Inhibition of Succinate Ubiquinone Reductase by an Extract of Acetone Treated Mitochondria*

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Abstract
The properties of rat liver mitochondria treated with acetone containing various proportions of water were investigated. Acetone containing one percent of water extracted ubiquinone (UQ) quantitatively from mitochondria without causing any significant change in the content of phospholipid. This preparation consumed oxygen with succinate as substrate on addition of cytochrome c alone, and UQ had little influence on this activity. Thus, oxygen seemed to be consumed via a UQ independent pathway. A phosphate buffer extract (F-S) of mitochondria which were pretreated with acetone containing one percent water inhibited the succinate-UQ reductase activity of the insoluble residue (R). From the effects of heat, gel filtration of Sephadex, and chymotrypsin [EC 3.4.4.5] treatment, the inhibitory factor in F-S seemed to be a protein. This protein factor was not readily solubilized unless mitochondria were pretreated with aqueous acetone. A marked inhibitory effect of F-S was observed when it was allowed to come into contact with UQ in a high ionic strength medium before incubation with R. The mechanism of inhibition of succinate-UQ reductase by F-S was discussed.