Molecular interactions of adenosine triphosphatase with the mitochondrial membrane as revealed by a spin label study

Mitochondrial ATPase complex has been spin labeled in the membrane using the inhibitor N (2,2,6,6 tetramethyl piperidyl 1 oxyl) N' (cyclohexyl)carbodiimide (NCCD). The amount of NCCD bound to mitochondrial fragments is 0.5 nmol/mg and cannot be dialyzed or extracted with ether, chloroform, or methanol. The electron paramagnetic resonance spectrum of NCCD bound to fragments is pH sensitive, a greater label immobilization occurring at pH values lower or higher than 7. Ether extraction removes the ATPase inhibition by NCCD without detaching the label. This effect appears to be the consequence of the dislocation of some components of the ATPase complex. Removal of F1 natural inhibitor or of F1 does not affect the spectrum of NCCD bound to fragments, while the removal of oligomycin sensitivity conferring protein produces an increase in the extreme splitting. Oligomycin sensitivity conferring protein may thus interact with the NCCD binding component of the membrane. The isolation of the NCCD binding proteolipid results in a large increase in the mobility of the label, but addition of dipalmitoyllecithin decreases the mobility of the label to the original level. Phospholipids are thus necessary to keep the NCCD binding proteolipid in the native conformation.