Photochemical Action Spectra, Not Absorption Spectra, Allow Identification of the Oxygen Sensor in the Carotid Body

Abstract

Carbon monoxide competitively inhibits reactions of many heme proteins with oxygen, including mitochondrial cytochrome oxidase, by forming a heme CO complex which can be dissociated with light. This results in an inhibition of O₂ consumption in the dark which is reversed by light. This light-induced reversal of respiratory inhibition is dependent on the wavelength of irradiating light because the amount of light absorbed by the CO complex depends on the absorption coefficient of the complex at each wavelength (photochemical action spectrum). This property was originally used to identify the mitochondrial oxidase (Kubowitz & Haas, 1932; Melnick, 1942; Warburg & Negelein, 1928; Warburg, 1949). In measurements of the oxygen sensory activity of the isolated, perfused-superfused carotid body, CO was observed to be competitive with respect to oxygen, an effect reversed by light. The dependence of the photo-reversal of the CO effect on the wavelength of light has been measured and shown to be characteristic of mitochondrial cytochrome oxidase (Wilson et al, 1994). Since the carotid body chemoreceptors show O₂-CO₂ interaction in which the hypoxic response is enhanced by CO₂, it is reasonable to expect that CO sensitivity would also show a similar interaction with CO₂. These characteristics of the cat carotid chemosensory fibers were examined and are presented in this paper.