Inhibition of electron transport and oxidative phosphorylation in plant mitochondria by gladiolic acid and structurally-related aromatic ortho dialdehydes

Abstract

Gladiolic acid (GA, 4-methoxy-5-methyl-o-phthalaldehyde-3-carboxylic acid), an antifungal aromatic ortho dialdehyde produced by Penicillium gladioli was found to be a potent inhibitor of electron transport and oxidative phosphorylation reactions in sweet potato and mung bean mitochondria. Similar results were also found with the naturally occurring ortho dialdehydes, cyclopaldic acid, quadrilineatin, and flavipin as well as the synthetic dialdehydes, 3-formyl opianic acid and o-phthalaldehyde. Because of their highly reactive ortho-diformyl grouping, GA and structurally related dialdehydes apparently act as multisite inhibitors affecting electron transport and oxidative phosphorylation (at each coupling site). Gladiolic acid has no uncoupling effect like 2,4-dinitrophenol and does not have the same point of interaction in the energy transfer process as oligomycin. Several "partial" reactions of phosphorylation (Mg+2–DNP-stimulated ATPase; ATP–Pi exchange) were strongly inhibited by the various dialdehydes. Flavipin and quadrilineatin are potent inhibitors (80% at a concentration of 25 μM) of site III phosphorylation. Gladiolic acid and related ortho dialdehydes inactivate the catalytic activity of native cytochrome c in vitro. Lysyl ε-NH2 rich cytochrome c may be a major site of GA action in the intact mitochondrion. In view of the high chemical reactivity of the ortho-diformyl group, it is suggested that mitochondrial function may be affected by aromatic ortho dialdehydes through a combination of reactions involving cross-linking of amino groups on membrane polypeptides and monofunctional reaction with free amino groups important for enzyme function, including ε-NH2 groups on cytochrome c. Cross-linking in mitochondrial membrane systems might affect function by interfering with molecular motion in the operation of the terminal portion of the electron-transport chain. The primary toxicological mode of action of GA and related dialdehydes appears to be due to inhibition of mitochondrial function.