Morphological and Biochemical Study of a P U R I F I E D Preparation of T H E Succinate and D P N H Oxidase System

I t has long been known that the cytochrome system, which is responsible for the major oxidative functions of the mammalian tissues, is associated with insoluble particulate matter of the cells. I t is furthermore well established that the mitochondria are the sites of the oxidative reactions of the cells and that the cytochrome system is localized in these cytoplasmic organelles. These two facts have led to the logical suggestion by numerous workers (e.g., 1-4) that this complex of oxidative enzymes may be fixed in the insoluble membranes of the mitochondria. The strongest support for this view has been recently presented in the publications of Watson and Siekevitz (5, 6). They have shown that, after disruption of isolated liver mitochondria, the oxidative enzymes are to be found in the insoluble residue and that electron micrographs of this residue show that it possesses some of the characteristics of mitochondrial membranes. We present data here which support and extend their observations. The material used for our studies is a preparation obtained from beef heart muscle which contains the cytochrome system and displays both succinate and reduced diphosphopyridinenucleotide (DPNH) oxidase activity. The properties of this enzyme preparation have been studied in some detail over the past 10 years (7-13). The procedure employed for the purification of this complex of oxidative enzymes was designed to yield a preparation of maximum activity per unit of dry weight; and no regard was paid, in developing the procedure, to the maintenance of the morphological integrity of any of the components of the cell. However, as will be seen from the electron micrographs to be presented here, the preparation has a morphological appearance which leads us to con-