Effect of pH on CN-resistant respiratory activity and regulation on Vigna uniguiculata mitochondria

Mitochondria from Vigna unguiculata cv. Vita 5 have a cyanide insensitive alternative oxidase cross-reacting with a monoclonal antibody raised against the alternative oxidase of Sauromatum guttatum. In the presence of NADH as substrate and dithiothreitol, the CN-resistant respiration of V. unguiculata intact mitochondria was found to be significantly influenced by assay pH with an optimum value of 6.25. This effect was still observed when nigericin, known to abolish ΔpH between the matrix and the intermembrane space, was present in the reaction medium. This pH effect was shown to be reversible. Alternative oxidase activation by pyruvate, also appeared to be pH dependent with an optimum pH value of 7.25. This modulation of the alternative oxidase activity by pH has been tentatively attributed to a new protonated-deprotonated status of this protein.