Biosynthesis and mass spectral fragmentation pathways of (13)C and (15)N labeled cytochalasin D produced by Xylaria arbuscula.

The fungus Xylaria arbuscula was isolated as an endophyte from Cupressus lusitanica and has shown to be a prominent producer of cytochalasins, mainly cytochalasins C, D and Q. Cytochalasins comprise an important class of fungal secondary metabolites that have aroused attention due to their uncommon molecular structures and pronounced biological activities. Due to the few published studies on the ESI-MS/MS fragmentation of this important class of secondary metabolites, in the first part of our work, we studied the cytochalasin D fragmentation pathways by using an ESI-Q-ToF mass spectrometer coupled with liquid chromatography. We verified that the main fragmentation routes were generated by hydrogen and McLafferty rearrangements which provided more ions than just the ones related to the losses of H2 O and CO as reported in previous studies. We also confirmed the diagnostic ions at m/z 146 and 120 as direct precursor derived from phenylalanine. The present work also aimed the production of structurally diverse cytochalasins by varying the culture conditions used to grow the fungus X. arbuscula and further insights into the biosynthesis of cytochalasins. HPLC-MS analysis revealed no significant changes in the metabolic profile of the microorganism with the supplementation of different nitrogen sources but indicated the ability of X. arbuscula to have access to inorganic and organic nitrogen, such as nitrate, ammonium and amino acids as a primary source of nitrogen. The administration of 2-13 C-glycine showed the direct correlation of this amino acid catabolism and the biosynthesis of cytochalasin D by X. arbuscula, due to the incorporation of three labeled carbons in cytochalasin chemical structure. Copyright © 2017 John Wiley & Sons, Ltd.