Stoichiometry of carbon monoxide binding by cytochrome c oxidase.

The stoichiometry of carbon monoxide binding to beef heart cytochrome c oxidase has been reinvestigated both by titration of the reduced oxidase with CO and by measuring the amount of carboxyhemoglobin that is formed after adding oxyhemoglobin to a solution of the CO-enzyme complex. In the titration experiments the ratio of CO bounds to total heme a present was always less than 0.50 while in the experiments where oxyhemoglobin was added the results were variable and of lower accuracy. These observations do not agree with the recent conclusion of Volpe, J.A., O'Toole, M.C., and Caughey, W.S. (1975) Biochem. Biophys. Res. Commun. 62, 48-53 that CO is bound in a 1:1 ratio with heme a. An explanation for their results is suggested.