Regulation of Succinate Dehyrogenase in Higher Plants: I. Some General Characteristics of the Membrane-bound Enzyme.

The spectrophotometric phenazine methosulfate assay of succinate dehydrogenase was adapted to use with cauliflower (Brassica oleracea) and mung bean (Phaseolus aureus) mitochondria with suitable modifications to overcome the permeability barrier to the dye. Procedures in the literature for the isolation and sonic disruption of mitochondria from these sources were modified to assure maximal yield and stability of the enzyme. In tightly coupled mung bean mitochondria, as isolated, about half of the succinate dehydrogenase is in the deactivated state, and the enzyme is further extensively deactivated on sonication or freeze-thawing. In cauliflower mitochondria most of the enzyme is in the deactivated form, and little or no further deactivation occurs on sonication or freeze-thawing. Incubation of mitochondria from either source with succinate leads to full activation of the enzyme. The energy of activation for the conversion of the deactivated to the activated form in membranal preparations under the influence of substrate is about 30,000 cal/mole, essentially the same value as in animal tissues. Activation of the enzyme also occurs under the influence of a variety of other agents, among which the action of anions as activators is documented in the present paper. Activation is accompanied by the release of very tightly bound oxaloacetate. As in animal tissues, the enzyme appears to contain covalently bound flavin (histidyl 8alpha-FAD), and the turnover number is 19,400 moles of succinate oxidized/mole of histidyl flavin at pH 7.5, 38 C.