Submitted to: Journal of Toxicology Toxins Reviews
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2004
Publication Date: 6/1/2004
Citation: Abou-Karam, M., Abbas, H.K., Shier, W.T. 2004. N-fatty acylation of hydrolized fumonisin B1, but not of intact fumonisin B1, strongly enhances in vitro mamillian toxicity. Journal of Toxicology Toxins Reviews. 23:123-151. Interpretive Summary: Fumonisins are toxins produced by a fungus which contaminates corn grown in hot, humid areas. These toxins are a concern because they are known to cause agriculturally-important diseases in horses and swine, and epidemiological studies have raised concerns that they may play roles in the cause of some types of cancer and birth defects in people. Normal food processing conditions used to manufacture corn-derived human food products such as tortilla chips change part of the fumonisins in corn into another form, called hydrolyzed fumonisins. This study has explored in more detail an unexpected observation made in the laboratory of Dr. A.H. Merrill at Emory University that attaching a long-chain fatty acid moiety onto the hydrolyzed fumonisin molecule makes it ten times as toxic as the original fumonisin in a laboratory mammalian cell line toxicity test system. In previous studies in this laboratory adding a short-chain fatty acid in the same way increased toxicity about two-fold relative to the original fumonisin in a similar laboratory test system. In the present study we have attached fatty acids with a range of chain lengths to the free amino group of hydrolyzed fumonisin B1 in the same way as was previously done in this laboratory and in Merrill's laboratory. We have observed that toxicity in the laboratory test system increased with increasing fatty acid chain length to about 2/3 the fatty acid chain length used by Merrill, after which it stayed at a similar toxicity, about ten times as toxic as fumonisins. Fatty acids can be attached to the free amino group of initial (unhydrolyzed) fumonisin B1 in the same manner as was done with hydrolyzed fumonisin B1. However, in the case of initial fumonisin B1, adding fatty acids (i.e. N-fatty acylation) completely destroyed toxicity in the laboratory. Also, no toxicity was observed for fatty acid compounds of initial or hydrolyzed fumonisins in a plant toxicity test system. Confirmation of the observation that adding fatty acids to hydrolyzed fumonisin B1 leads to a ten-fold increase in toxicity in the laboratory would be important, if fatty acids are added to products such as tortilla chips during frying. There is no evidence that animals hydrolyze fumonisin B1, and hydrolyzed fumonisin B1 is considerably less toxic orally than initial fumonisin B1.
Technical Abstract: Fumonisin B1 (FB1) is the most abundant of a series of sphingosine-analog mycotoxins produced by Fusarium verticillioides, a major fungal contaminant of stored corn (maize) world-wide. Fumonisins were originally isolated as environmental tumor promoters, and they remain a concern because they are frequent contaminants of corn-derived food products intended for direct human consumption. FB1 inhibits ceramide synthase, which may account for its acute toxic effects, but understanding of its tumor promotion mechanism has been limited by the general lack of understanding in the field. There is no evidence for functional metabolism of fumonisins in mammals, but abiogenic conversions during food processing are a concern because some known conversion products retain biological activity, including hydrolyzed FB1 (HFB1). HFB1, formed by alkaline removal of FB1 side chains, is a frequent contaminant of lime-treated corn products such as tortillas and tortilla chips. Humpf et al. (J. Biol. Chem., 273, 19060, 1998) observed that HFB1 not only inhibits ceramide synthase, but it is converted to a ceramide analog with about ten times the in vitro mammalian toxicity of intact FB1. In the present study we have confirmed this observation by preparing a series of ceramide analogs of HFB1 with varying fatty acid chain lengths and degree of unsaturation. Optimal in vitro mammalian toxicity was observed with fatty acid chain lengths of 10-14 carbons. However, ceramide analogs of HFB1 were not phytotoxic in vitro, and ceramide analogs of FB1 were not toxic in either mammalian or plant in vitro bioassays.