Re-evaluation of the near infrared spectra of mitochondrial cytochrome c oxidase: Implications for non invasive in vivo monitoring of tissues.

We re-determined the near infrared (NIR) spectral signatures (650-980nm) of the different cytochrome c oxidase redox centres, in the process separating them into their component species. We confirm that the primary contributor to the oxidase NIR spectrum between 700 and 980nm is cupric CuA, which in the beef heart enzyme has a maximum at 835nm. The 655nm band characterises the fully oxidised haem a3/CuB binuclear centre; it is bleached either when one or more electrons are added to the binuclear centre or when the latter is modified by ligands. The resulting 'perturbed' binuclear centre is also characterised by a previously unreported broad 715-920nm band. The NIR spectra of certain stable liganded species (formate and CO), and the unstable oxygen reaction compounds P and F, are similar, suggesting that the latter may resemble the stable species electronically. Oxidoreduction of haem a makes no contribution either to the 835nm maximum or the 715nm band. Our results confirm the ability of NIRS to monitor the CuA centre of cytochrome oxidase activity in vivo, although noting some difficulties in precise quantitative interpretations in the presence of perturbations of the haem a3/CuB binuclear centre.