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.

Structural Diversity and Biological Activities of Indole Diketopiperazine Alkaloids from Fungi

Indole diketopiperazine alkaloids are secondary metabolites of microorganisms that are widely distributed in filamentous fungi, especially in the genera Aspergillus and Penicillium of the phylum Ascomycota or sac fungi. These alkaloids represent a group of natural products characterized by diversity in both chemical structures and biological activities. This review aims to summarize 166 indole diketopiperazine alkaloids from fungi published from 1944 to mid-2015. The emphasis is on diverse chemical structures within these alkaloids and their relevant biological activities. The aim is to assess which of these compounds merit further study for purposes of drug development.

Occurrence of halogenated alkaloids

Once considered to be isolation artifacts or chemical "mistakes" of nature, the number of naturally occurring organohalogen compounds has grown from a dozen in 1954 to >5000 today. Of these, at least 25% are halogenated alkaloids. This is not surprising since nitrogen-containing pyrroles, indoles, carbolines, tryptamines, tyrosines, and tyramines are excellent platforms for biohalogenation, particularly in the marine environment where both chloride and bromide are plentiful for biooxidation and subsequent incorporation into these electron-rich substrates. This review presents the occurrence of all halogenated alkaloids, with the exception of marine bromotyrosines where coverage begins where it left off in volume 61 of The Alkaloids. Whereas the biological activity of these extraordinary compounds is briefly cited for some examples, a future volume of The Alkaloids will present full coverage of this topic and will also include selected syntheses of halogenated alkaloids. Natural organohalogens of all types, especially marine and terrestrial halogenated alkaloids, comprise a rapidly expanding class of natural products, in many cases expressing powerful biological activity. This enormous proliferation has several origins: (1) a revitalization of natural product research in a search for new drugs, (2) improved compound characterization methods (multidimensional NMR, high-resolution mass spectrometry), (3) specific enzyme-based and other biological assays, (4) sophisticated collection methods (SCUBA and remote submersibles for deep ocean marine collections), (5) new separation and purification techniques (HPLC and countercurrent separation), (6) a greater appreciation of traditional folk medicine and ethobotany, and (7) marine bacteria and fungi as novel sources of natural products. Halogenated alkaloids are truly omnipresent in the environment. Indeed, one compound, Q1 (234), is ubiquitous in the marine food web and is found in the Inuit from their diet of whale blubber. Given the fact that of the 500,000 estimated marine organisms--which are the source of most halogenated alkaloids--only a small percentage have been investigated for their chemical content, it is certain that myriad new halogenated alkaloids are awaiting discovery. For example, it is estimated that nearly 4000 species of bryozoans have not been examined for their chemical content. The few species that have been studied contain some extraordinary halogenated alkaloids, such as hinckdentine A (610) and the chartellines (611-613). Of the estimated 1.5 million species of fungi, secondary metabolites have been characterized from only 5000 species. The future seems bright for the collector of halogenated alkaloids!

Failure of immunisation against sporidesmin or a structurally related compound to protect ewes against facial eczema

Sixty ewes, divided randomly into four groups of 15, were immunised subcutaneously against sporidesmin (sdm) -bovine thyroglobulin (BTG) or 2-amino-5-chloro-3,4-dimethoxy benzyl alcohol (ACDMBA) coupled to heat killed staphylococci or to bovine gamma globulin. Fifteen ewes served as untreated controls. Approximately 10 weeks after inoculation ewes were dosed orally with sdm at a rate of 0.1 mg/kg body weight/day for three consecutive days. Sdm antibody binding values.in plasma collected before dosing were higher in ewes immunised with sdm-BTG than ewes given the ACDMBA-complexes. Levels in the 15 untreated ewes were all very low. However, despite the presence of antibodies, the immunised ewes were not protected against sdm challenge; and cholesterol and bilirubin levels in serum and liver and urinary bladder damage scores, at slaughter, were all significantly higher (P<0.05) in the immunised compared to the control ewes six weeks after dosing. It is concluded from these results that subcutaneous immunisation against sdm or the structurally related substance used did not protect sheep against sdm dosing.

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.

Studies on the mechanism of toxicity of the mycotoxin, sporidesmin. I. Generation of superoxide radical by sporidesmin

Sporidesmin (SDMS2), the mycotoxin responsible for 'facial eczema' in ruminants, contains a disulphide group which appears to be intimately involved in its toxic action. The reduced (dithiol) form of sporidesmin has been shown readily to undergo autoxidation in vitro in a reaction which generates superoxide radical (O2-). The autoxidation reaction, which takes place over a wide pH range, is strongly catalysed by trace amounts of copper, although the reaction was inhibited at high concentrations of this metal. Inhibition of the autooxidation of reduced sporidesmin (SDM(SH)2) was also observed in the presence of nickel, cobalt and manganese. Superoxide radical is also generated from SDMS2 itself in a cyclic reduction/autoxidation reaction with glutathione and other thiols; in view of the known toxicity of superoxide and its derivatives, it is suggested that oxygen-free-radicals may be involved in the initiation of the deleterious effects of the mycotoxin.

The growth of Bacillus subtilis in media containing chetomin, sporidesmin, and gliotoxin

Sporidesmin, gliotoxin, and chetomin increase the lag phase of growth of Bacillus subtilis. By daily subculture of the organism in media that contained one of the antibiotics in sufficient concentration to increase the lag phase of growth by 300%, organisms were obtained whose lag phase of growth was the same as that of the parent strain (HLX 373). Repetition of this process has made it possible to obtain cultures which have a lag phase of growth of 300 ± 20 min in the presence of chetomin (0.15 μg/ml), gliotoxin (3.0 μg/ml), or sporidesmin (30 μg/ml). The culture which could grow in the presence of gliotoxin lost this ability on subcultivation in the absence of the antibiotic. Sufficient cross resistance was shown to indicate the possibility that these three antibiotics had a similar mode of action.