Studies on the Terminal Electron Transport System

The present communication is devoted to the description of the isolation, properties, and composition of a particulate unit which we shall refer to as the succinic dehydrogenase complex (SDC). The working hypothesis underlying this and subsequent communications in this series is that SDC is a unit of enzymatic action in the same sense as any well defined soluble enzyme. Since the discovery of succinic dehydrogenase (l), a vast literature on this enzyme has accumulated, much of which has been amply reviewed in recent years (24). The important investigations which are pertinent to or form the basis of the observations reported in this paper may be classified as follows: relationship of succinic dehydrogenase to cytochrome c and the cytochromes generally (5-9), 2,3-dimercapto-1-propanol (BAL) sensitivity of succinic dehydrogenase (lo-14), role of cytochrome b in succinic dehydrogenation (15-17)) bacterial succinic dehydrogenase (18), development of new techniques for the separation of particulate enzymes (19-27)) purification of succinic dehydrogenase by ammonium sulfatecholate mixtures (2%30), and preparation of a soluble succinic dehydrogenase (31). Most recently Neufeld et al. (29) have described the isolation of two purified (particulate?) preparations of succinic dehydrogenase from heart muscle extracts. The first (Preparation A), obtained by fractionation with ammonium sulfate in the presence of cholate and by selective heat denaturation, is about 20 times more active than the whole homogenate of heart muscle. It is free of cytochrome c and cytochrome oxidase. The Qoz at 30” is 46. The reaction of Preparation A with cytochrome c is relatively slow compared to the reaction with indophenol. The second (Preparation B), obtained by extensions of the procedures applied in the making of Preparation A, is 30 times more active than the starting material. It reacts with indophenol but not with cytochrome c. The