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Abstract
HC-toxin, a host-specific toxin produced by Helminthosporium carbonum, race 1, is a cyclic tetrapeptide, cyclo (L-Ala-D-Ala-L-Aoe-D-Pro). Aoe stands for an unusual amino acid, 2-amino-8-oxo-9,10-epoxydecanoic acid. Except for the epoxide group, structural features for the toxicity and host-specificity of HC-toxin were uncertain. Therefore, the contributions of various structural features to the biological activities of HC-toxin were investigated by chemical modification of HC-toxin and by comparing the action of HC-toxin to other naturally occurring cyclic tetrapeptides containing Aoe.
Two stereoisomers, produced by reduction of the ketone function in Aoe had no biological activity, suggesting that HC-toxin requires the ketone group for toxicity as well as the epoxide group.
A minor toxin in which glycine replaces the D-alanine residue was isolated. The structure of this minor toxin, called Gly-HC-toxin, was determined to be cyclo (L-Ala-Gly-L-Aoe-D-Pro). Bioassays indicated that the Gly-HC-toxin was only one-thirty-fifth as toxic as HC-toxin, but the toxicity was specific for HC-toxin susceptible maize cultivars. Effectively, the substitution of a hydrogen atom for a methyl group caused a significant decrease in toxicity. NMR analysis indicated that the ring conformations of HC-toxin and Gly-HC-toxin were nearly identical.
The related cyclic tetrapeptides, CYL-2 and chlamydocin were isolated and their biological activities determined. Chlamydocin and CYL-2 were nearly as toxic as HC-toxin against maize. However, their activities were non-specific. Thus, molecular features that determine toxicity and cultivar specificity appear to be different. Specificity may be due to the overall conformation of the peptide ring.
A chlamydocin analog, containing an alanine residue instead of (alpha)-amino isobutyric acid, residue was isolated and characterized.
The binding of HC-toxin to a guanosine, GMP and isolated maize DNA was determined by fluorescence spectroscopy. Changes in fluorescence indicated that HC-toxin alkylated nucleotides and DNA non-specifically.
