Structural Diversity and Biological Activities of the Cyclodipeptides from Fungi

Cyclodipeptides, called 2,5-diketopiperazines (2,5-DKPs), are obtained by the condensation of two amino acids. Fungi have been considered to be a rich source of novel and bioactive cyclodipeptides. This review highlights the occurrence, structures and biological activities of the fungal cyclodipeptides with the literature covered up to July 2017. A total of 635 fungal cyclodipeptides belonging to the groups of tryptophan-proline, tryptophan-tryptophan, tryptophan-Xaa, proline-Xaa, non-tryptophan-non-proline, and thio-analogs have been discussed and reviewed. They were mainly isolated from the genera of Aspergillus and Penicillium. More and more cyclodipeptides have been isolated from marine-derived and plant endophytic fungi. Some of them were screened to have cytotoxic, phytotoxic, antimicrobial, insecticidal, vasodilator, radical scavenging, antioxidant, brine shrimp lethal, antiviral, nematicidal, antituberculosis, and enzyme-inhibitory activities to show their potential applications in agriculture, medicinal, and food industry.

Thiodiketopiperazines produced by the endophytic fungus Epicoccum nigrum

Thirteen new thiodiketopiperazines, epicoccin I (1), ent-epicoccin G (2), and epicoccins J-T (3-13), together with six known diketopiperazines (14-19), have been isolated from the endophytic fungus Epicoccum nigrum. The structures of 1, 2, and 10 were confirmed by X-ray crystallography, and the absolute configurations of 2, 4, 6, and 8 were assigned using Moshers' method. Compounds 2, 6, 12, and 17 showed potent activities in vitro against the release of beta-glucuronidase in rat polymorphonuclear leukocytes induced by platelet-activating factor, with IC(50) values of 3.07, 4.16, 4.95, and 1.98 microM, respectively. None of the 19 compounds exhibited detectable cytotoxic activities toward six tumor cell lines (A549, Be-l7402, BGC-823, HCT-8, HCT-116, and A2780) in the MTT assay.

Urinary excretion of immunoreactive sporidesmin metabolites in sheep in relation to factors influencing susceptibility to sporidesmin intoxication

AIM

To study the urinary disposition of orally administered sporidesmins A and D in sheep and identify factors influencing their kinetics, particularly the influence of breeding for resistance and susceptibility to sporidesmin, the mycotoxin responsible for the hepatogenous photosensitisation, facial eczema.

METHODS

A competitive ELISA was used to monitor urinary output of immunoreactive metabolites after the intraruminal administration, to female Romney sheep, of either sporidesmin A or sporidesmin D, the nontoxic analogue. Preliminary characterisation of metabolites was carried out using HPLC with fractions monitored by ELISA.

RESULTS

Maximum urinary excretion rates of immunoreactive metabolites occurred 2-8 h after dosing with sporidesmin D and 15-30 h after dosing with sporidesmin A. Sporidesmin D caused no liver injury, as detected by changes in serum enzyme activity, while the liver injury caused by sporidesmin A was greatest for the sheep with the highest cumulative output of metabolite. When sporidesmin D was administered in two separate doses to sheep bred for either resistance or susceptibility to facial eczema, the variability of metabolic output between sheep within groups was much less after the second dose. The mean urinary metabolite excretion was greater for the susceptible than the resistant sheep but the difference was not significant. Potentiation (caused by pre-administration of small doses of sporidesmin A) resulted in a more severe reaction to the dosed sporidesmin A. Urinary output of metabolite was less in the potentiated than in the unpotentiated sheep. When resistant and susceptible sheep were dosed with sporidesmin A after potentiation there was no difference between them in their cumulative totals or excretion rates of immunoreactive metabolites. However, the volume of urine produced by the susceptible sheep was lower and less variable than the resistant sheep and consequently the concentration of their urinary metabolites was higher. Preliminary ELISA examination of HPLC-fractionated urine from a sheep dosed with sporidesmin A indicated the presence of several metabolites of sporidesmin.

CONCLUSION

Sporidesmin A and metabolites are rapidly excreted in urine but not as rapidly as sporidesmin D and its metabolites. Only minor differences between sheep bred for resistance and susceptibility were seen. Potentiation caused a more severe reaction to sporidesmin A and less urinary excretion of the sporidesmin and its metabolites.

CLINICAL RELEVANCE

This work is part of a programme with the aim of identifying FE-resistant animals without the need for sporidesmin dosing.

Metal complexes of sporidesmin D and dimethylgliotoxin, investigated by electrospray ionisation mass spectrometry

Electrospray ionisation mass spectrometry (ES-MS) has been used to probe the coordination chemistry of metabolites such as sporidesmin D (spdD), found in the saprophytic fungus Pithomyces chartarum, and the related bisdethiobis(methylthio)gliotoxin (dimethylgliotoxin, Megtx). SpdD forms complexes of the type [spdD+M(MeCN)] and [2spdD+M]+ (M=Cu, Ag) and, at higher cone voltages, [spdD+M]+. The bis(ligand) ion [2spdD+M]+ was observed at very high cone voltages, indicating it has appreciable stability; the proposed structure of this species has a four-coordinate metal ion with two bidentate spdD ligands, coordinated through their SMe groups. 1H NMR titrations of spdD with K+, Ag+ and Cu+ provided additional evidence for complex formation with the soft metals. SpdD forms only relatively weak complexes with Zn2+, Cd2+, Co2+ and Mn2+, in keeping with the known reduced tendency of these metals to form stable thioether complexes. ES-MS studies of Megtx showed similar results to spdD, with stable adducts formed with Cu+ and Ag+ ions. The X-ray crystal structure of spdD is also reported.