated FB1 yielded 99% pure FA1, which produced Sa:So ratios in liver slices of 0.08-0.18. After storage for 1-2 years as either a dry powder in a desiccator, or as a dried film at 4 degree C, the purified FA1 again contained O-acetylated FB1, and was capable of ceramide synthase inhibition. FA1 was most stable in neutral solution. In acidic solution, the equilibrium shifts toward the O-acetylated forms. FA1 in solid form also rearranges, but more slowly than in acid solution. Since FA1 is considerably less cytotoxic than FB1, these results support the hypothesis that a primary amino group is necessary for both ceramide synthase inhibition and toxicity.