Malate Oxidation, Rotenone-Resistance, and Alternative Path Activity in Plant Mitochondria

Abstract

The effect of cyanide and rotenone on malate (pH 6.8), malate plus glutamate (pH 7.8), citrate, .alpha.-ketoglutarate and succinate oxidation by cauliflower (Brassica oleracea L.) bud, sweet potato (Ipomoea batatas L.) tuber and spinach (Spinacia oleracea and Kalanchoe 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 1/3. Addition of succinate to mitochondria oxidizing malate stimulated O2 uptake, but adding citrate and .alpha.-ketoglutarate did not. Evidently, there is no direct link between malic enzyme and the rotenone- and cyanide-resistant respiratory pathways and there is no need to postulate separate compartmentation of malic enzyme and the other NAD-linked enzymes in the matrix.