Studies on the succinate dehydrogenating system. II. Reconstitution of succinate-ubiquinone reductase from the soluble components.

1. A protein fraction containing three polypeptides (the major one with Mr < 13 000) was isolated by means of Triton X-100 extraction of submitochondrial particles specifically treated to remove succinate dehydrogenase. 2. The mixing of the protein fraction with the soluble reconstitutively active succinate dehydrogenase results in formation of highly active succinate-DCIP reductase which is sensitive to thenoyltrifluoroacetone or carboxin. 3. The maximal turnover number of succinate dehydrogenase in the succinate-DCIP reductase reaction revealed in the presence of a saturating amount of the protein fraction is slightly higher than that measured with phenazine methosulfate as artificial electron acceptor. 4. The protein fraction greatly increases the stability of soluble succinate dehydrogenase under aerobic conditions. 5. The titration of soluble succinate dehydrogenase by the protein fraction shows that smaller amounts of the protein fraction are required to block the reduction of ferrycyanide by Hipip center than that required to reveal the maximal catalytic capacity of the enzyme. 6. The apparent Km of the reconstituted system for DCIP depends on the amount of protein fraction; the more protein fraction added to the enzyme, the lower the Km value obtained. 7. A comparison of different reconstituted succinate-ubiquinone reductases described in the literature is presented and the possible arrangement of the native and reconstituted succinate-ubiquinone region of the respiratory chain is discussed.