The reactivity of thiol groups in bovine heart cytochrome c oxidase towards 5,5'-dithiobis(2-nitrobenzoic acid)

The stimulation of succinate-fueled respiration of rat liver mitochondria in state 4 by α,ω-hexadecanedioic acid without induction of proton conductivity of the inner membrane. Intrinsic uncoupling of the bc1 complex

The paper shows that natural α,ω-dioic acid, α,ω-hexadecanedioic acid (HDA), is able to stimulate the respiration of succinate-fueled rat liver mitochondria in state 4 without induction of proton conductivity of the inner membrane. This effect of HDA is less pronounced in glutamate/malate-fueled mitochondria, as well as in the case of ascorbate/TMPD or ascorbate/ferrocyanide substrate systems, which transfer electrons directly to cytochrome c. It is noted that HDA-induced stimulation of respiration is not associated with damage to the inner membrane in a part of mitochondria and with shunting of electrons through the bc₁ complex. Therefore, HDA can be considered as a natural decoupling agent. Specific inhibitors of the bc₁ complex (antimycin A and myxothiazole) as well as malonate and dithionitrobenzoate were used in the inhibitory analysis. These and other experiments have shown that during the oxidation of succinate in liver mitochondria, the decoupling effect of HDA is mainly carried out at the level of the bc₁ complex. We hypothesized that HDA is capable of promoting the cyclic transport of protons within the bc₁ complex and thus switch this complex to the idle mode of operation (intrinsic uncoupling of the bc₁ complex). Induction of free respiration in liver mitochondria by HDA at the level of the bc₁ complex is considered as one of the "rescue pathways" of hepatocytes in various pathological conditions, accompanied by disorders of carbohydrate and lipid metabolism and increased oxidative stress.

Cytochrome-c oxidase. Subunit structure and proton pumping

This article reviews the significance of the subunit structure of cytochrome-c oxidase in proton pumping and in particular summarizes available evidences for or against a role of subunit III in the control of this important function of the enzyme.

Tissue-specific and species-specific distribution of -SH groups in cytochrome c oxidase subunits

Cytochrome c oxidase was isolated from pig, bovine, rat and human tissues including liver, heart, diaphragm and kidney. The native and the sodium-dodecyl-sulfate (SDS)-dissociated enzymes were labelled under optimal conditions with N-ethyl-[2,3-14C]maleimide before and after reduction with dithiothreitol, separated into 13 subunits by SDS gel electrophoresis and the radioactive bands were visualized by fluorography. In some cases the radioactive bands were cut out and counted. All isozymes were labelled in subunits I, III, Va and VIIb, and in subunit II after reduction. Labelling of subunit Vb was equivocal, and in no case were subunits IV and VIc labelled. All other subunits were labelled tissue-specifically and/or species-specifically. No differences were found between labelling of the native and SDS-dissociated enzyme. By relating the molar amount of bound N-ethylmaleimide to the known amount of cysteines in subunits of bovine heart cytochrome c oxidase, the percentage of -SH group reactivity was calculated. Only the cysteine of subunit Va was found to be 100% reactive. The distinct and different reactivity of subunit VIIb as compared to subunits VIIa and VIIc clearly establishes this polypeptide as an independent subunit of mammalian cytochrome c oxidase.

Selective labeling of beef heart cytochrome oxidase subunit III with eosin-5-maleimide

Cytochrome c oxidase has been isolated from beef heart mitochondria and labeled with the fluorochrome eosin-5-maleimide (EMA) after pretreatment with mersalyl. On SDS-polyacrylamide gels, EMA fluorescence and absorption occurred at a single band corresponding to subunit III. Since only Cys 115 of the two cysteinyl residues of subunit III had been shown to be reactive towards water-soluble SH-reagents, it was concluded that this residue was the one labeled by EMA. The EMA/enzyme ratio was about 1. Gel filtration experiments have shown that upon treatment with dicyclohexylcarbodiimide, subunit III was loosened from the complex; this result suggests that the inhibitory effect of dicyclohexylcarbodiimide on the H+-translocation activity may be related to such a phenomenon.