Malate oxidation, rotenone-resistance, and alternative path activity in plant mitochondria

The effect of cyanide and rotenone on malate (pH 6.8), malate plus glutamate (pH 7.8), citrate, α-ketoglutarate, and succinate oxidation by cauliflower (Brassica oleracea L.) bud, sweet potato (Ipomoea batatis L.) tuber, and spinach (Spinacia oleracea and Kalanchoë daigremontiana leaf mitochondria was investigated. Cyanide inhibited all substrates equally with the exception of malate plus glutamate; in this case, inhibition of O₂ uptake was more severe due to an effect of cyanide on aspartate aminotransferase. Azide and antimycin A gave similar inhibitions with all substrates. Subsequent addition of NAD had no effect with any substrate. Providing that oxalacetate accumulation was prevented, rotenone inhibited all NAD-linked substrates equally and caused ADP:O ratios to decrease by one-third. Addition of succinate to mitochondria oxidizing malate stimulated oxygen uptake, but adding citrate and α-ketoglutarate did not. These results indicate that there is no direct link between malic enzyme and the rotenone- and cyanide-resistant respiratory pathways, and that there is no need to postulate separate compartmentation of malic enzyme and the other NAD-linked enzymes in the matrix.