Proton translocation coupled to quinone reduction by reduced nicotinamide–adenine dinucleotide in rat liver and ox heart mitochondria
Measurements were made of the stoicheiometry of proton-translocation coupled to NAD(P)H oxidation by several quinones (duroquinone, ubiquinone0, ubiquinone1, ubiquinone2) in mitochondria from rat liver and ox heart. Observed stoicheiometries of protons translocated per mol of NADH oxidized (→H+/2e− ratios; Mitchell, 1966) ranged from 0.75 (rat liver mitochondria with ubiquinone1) to 1.55 (ox heart mitochondria with ubiquinone1 or ubiquinone2). Only the rotenone-sensitive pathway of NADH oxidation by quinone was able to support proton translocation. Correction of the observed →H+/2e− ratios for the loss of reducing equivalents to the rotenone-insensitive pathway increased their value to approx. 2.0. It is concluded that the rotenone-sensitive NADH– ubiquinone reductase activity of the respiratory chain may be organized in the mitochondrial membrane as a proton-translocating oxidoreduction loop. The number of such loops between NADH and ubiquinone is one, and not two, as initially proposed by Mitchell (1966).