THE SPECIFICITY OF MITOCHONDRIAL COMPLEX-I FOR UBIQUINONES

We report the first detailed study on the ubiquinone (coenzyme Q; abbr eviated to Q) analogue specificity of mitochondrial complex I, NADH:Q reductase, in intact submitochondrial particles. The enzymic function of complex I has been investigated using a series of analogues of Q as electron acceptor substrates for both electron transport activity and the associated generation of membrane potential. Q analogues with a s aturated substituent of one to three carbons at position 6 of the 2,3- dimethoxy-5-methyl-1,4-benzoquinone ring have the fastest rates of ele ctron transport activity, and analogues with a substituent of seven to nine carbon atoms have the highest values of association constant der ived from NADH:Q reductase activity. The rate of NADH:Q reductase acti vity is potently but incompletely inhibited by rotenone, and the resid ual rotenone-insensitive rate is stimulated by Q analogues in differen t ways depending on the hydrophobicity of their substituent. Membrane potential measurements have been undertaken to evaluate the energetic efficiency of complex I with various Q analogues. Only hydrophobic ana logues such as nonyl-Q or undecyl-Q show an efficiency of membrane pot ential generation equivalent to that of endogenous Q. The less hydroph obic analogues as well as the isoprenoid analogue Q-2 are more efficie nt as substrates for the redox activity of complex I than for membrane potential generation. Thus the hydrophilic Q analogues act also as el ectron sinks and interact incompletely with the physiological Q site i n complex I that pumps protons and generates membrane potential.