Christiaan J. Rabie

Characterization of magnesium and calcium tenuazonate from Phoma sorghina

Abstract Phoma sorghina Sacc., the fungus implicated in the aetiology of onyalai, a haematologic disorder, produces magnesium and calcium tenuazonate as toxic constituents.

Toxigenicity of Rhizopus species

Abstract The toxigenicity of 14 isolates representing 14 species of Rhizopus isolated from sorghum malt, fermented foods and various other foodstuffs, was evaluated in ducklings and rats. Moulded maize meal was prepared by growing single-spored isolates on autoclaved whole yellow maize for 21 days at 25 and 37°C. Of 104 isolates fed ad libitum to ducklings at a 50% (w/w) moulded meal dose level, 47 were toxigenic (killing 3 4 or 4 4 in 14 days). Toxigenic strains of the following species were isolated: R. chinensis, R. formosanensis, R. japonicus, R. japonicus, R. javanicus, R. microsporus, R. nigricans, R. oryzae, R. rhizopodiformis and R. tonkinensis. Fifteen of the isolates toxic to ducklings were tested in rats. Two were acutely toxic, viz. R. microsporus and R. chinensis, while all seven isolates of R. oryzae, caused growth retardation. Rhizonin A, the sole toxin chemically characterised in the genus Rhizopus, could only be detected in culture material of R. microsporus of the 11 species that were tested. Unknown toxin(s) must be responsible for the toxigenicity of these other species. The growth substrate (maize vs. soy-beans) had an influence on the toxigenicity of culture material of some isolates to ducklings. R. oryzae and R. japonicus were much more toxic when cultured on maize than on soy-beans.

Cytochalasins E and K, toxic metabolites from Aspergillus clavatus

The isolation of cytochalasin E and cytochalasin K from Aspergillus clavatus is described. The structure of cytochalasin K, a new metabolite, has been determined by spectroscopic data and chemical reactions. The relationship between cytochalasin E and cytochalasin K was confirmed by chemical conversion. Chemical reactions on cytochalasins E and K, both of which contain the unique vinyl carbonate moiety, are described.

X-Ray crystal structure of asteltoxin, a novel mycotoxin from aspergillus stellatus curzi

The structure of a new mycotoxin, asteltoxin, isolated from cultures of Aspergillus stellatus is proposed, based on 13C and 1H n.m.r. spectral data and X-ray crystallography.

The mycoflora of sorghum malt

A quantitative and qualitative study of the mycoflora of both commercial and industrial sorghum malt was undertaken. Samples were collected from three industrial (22 samples) and 10 commercial (65 samples) maltsters over a period of three years. Fungal propagule counts were made of surface sterilized kernels on four different media (potato dextrose agar, acidified Czapeks-Dox medium, malt-salt agar and pentachloronitrobenzene medium) using both the dilution plate and the whole kernel method. Counts were unacceptably high in many samples, especially in certain commercial malts. Counts differed between malts from different industrial malt producers, some consistently showing higher contamination levels than others. Differences were found between the dominant fungi occurring in the two malt types and the two evaluation methods also gave different results regarding certain taxa. In all, 61 different species representing 29 genera were found to occur in sorghum malt. Fungi most frequently encountered were: Yeasts, Rhizopus rhizopodiformis (Cohn) Zopf, R. oryzae Went & Prinsen Geerligs. Aspergillus clavatus Desmazieres. A. flavus Link, Fusarium moniliforme Sheldon, F. moniliforme Sheldon var. subglutinans Wollenw. & Reinking, F. chlamydosporum Wollenw. & Reinking and Phoma sorghina (Sacc.) Boerema, Dorenbosch & Van Kesteren.

Metabolites of Aspergillus ustus. Part 3. Structure elucidation of austalides G–L

The structure elucidation of austalides G–L, based on a study of their 1H and 13C n.m.r. spectra and chemical derivatization, is described.

The isolation, structure, and absolute configuration of the mycotoxin, rhizonin A, a novel cyclic heptapeptide containing N-methyl-3-(3-furyl)alanine, produced by Rhizopus microsporus

Rhizonin A, the main mycotoxin from Rhizopus microsporus, is a cyclic heptapeptide containing valine, allo-isoleucine, leucine, N-methylalanine, N-methyl-3-(3-furyl)alanine (2 : 1 : 1 : 1 : 2), its amino-acid sequence having been established by mass spectrometry; X-ray crystallography verified the structure of rhizonin A and afforded its conformation and relative configuration, and its absolute configuration was obtained from high performance liquid chromatographic studies of the dansyl-derivatised amino-acids.

Metabolites of Aspergillus ustus. Part 1. Application of the heteronuclear selective population inversion (SPI) n.m.r. technique to the structure elucidation of the austalides A–F, novel ortho ester meroterpenoids

The isolation and characteristics of 12 biosynthetically related metabolites, austalides A–L from cultures of Aspergillus ustus are reported. The structure elucidation of the austalides A–F is based on a detailed study of their high-field 1H and 13C n.m.r. spectra and especially on the three- and two-bond (C,H) connectivity pattern as determined by heteronuclear 13C-{1H} selective population inversion (SPI) experiments. The conformation and relative configuration of austalides A and D were deduced from the observed proton–proton nuclear Overhauser effects (n.O.e.s) and the magnitude of the proton–proton coupling constants. Base-catalysed hydrogen–deuterium exchange on the diketone derivative (18), obtained by Jones oxidation of austalide F, leads to the regio- and stereo-specific incorporation of deuterium atoms at C-12, C-18, and C-21. A mechanism is proposed to explain the incorporation of a deuterium atom at C-21, a position γ to the C-13 carbonyl group.

Structures of the austalides A–E, five noval toxic metabolites from Aspergillus ustus

The structures of austalides A–E, five new mycotoxins isolated from cultures of Aspergillus ustus, are based on the X-ray crystallographic study of austalide A, chemical derivatization, and correlation of their 13C and 1H n.m.r spectral data.

Structure elucidation of a novel trichothecene glycoside using 1H and 13C nuclear magnetic resonance spectroscopy

The structure elucidation of a novel trichothecene glycoside, 15-acetoxy-3α-hydroxy-4β-(α-D-glucopyranosyloxy)-12,13-epoxytrichothec-9-ene, is based on a detailed study of its one- and two-dimensional 1H and 13C n.m.r. spectra, and chemical reactions. The metabolite displays a reduced toxicity when compared with other trichothecene metabolites in several biological systems.