Scott S. Sibbett and James K. Hurst* abstract: Soret excitation of canine myeloperoxidase (MPO) gives rise to a complex resonance Raman (RR) spectrum characterized by multiple bands in the core size and oxidation state marker regions. Relative intensities of the bands obtained by 406-and 454-nm laser excitation were nearly identical and were temperature independent from 77 to 273 K. Spectra of dithionite-reduced and cyanide-coordinated derivatives are also reported. In the native and dithionite-reduced enzyme, there are no detectable bands between 1620 and 1700cm" 1, indi-IN^ yeloperoxidase (MPO) 1 has been implicated in the an-timicrobial and cytotoxic reactions of neutrophils and mono-cytes (Klebanoff & Clark, 1978). This enzyme is functionally unique in its capacity to catalyze the peroxidation of chloride ion (Harrison & Schultz, 1976). The reaction produces hypochlorous acid (HOC1), whose ability to cause extremely rapid oxidative degradation of a wide variety of biological substrates, including porphyrins, hemes, and heme proteins (Albrich et al., 1981), suggests that it is the ultimate MPO-generated toxin (Klebanoff & Clark, 1978; Albrich et al., 1981).