Behavior of the Copper Band of Cytochrome C Oxidase in Rat Brain During FC-43-for-Blood Substitution

Abstract

Over the past few years, it has become possible to monitor tissue oxygenation in situ by measuring changes in oxygen-dependent absorption of near infrared (NIR) light by tissue chromophores. Soft tissues and bone are relatively translucent to near infrared light. Hemoglobin (Hb), oxyhemoglobin (HbO₂) and the oxidized copper band of cytochrome c oxidase (cytochrome a,a₃) have oxygen-dependent absorption spectra in the 700-900 nm wavelength region. These concepts were first reported when Jöbsis (1977) demonstrated transmission of NIR light through intact tissues and the feasibility of NIR monitoring of changes in both the oxygenation of hemoglobin and the oxidation level of cytochrome a,a ₃ in living brain. NIR signals acquired from intact brain must be partitioned into absorption changes proportional to the concentrations of each of the three absorbers because of overlap of the NIR absorption spectra of Hb, HbO₂ and oxidized cytochrome a,a ₃. The NIR contributions of Hb, HbO₂, and cytochrome a,a ₃ to the spectrum of each of the other absorbers in brain tissue (t) can be accounted for by means of algorithms that calculate changes in the relative amounts of each component. The algorithms resolve changes in absorption at several wavelengths according to the relative contribution of each absorber at each wavelength. Since three overlapping absorption spectra must be deconvoluted, spectral data from at least three NIR wavelengths are necessary to correct for the contributions of the three molecular species. In some situations, four wavelength algorithms appear to provide more accurate approximations of the NIR absorption properties of living tissues.