Molecular Features Affecting the Biological Activity of the Host-Selective Toxins from Cochliobolus Victoriae
The structures of the toxins produced by Cochliobolus victoriae, victorin B, C, D, E, and victoricine have recently been established. These toxins and modified forms of victorin C were tested for their biological activity. Half-maximal inhibition of dark CO2 fixation in susceptible oat genotypes occurred with the native toxins in the range of 0.004 – 0.546 μM. An essential component for the inhibitory activity of victorin is the glyoxylic acid residue. Removal of glyoxylic acid completely abolished the inhibitory activity of victorin and the reduction of the aldehyde group transformed the toxin into a protectant. Conversion of victorin to its methyl ester reduced the inhibitory activity to 10% of the original activity and derivatization of the ε-amino group of the OHlys moiety decreased the inhibitory activity to 1% of victorin C. In addition, the opening of the macrocyclic ring of the toxin drastically reduced the inhibitory activity.