Thyroid peroxidase (TPO) is a membrane-bound heme enzyme that catalyzes iodination and coupling of thyroglobulin tyrosine residues in the biosynthesis of thyroid hormones. The soluble heme enzyme lactoperoxidase (LPO) also iodinates tyrosine efficiently and serves as a functional model for TPO. Whereas horseradish peroxidase oxidizes sulfur-containing compounds, e.g., cysteine and glutathione, these compounds interact with LPO and TPO without oxidation. Indeed, clinical treatment of thyroid disease involves the use of sulfur-containing drugs (goitrogens) to inactivate TPO. Reactions of TPO and LPO with goitrogens result in enzyme inhibition, in a process similar to the conversion of myoglobin to sulf-myoglobin (sulf-Mb). We have examined the electronic absorption and resonance Raman (RR) spectral properties of LPO treated with the potent antithyroid drug methylmercaptoimidazole (MMi). Interaction of LPO with MMi yields a stable, emerald green complex (SLPO (MNi]). The electronic absorption spectra of SLPO [MMi] are similar to those of sulf-Mb (which has a sulfur-modified iron chlorin macrocycle), and to those of the naturally occurring Escherichia coli and Neurospora crassa chlorin catalases. The Soret excitation RR spectrum of the SLPO [DIM] complex is distinct from that of native LPO, but is consistent with the spectral pattern for metallochlorins established by our laboratory. The SLPO [NW RR pattern is also analogous to that of chlorin-containing proteins such as the isomeric sulf-Mb's, myeloperoxidase, and the E. coli and N. crassa catalases. These data strongly suggest that MMi treatment of LPO results in conversion of the porphyrin prosthetic moiety to an iron chlorin. We infer that clinical treatment of thyroid disorders with the goitrogen drug MMi similarly converts the porphyrin prosthetic moiety of thyroid peroxidase to an iron chlorin. Spectral differences between sulf-Mb and SLPO [MMI] are particularly intriguing and suggest structural differences between the two systems. Further investigation of the interaction(s) of sulfur compounds with heme systems is in progress.