Effect of membrane environment on succinate dehydrogenase activity.

The turnover number of succinate dehydrogenase from mammalian heart determined by the spectrophotometric phenazine methosulfate assay, after complete activation, is approximately 21,000 mol of succinate oxidized/min/mol of histidyl flavin at 38 degrees in relatively intact inner membrane preparations and mitochondria. Reconstitutively active soluble preparations, extracted anaerobically in the presence of succinate from inner membrane preparations show turnover numbers of 11,500 to 14,500 and a significantly lower apparent Km for phenazine methosulfate than the parent particles. The decline of both the turnover number and of the Km occurs during the brief period when the enzyme is detached from the membrane. The observed values represent the activities in the soluble extract of both the reconstitutively active and reconstitutively inactive enzyme. The latter may be from 10 to 40% even in the most carefully prepared enzyme; it has a lower turnover number in the phenazine methosulfate assay than the average for the solution and is devoid of catalytic activity in the "low Km" ferricyanide assay (Vinogradov, A. D., Ackrell, B.A.C., and Singer, T.P. (1975) Biochem. Biophys. Res. Commun. 67, 803-809). The reconstitutively active form of the soluble enzyme has a turnover number of at least 15,000 and an equal activity in the low Km ferricyamide assay. When recombined with the membrane the total activity of the enzyme is increased by over 60% and it regains the original turnover number, Km for phenazine methosulfate, and sensitivity of the phenazine methosulfate reductase activity to thenoyltrifluoroacetone, carboxamides, and cyanide. It appears, therefore, that the membrane environment or some component of it exerts a positive modulating influence on the enzyme even in the fully activated state. In certain particulate sources (Keilin-Hartree preparations, Complex II) the enzyme shows lower turnover numbers (11,000 to 12,500) than in more intact inner membranes. This seems to be due to inactivation in the course of preparation and, in the case of Complex II, in part also to loss of the normal membrane environment or of a membrane component, possibly Q-10, during isolation.