Sphinganine-Analog Mycotoxins (SAMs): Chemical Structures, Bioactivities, and Genetic Controls

Sphinganine-analog mycotoxins (SAMs) including fumonisins and A. alternata f. sp. Lycopersici (AAL) toxins are a group of related mycotoxins produced by plant pathogenic fungi in the Fusarium genus and in Alternaria alternata f. sp. Lycopersici, respectively. SAMs have shown diverse cytotoxicity and phytotoxicity, causing adverse impacts on plants, animals, and humans, and are a destructive force to crop production worldwide. This review summarizes the structural diversity of SAMs and encapsulates the relationships between their structures and biological activities. The toxicity of SAMs on plants and animals is mainly attributed to their inhibitory activity against the ceramide biosynthesis enzyme, influencing the sphingolipid metabolism and causing programmed cell death. We also reviewed the detoxification methods against SAMs and how plants develop resistance to SAMs. Genetic and evolutionary analyses revealed that the FUM (fumonisins biosynthetic) gene cluster was responsible for fumonisin biosynthesis in Fusarium spp. Sequence comparisons among species within the genus Fusarium suggested that mutations and multiple horizontal gene transfers involving the FUM gene cluster were responsible for the interspecific difference in fumonisin synthesis. We finish by describing methods for monitoring and quantifying SAMs in food and agricultural products.

Digestibility and absorption of deoxynivalenol-3-ß-glucoside in in vitro models

Certain mycotoxins may be present in plant materials as their glucosides. The question is whether these glucosides may be hydrolysed into their parent compounds in the gastro-intestinal tract (GI-tract), thus increasing the exposure. Therefore, the potential hydrolysis of deoxynivalenol-3-ß-glucoside (DON-3G) to deoxynivalenol (DON) was assessed in two in vitro models representing the human upper GI-tract (mouth, stomach and small intestine). In a fed digestion model, there was no evidence of release of DON from DON-3G, spiked at a level of 2,778 μg DON- 3G/kg food. This shows that the conditions in the GI-tract do not result in hydrolysis of this glucoside into the original mycotoxin. The absorption and transformation of DON-3G in the small intestine was assessed in an in vitro model with human Caco-2 cells in a Transwell system. No evidence was found for the transformation of DON-3G to DON by the Caco-2 cells in both the apical or basolateral side in 24 hours (cells were exposed to 2.4 nmol DON- 3G/ml medium). However, when DON itself was added to the apical side an amount of 23% of the spiked DON was detected in the basolateral side after 24 hours (cells were exposed to 2.3 nmol/ml medium). In conclusion, no evidence was found in the in vitro experiments for significant elevated exposure of humans to DON, since DON- 3G was not hydrolysed to DON in the digestion model representing the upper part of the GI-tract and DON-3G was not hydrolysed to DON by the intestinal epithelial Caco-2 cells. It was shown that bioavailability of DON-3G in humans may be low as compared to DON since Caco-2 cells did not absorb DON-3G, in contrast to DON.

Free and bound fumonisins in gluten-free food products

In this work a multiresidual LC-ESI-MS/MS method for the simultaneous detection of free and bound fumonisins is described, which allowed for a very low LOD and a very good recovery for all the analytes. The method was applied to the determination of free and bound fumonisins in several gluten-free products from the Italian market. Free fumonisins were found to occur in 90% of the samples: the overall median value was below the EU legal limit for foods for human consumption (800 microg/kg). Nonetheless, fumonisins occurred in several samples at concentrations above the legal limit, reaching also very strong contamination levels (maximum concentration level: 3310 microg/kg). Anyway, considering the limited diet of people suffering of the celiac disease or allergic to other wheat proteins, the incidence of fumonisin contamination may be envisaged as problematic. Furthermore, bound fumonisins were found to be present in all the analysed samples at similar or even higher amounts than the free forms. In many cases the sum of free and bound fumonisins exceeded the EU legal limit for total fumonisins also for those samples characterized by a low contamination of free fumonisins, thus opening a new important task to be addressed for the risk assessment in this field.

A LC/MS/MS method for the simultaneous quantification of free and masked fumonisins in maize and maize-based products

An LC-ESI-MS/MS method for the simultaneous detection of the main fumonisins and their hydrolysed derivatives is described, allowing for a simplified sample preparation without previous clean up. The method has a very low quantification limit (10 µg/kg for FB1, 12 µg/kg for FB2 and FB3, 70 µg/kg for HFB1, HFB2 and HFB3 in maize flour) and a very good recovery for all the analytes. The method has been applied to check several maize-based foods for the presence of free and bound forms of fumonisins, the latter being determined after alkaline hydrolysis as hydrolysed derivatives. Bound fumonisins were found to be present not only in thermally treated maize-based products but also in mild processed or even raw products (pasta, bread, cakes, crisps, flour) and they were always present in almost similar or even higher amounts than the free forms. Osborne fractions of maize proteins showed that fumonisins were particularly bound to prolamins and glutelins. Model systems and extracts of these protein fractions gave positive response to ELISA tests, thus confirming the cross reactivity of these masked forms.

Analysis of heat-processed corn foods for fumonisins and bound fumonisins

Thirty retail samples of heat-processed corn foods, i.e. corn flakes, corn-based breakfast cereals, tortilla chips and corn chips, were analysed for fumonisins--fumonisin B1 (FB1), fumonisin B2 (FB2) and hydrolysed FB1 (HFB1)--as well as for protein- and total-bound FB1. Bound (hidden) fumonisins cannot be detected by conventional analysis. Improved methods for the determination of bound FB1 were developed. The protein-bound FB1 was extracted with 1% sodium dodecylsulfate (SDS) solution. The SDS, which interfered with high-performance liquid chromatography (HPLC) analysis, was then separated from protein-bound FB1 by complexing with methylene blue followed by solvent extraction and hydrolysis with 2 N KOH. To measure total-bound FB1, the sample itself was hydrolysed with KOH. In both cases, clean-up was accomplished on an OASIS polymeric solid-phase extraction column and the bound fumonisins were determined by HPLC measurement of HFB1. Fourteen of 15 samples of corn flakes and other corn-based breakfast cereals analysed contained detectable levels of FB1 with a mean in positive samples of 67ng g(-1) (13-237 ng g(-1)). Two samples also had detectable levels of FB2 (21-23ng g(-1)). Bound FB1 was found in all samples; the mean protein-bound FB1 measured was 58 ng g(-1) (22-176 ng g(-1)) and the mean total-bound FB1 measured was 106 ng g(-1) (28-418 ng g(-1)), reported as FB1 equivalents after correction for recoveries of HFB1. There was an average of about 1.3 times more FB1 in the bound form compared with extractable FB1, and this was about twice as much as protein-bound FB1. Seven of the 15 samples of alkali-processed corn-based foods, such as tortilla chips and corn chips, contained FB1 and three contained HFB1 with means in measurable positive samples of 78 (48-134) and 29 (13-47) ng g(-1), respectively. Five of these alkali-processed corn foods contained bound FB1; the mean measurable protein-bound FB1 was 42 ng g(-1) (39-46 ng g(-1)) and the mean measurable total-bound FB1 was 100 ng g(-1) (54-209 ng g(-1)). HFB1 derived from bound FB1 in selected samples was confirmed by HPLC with mass spectrometry (MS).