Comparison of Activities of the Host-Specific Toxin of Helminthosporium maydis, Race T, and a Synthetic C41 Analog

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Abstract
It was previously proposed that the host-selective toxin of H. maydis race T consists of a series of unusual linear (C35-C45)polyketols, of equal toxicity on a weight or molar (10-8 to 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 to 10-7) basis. In the present study, a C41 analog with 4 .beta.-ketol units spaced by CH2 bridges as in native toxin was 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, providing 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. 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.