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One approach to the study of the mechanism of multienzyme systems is resolution of the constituent parts and reconstitution of the system from the purified components. This has proved to be difficult in the case of oxidative phosphorylation because the catalytic units also form an integral part of the mitochondrial structure and thus appear to have a dual function. Resolution is further complicated by the association of the enzymes with the insoluble structural protein and phospholipid. The resolved components often show alterations in properties due either to changes produced by the isolation methods or to a change in environment from a particle-bound state to a free soluble protein. A major step in this direction was made in 19581, 2 when it was found that a soluble protein fraction from mitochondria stimulated phosphorylation in submitochondrial particles (see Table 1). Since these soluble proteins were concerned with reactions leading to ATP synthesis, they were given the operational name of "coupling" factors. A more appropriate name would be "energy transfer" factors since they appear to be acting in the steps subsequent to the coupling event. A factor that has been studied in detail is F1 which has high ATPase activity.3 Another preparation, Factor A, which shows similar stimulatory activity in several energy-linked reactions has been isolated in our laboratory.4' 5 This protein has no ATPase activity but can be converted to as active an ATPase as F1 by exposure to elevated temperatures. It has been suggested that the factor, as it occurs in mitochondria, has no inherent ATPase activity,3' 5 but the hydrolytic property is induced as a result of changes in environmental conditions during the isolations At least three other factors closely related to F1 have been reported (Table 1). The claims for additional factors have proliferated during the last few years. However, the only other extensively purified protein is Factor B. The role of the purified ADP-specific nucleotide diphosphokinase (ADP-ATP exchange enzyme) and of the phosphoryl-transferase in oxidative phosphorylation needs further exploration. The status of the remaining factors is uncertain since they have not been adequately purified, and milligram quantities are needed for significant effect on similar amounts of particle protein. The stimulation of coupled phosphorylation under these conditions may be caused by contaminants present in the preparation. The activity of F4, for example, is now attributed to contaminating F2 and F3,6 and McLennon and Tzagoloff have been able to prepare OSCP (Table 1) from F4. It is often difficult to see the relationship between the different factor preparations since several types of deficient particles are in use, most of them having a considerable complement of bound factors resulting in rather high basal activity. The significance of small changes in P/O above the basal level or above the level after partial supplementation becomes difficult to evaluate. Sometimes the superficial analogies drawn between different factors

The role of Factor B in the energy transfer reactions of oxidative phosphorylation.