Identification of the subunits of bovine heart mitochondrial NADH dehydrogenase that are exposed to the phospholipid bilayer by photo-labelling with 5-iodonaphth-1-yl azide.

Mitochondrial NADH dehydrogenase may be isolated from bovine heart as a lipoprotein complex (Complex I or NADH-ubiquinone oxidoreductase). Polypeptide subunits that are exposed to the hydrophobic region of the phospholipid bilayer were identified by photolabelling with the hydrophobic probe, 5-[125I]iodonaphth-1-yl azide. Chaotropic resolution of the labelled enzyme showed that the hydrophilic flavoprotein and iron-protein fragments of the enzyme were not in contact with the phospholipid bilayer. When complex I that had been partially depleted of phospholipids was photolabelled, incorporation of radioactivity into certain polypeptides was increased, indicating either conformational changes in protein or preferential association of these polypeptides with residual cardiolipin. A model NADH dehydrogenase structure is proposed on the basis of these results and those obtained with hydrophilic probes by Smith & Ragan (1980) Biochem. J. 185, 315-326.