Metabolic studies on the transformation of trichodiene to trichothecene mycotoxins

Hesketh, Andrew R. (1991) Metabolic studies on the transformation of trichodiene to trichothecene mycotoxins. PhD thesis, University of Nottingham.



Trichodiene and [14C]trichodiene have been produced in high yields by treatment of Fusarium culmorum CMI 14764 cultures with the furanocoumarin xanthotoxin. Smaller amounts of isotrichodermin (ITD) and the unsubstituted trichothecene 12,13-epoxytrichothec-9-ene (EPT) were obtained in the same way. EPT was also produced by semi-synthesis from ITD.

Trichodiene (TDN) was shown to be a precursor of the trichothecene mycotoxins in F. culmorum, including EPT, ITD, calonectrin (CAL), 7a-hydroxycalonectrin (7-hydroxyCAL), 15-deacetylcalonectrin, 3-acetyldeoxynivalenol (3-AcDON) and 7,8-dihydroxycalonectrin (DHC). When large amounts of TDN were supplied, a new trichodiene metabolite was found to accumulate which was fully characterised as 12,13-epoxy-2a, 11 adihydroxytrichodiene, and given the trivial name isotrichodiol. A method for the production of 14C-labelled isotrichodiol (ITdiol) was developed, and the incorporation of ["C]ITdiol into 3-AcDON, DHC and 7-hydroxyCAL was demonstrated. Slow, acid-catalysed cyclisation of ITdiol to EPT and pre-sambucoin was demonstrated, and allylic isomerisation to both 9a- and 9p-trichodiol was also detected. Labelled pre-sambucoin was incorporated into sambucoin by F. culmorwn, and ITdiol is thus proposed as a precursor to both sambucoin and sambucinol, aswel as to the trichothecenes.

A range of semi-synthetic derivatives of TDN were prepared and tested as possible inhibitors of the post-TDN biosynthesic pathway to trichothecenes in F. culmorum. In whole-cell systems all the derivatives inhibited the incorporation of labelled TDN into trichothecenes, and also initiated the production of ITdiol. One derivative, 9P, 10ß-epoxytrichodiene, was shown to be biotransformed by the fungus, undergoing 12,13-epoxidation with subsequent hydroxylation at C-3 producing 3a-hydroxy-9(3,10(3; 12,13-diepoxytrichodiene.

9ß-Trichodiol was isolated from Trichothecium roseum, and its slow, acidcatalysed cyclisation to EPT was demonstrated. 9a-Trichotriol, 9ß-trichotriol and isotrichotriol were isolated from F. culmorum for the first time, and literature assignments for the stereochemistry of the C-9 hydroxyl in trichodiol and trichotriol are reassessed. The incorporation of [`4C]ITdiol into trichothecenes in F. culmorum was found to be approximately 5 times greater than the incorporation of [14C]-913-trichotriol, and was shown to be inhibited by isotrichotriol but not by 9ß-trichodiol and 9ß-trichotriol. It is proposed that trichodiol and trichotriol are not biosynthetic intermediates in the pathway to the trichothecenes, and that they are non-enzymic metabolites produced from ITdiol and isotrichotriol, respectively, by acid-catalysed isomerisations. A new scheme for the biosynthesis of trichothecenes is proposed in which ITdiol and isotrichotriol are intermediates in the production of isotrichodermol from TDN.

Two novel compounds, 15-deacetyl-7,8-dihydroxycalonectrin (15-deacetylDHC) and 8a-hydroxyisotrichodiol were isolated from F. culmorum, and 15-deacetylDHC and DHC were shown to be precursors to 3-AcDON. It is proposed that the post-cyclisation biosynthesis of 3-AcDON involves sequential oxygenation of isotrichodermol at C-15, C-7 and C-8 producing DHC, which then undergoes deacylation to 15-deacetylDHC followed by oxidation at C-8 to 3-AcDON.

Item Type:Thesis (PhD)
Supervisors:Bycroft, B.W.
Dewick, P.M.
Faculties/Schools:UK Campuses > Faculty of Science > School of Pharmacy
ID Code:2521
Deposited By:Mrs Olga Lashkova
Deposited On:11 Apr 2012 10:52
Last Modified:11 Apr 2012 10:52

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