Figure 4.

Mitochondrial enzyme function studies. A, Spectrophotometric assay of respiratory-chain enzyme activities in tumors with (first trace) and without (second trace) mutations in SDHD. The values given on the traces are nmol/min/mg protein. Mean ± 1 SD is indicated in brackets. LM = lauryl maltoside; DUQH₂ = decylubiquinol. B, Enzyme activities in subject II:2 (inherited pheo) and controls (sporadic pheos). SCCR = malonate-sensitive succinate cytochrome c reductase activity (complexes II+III). SQDR = succinate quinone dichlorophenolindophenol reductase (complex II). SPDR = succinate phenazine methosulfate dichlorophenolindophenol reductase (succinate dehydrogenase). C, Schematic representation of the principal electron paths through the various subunits of complex II. The acceptor sites for quinones and phenazine methosulfate are indicated as well as the thenoyltrifluoroacetone inhibition site. Aa = antimycin; C = SDHC; CIII = complex III; CIV = complex IV; D = SDHD; DCPIP = dichlorophenolindophenol; DUQ = decylubiquinone; DUQH₂ = decylubiquinol; FAD = flavine adenosine; Fp = flavoprotein; Ip = iron protein; KCN = potassium cyanide; PMS = phenazine methosulfate; Q = ubiquinone; Q^▪− = semiubiquinol; QH₂ = ubiquinol. Q, Q^▪−, and QH₂ are the various forms of the ubiquinone present in the mitochondrial membrane.