Partial Resolution of the Enzymes Catalyzing Oxidative Phosphorylation. Iii. a New Coupling Factor Required by Submitochondrial Particles Extracted with Phosphatides.

1. The reaction of mitochondrial adenosine triphosphatase (coupling factor 1) with iodine resulted in rapid loss of ATPase activity, disappearance of sulfhydryl groups, binding of iodine to the protein, and partial dissociation of the molecule. The extent of the changes observed depended on the molar ratio of iodine to protein in the reaction mixture. At a molar ratio of 50, more than 99% of the ATPase activity was inhibited, about 20 atoms of iodine were bound per mole of enzyme and about 40% of the original protein (SZO,~ = 11.9 S) was converted to a subunit with a sedimentation coefficient of 3.5 S. 2. The iodinated enzyme, which contained less than 0.5% of the original ATPase activity, stimulated oxidative phosphorylation, the s2Pi-ATP exchange reaction and the ATPdependent reduction of DPN catalyzed by submitochondrial particles which were partially but not completely devoid of coupling factor 1. 3. Reaction of coupling factor 1 with dicyclohexylcarbodiimide also resulted in virtually complete inhibition of ATPase activity. The inhibition was not accompanied by dissociation of the molecule or change in sulfhydryl content. Similar to the iodine-treated enzyme, dicyclohexylcarbodiimidetreated coupling factor 1 stimulated oxidative phosphorylation only in submitochondrial particles which contained endogenous coupling factor 1. 4. The failure of chemically modified ATPase to stimulate coupled phosphorylation in submitochondrial particles completely resolved with respect to coupling factor 1 supports the suggestion of a dual role of the coupling factor in oxidative phosphorylation.