Evidence for a lipid dependence of mitochondrial nicotinamide nucleotide transhydrogenase.

1. The lipid dependence of mitochondrial nicotinamide nucleotide transhydrogenase from beef heart was investigated. With submitochondrial particles digestion of phospholipids by phospholipases A and C led to a partial inhibition that could not be readily reversed by phospholipids. 2. Extraction of neutral lipids including ubiquinone from lyophilized submitochondrial particles with pentane did inhibit the transhydrogenase, whereas further extraction with water/acetone led to a complete and apparently irreversible inhibition. 3. A partially purified preparation of transhydrogenase, depleted of lipids (and inactivated) by treatment with cholate and ammonium sulphate, was reactivated by various purified phospholipids but not by detergents or triacylglycerols. 4. It is concluded that mitochondrial transhydrogenase, catalyzing the nonenergy-linked transhydrogenase reaction, requires phospholipids specifically for its catalytic activity and not as dispersing agents. A mixture of phospholipids appears to fulfill this requirement better than the individual phospholipids.