Relation of Binding of Diphenylene[1251]iodonium to Mitochondria to the Extent of Inhibition of Oxygen

1. Several ring-substituted derivatives of diphenyleneiodonium catalyse the exchange of Clh and OHions across the inner membrane of rat liver mitochondria. They also inhibit state 3 and state 3u oxidations of glutamate plus malate in the presence of Cl more than in its absence. Most have activities similar to diphenyleneiodonium, although 2,4dichlorodiphenyleneiodonium is up to 50 times more active. 2. Diphenyleneiodonium inhibits soluble rat liver NADH dehydrogenase and NADH oxidation by rat liver submitochondrial particles directly; 2,4-dichlorodiphenyleneiodonium is only about twice as inhibitory. 3. Liver mitochondria contain two classes of binding sites for diphenylene[1251]iodonium, namelyhigh-affinity sites with anaffinityconstant of3 x 105 M-1 (1-2nmol/ mg of protein), and low-affinity sites with an affinity constant of 1.3 x 103M-1 (80nmol/mg of protein). Both sites occur in hepatocytes with a relative enrichment of the low-affinity site. NADH dehydrogenase preparations only apparently contain high-affinity binding sites. Only low-affinity sites occur in erythrocytes. 4. 2,4-Dichlorodiphenyleneiodonium competes with diphenylene[1251]iodonium for both lowand high-affnity sites, whereas tri-n-propyltin only competes for the low-affinity sites. 5. The high-affinity sites are apparently associated with NADH dehydrogenase and the low-affinity sites probably represent electrostatic binding of diphenylene[1251]iodonium to phospholipids. The highaffinity site does-not appear to be associated with a rate-limiting stage ofNADH oxidation.