Room temperature, CO-difference spectra of intact rat polymorphonuclear leucocytes (neutrophils) revealed the presence of a number of CO-binding haemoproteins. Absorption maxima at 413, 540 and 570 nm were attributed to the CO-complex of cytochrome b-245 whereas an absorption maximum at 595 nm was assigned to the contribution from a myeloperoxidase complex, since an identical absorption maximum was observed in CO-difference spectra of purified myeloperoxidase in the presence of H2O2. Photochemical action spectra for the relief of CO-inhibited O2 uptake revealed contributions from both cytochrome b-245 and myeloperoxidase. The potential of these two O2- and CO-binding haemoproteins to function as oxidases during the respiratory burst is discussed.

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