STIMULATION OF THE ALTERNATIVE PATHWAY BY SUCCINATE AND MALATE

Stimulation of the cyanide-resistant oxidation of exogenous NADH in po tato (Solanum tuberosum L. cv Bintje) tuber callus mitochondria was ob tained with succinate, malate, and pyruvate. Half-maximal stimulation was observed at a succinate or malate concentration of 3 to 4 mM, whic h is considerably higher than that found for pyruvate (0.128 mM). No e ffect of succinate or malate addition was found when duroquinone was t he electron acceptor. Duroquinol oxidation via the alternative pathway was poor and not stimulated by organic acids. Under stimulating condi tions, no swelling or contraction of the mitochondria could be observe d. Conversely, variation of the osmolarity did not affect the extent o f stimulation. However, the assay temperature had a significant effect : no stimulation occurred at temperatures below 16 to 20 degrees C. Me mbrane fluidity measurements showed a phase transition at about 17 deg rees C. Ubiquinone reduction levels were not significantly higher in t he presence of succinate and malate, but the kinetics of the alternati ve oxidase were changed in a way comparable to that found for the stim ulation by pyruvate. At low temperatures the alternative oxidase displ ayed ''activated'' kinetics, and a role for membrane fluidity in the s timulation of the alternative pathway by carboxylic acids is suggested .