Mitochondrial respiratory chain of Tetrahymena pyriformis. The thermodynamic and spectral properties

Abstract

The mitochondria isolated from the ciliate protozoon Tetrahymena pyriformis carry an oxidative phosphorylation with P/O ratio of 2 for succinate oxidation and P/O ratio of 3 for the oxidation of the NAD-linked substrates. The respiration is more than 90% inhibited with 1 mM cyanide while antimycin A and rotenone inhibit at concentrations of 1000-fold higher than those effective in mammalian mitochondria. Using a combination of spectral studies and potentiometric titrations, the components of the respiratory chain were identified and characterized with respect to the values of their half-reduction potentials. In the cytochrome bc1 region of the chain a cytochrome c was present with an Em7.2 of 0.225 V and two components with absorption maxima at 560 nm and the half-reduction potential values of -0.065 and -0.15 V at pH 7.2. The cytochrome with the more positive half-reduction potential was identified as the analogue of the cytochrome(s) b present in mitochondria of higher organisms, while the cytochrome with the more negative half-reduction potential was tentatively identified as cytochrome o. In addition ubiquinone was present at a concentration of approx. 4 nmol per mg mitochondrial protein. In the spectral region where cytochromes a absorb at least three cytochromes were found. A cytochrome with an absorption maximum at 593 nm and a midpoint potential of -0.085 V at pH 7.2 was identified as cytochrome a1. The absorption change at 615-640 nm, attributed usually to cytochrome a2, was resolved into two components with Em7,2 values of 0,245 and 0.345 V. It is concluded that the terminal oxidase in Tetrahymena pyriformis mitochondria is cytochrome a2 which in its two component structure resembles cytochrome aa3.