Component of the alternative oxidase localized to the matrix surface of the inner membrane of plant mitochondria

In mitoplasts from Arum maculatum spadices, succinate dehydrogenase (EC 1.3.99.1) and the alternative, cyanide‐resistant oxidase activity (measured as m‐chlorobenzhydroxamic acid‐sensitive duroquinol oxidation) was unaffected by treatment with trypsin. In contrast, when 85% inside‐out submitochondrial particles were treated with trypsin the alternative oxidase activity was inhibited by about 50% and succinate dehydrogenase activity by about 40%. Thus, a trypsin‐sensitive component of the alternative pathway is located on the inner surface of the inner mitochondrial membrane. After trypsin treatment of the inside‐out submitochondrial particles the inhibited alternative oxidase activity was partly restored by including 0.7 M citrate in the assay medium. This indicates that trypsin does not destroy the active site but merely causes a conformational change in the enzyme, thereby lowering its activity.