A Naturally Occurring Inhibitor of Mitochondrial Adenosine Triphosphatase

Abstract

Freshly prepared mitochondria have little or no adenosine triphosphatase (ATPase) activity (1). On aging, or in the presence of uncoupling agents such as 2,4-dinitrophenol, considerable activity develops (l-4). The striking similarities between conditions which evoke ATPase activity and those which uncouple oxidative phosphorylation have led to the assumption that the ATPase activity is a reflection of the hydrolysis of some high energy intermediate in the coupling mechanism (cf. reviews by Racker (5) and Lehninger and Wadkins (6)). Previous reports from this laboratory described a physically homogeneous protein, coupling Factor 1 (F,), which was essential for recoupling phosphorylation to oxidation in submitochondrial particles (7), and which also catalyzed the hydrolysis of ATP (S).l It was concluded from these studies that the hydrolytic activity of coupling Factor 1 was an aberrant activity resulting from the displacement of the protein from its environment in the mitochondrial structure (7). ATPase activity in mitochondria is either masked or, when expressed, is inhibited by oligomycin and is cold-stable. On the other hand, the dissociated purified enzyme is rapidly inactivated at low temperatures (7,s) and insensitive to oligomycin. As an approach to understanding the mode of action of factors involved in oxidative phosphorylation, we have sought to restore to the solubilized ATPase, by the addition of mitochondrial components, some of the properties it exhibits in intact mitochondria. Factors which restore the cold stability (10) and oligomycin sensitivity (11) of the soluble enzyme have already been reported. It is the purpose of this paper to describe the purification and properties of a low molecular weight mitochondrial protein which inhibits the soluble ATPase and increases cold activity. A (12). stability, but does not interfere with its coupling preliminary report of this work has been presented