Comparison of Activities of the Host-Specific Toxin of Helminthosporium maydis, Race T, and a Synthetic C(41) Analog

It previously had been proposed that the host-selective toxin of Helminthosporium maydis race T consists of a series of unusual linear (C₃₅ to C₄₅)polyketols, of equal toxicity on a weight or molar (10⁻⁸−10⁻⁹) basis. Previous laboratory synthesis of T-toxin analogs was limited to shorter (C₁₅ to C₂₆) versions which possessed the requisite specificity for susceptible corn (Zea mays) but were less toxic on a weight or molar (10⁻⁶−10⁻⁷) basis. In the present study, a C₄₁ analog with four β-ketol units spaced by CH₂ bridges as in native toxin has been synthesized. On a weight or molar basis, it is as effective as native toxin or its purified components in stimulating NADH oxidation of mitochondria from susceptible corn, thus providing firm evidence for the correctness of the proposed structures of T-toxin. Additional support derives from the observation that C₂₄ and C₂₆ analogs with -(CH₂)₄- and -(CH₂)₆- bridges between ketol groups are not as effective in stimulating NADH oxidation as are C₂₃ and C₂₅ analogs with the -(CH₂)₃- and -(CH₂)₅- bridges of native T-toxin.

It was calculated that a single molecule of the C₄₁ analog is at least 300 times more effective in stimulating mitochondrial oxidation than a molecule of the C₂₃ or C₂₅ analogs. This emphasizes the importance of chain length for toxicity, perhaps through perturbation of membrane functions of mitochondria and/or chloroplasts.