Dissipation of the Membrane Potential in Susceptible Corn Mitochondria by the Toxin of Helminthosporium maydis, Race T, and Toxin Analogs

We have tested directly the effect of Helminthosporium maydis T (Hmt) toxin and various analogs on the membrane potential formed in mitochondria isolated from a Texas (T) cytoplasmic male-sterile and a normal (N) corn. ATP, malate or succinate generated a membrane potential (negative inside) as monitored by the absorbance change of a cationic dye, safranine. The relative membrane potential (Deltapsi) could also be detected indirectly as (45)Ca(2+) uptake. Hmt toxin added to T mitochondria dissipated the steady state Deltapsi similar to addition of a protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Toxin analogs (Cpd XIII: C(41)H(68)O(12) and Cpd IV: C(25)H(44)O(6)), reduced native toxin (RT2C: C(41)H(84)O(13)) and Pm toxin (band A: C(33)H(60)O(8), produced by the fungus, Phyllosticta maydis) were effective in dissipating Deltapsi and decreasing Ca(2+) uptake with the following order: Pm (100) >> HmT (23-30) > Cpd XIII (11-25) >> RT2C (0-4-1.8) > Cpd IV (0.2-1.0). In contrast, the toxins and analogs had no effect on Deltapsi formed in N mitochondria. The striking similarities of the HmT toxin (band 1: C(41)H(68)O(13)) and Cpd XIII on T mitochondrial activities provide strong evidence supporting the correctness of the polyketol structure assigned to the native toxin. Since the Deltapsi in energized mitochondria is caused mainly by the electrogenic extrusion of H(+), the results support the idea that HmT toxin increases membrane permeability of T mitochondria to H(+). The host specificity of the toxin suggests that an interaction with unique target site(s) on the inner mitochondrial membrane of T corn causes H(+) leakage.