Changes in the redox state of the alternative oxidase regulatory sulfhydryl/disulfide system during mitochondrial isolation: Implications for inferences of activity in vivo
The cyanide-resistant alternative oxidase of plants exists as a homodimer in the inner mitochondrial membrane. When the subunits of the dimer are connected by an intermolecular disulfide bond, the enzyme is relatively inactive. When this bond is reduced to its constituent sulfhydryls, the enzyme becomes more active and can be further activated by α-keto acids. We have attempted to correlate the proportions of oxidized and reduced alternative oxidase with the physiological state of the plant by using immunoblots to visualize the ratio of alternative oxidase species present. However, during the process of mitochondrial isolation from Sauromatum guttatum and Glycine max, the alternative oxidase underwent oxidation of the intermolecular sulfhydryl/disulfide system with the result that the proportion of oxidized and reduced alternative oxidase species in isolated mitochondria differed from that of the starting plant material. The presence of the sulfhydryl reagents, N-ethylmaleimide and iodoacetate, during mitochondrial isolation prevented alternative oxidase sulfhydryl/disulfide oxidation, but also led to reduction of the oxidized protein species. Thus, it appears not possible to determine the redox state of the alternative oxidase sulfhydryl/disulfide system in vivo. Further, due to the spontaneous formation of the oxidized alternative oxidase species during isolation, alternative oxidase activity will be less in isolated mitochondria than in the plant tissue from which the mitochondria were derived.
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