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21.
The significance of laboratory sample preparation for the determination of two important mycotoxins, ochratoxin A (OTA) and deoxynivalenol (DON), in wheat was investigated by comparing water-slurry mixing and dry-milling procedures. The distribution of OTA and DON in 10 kg samples of naturally contaminated wheat was established by analyzing one hundred 100 g subsamples of each sample. A normal distribution and a good repeatability of DON measurements was observed for both water-slurry mixing (mean 2290 microg/kg, CV 4.6%, median 2290 microg/kg) and dry milling (mean 2310 microg/kg, CV 6.4%, median 2290 microg/kg) procedures. For OTA determinations, reliable results could be obtained only by slurry mixing sample preparation (mean 2.62 microg/kg, CV 4.0%, median 2.62 microg/kg), whereas dry-milling comminution resulted in an inhomogeneous distribution with a high variability (mean 0.83 microg/kg, CV 75.2%, median 0.60 microg/kg) and a positive skewness (2.12). Ad hoc experiments were performed on different size portions of the same sample (10 kg) to assess accuracy and precision of the comminution/homogenization procedures (slurry mixing and dry milling). Very good results were obtained for DON determination with both procedures in terms of accuracy (>98.7% of the "weighted value") and precision (CV <3%). For OTA determination good results were only obtained by slurry mixing (99.4% of the "weighted value," CV 10%), whereas dry milling provided results with low accuracy (43.2% of the "weighted value") and high variability (CV 110%). This study clearly demonstrated that sample preparation by slurry mixing is strictly necessary to obtain reliable laboratory samples for OTA determination in wheat to minimize misclassification of acceptable/rejectable lots, mainly within official control. 相似文献
22.
Identification of NanE as the thioesterase for polyether chain release in nanchangmycin biosynthesis
Liu T You D Valenzano C Sun Y Li J Yu Q Zhou X Cane DE Deng Z 《Chemistry & biology》2006,13(9):945-955
The polyketide synthase (PKS) for the biosynthesis of the polyether nanchangmycin lacks an apparent thioesterase comparable to the type I thioesterase domains of the modular PKSs responsible for macrolide biosynthesis. Three candidate polyether chain-releasing factors were examined. Both the putative CR domain and the NanE protein appeared to be genetically relevant. Among the three heterologously expressed soluble proteins (recombinant CR domain, the ACP-CR didomain, and NanE) tested, only NanE hydrolyzed the polyether-SNAC. By contrast, recombinant DEBS TE from the erythromycin pathway, and the recombinant MonAX, a type II TE associated with the polyether monensin biosynthesis for which a homolog has not been detected in the nanchangmycin cluster, hydrolyzed a diketide-SNAC but not the polyether-SNAC. We could thus conclude that NanE is a dedicated thioesterase mediating the specific release of the polyether chain during nanchangmycin biosynthesis. 相似文献