Actinomycetes secrete into their surroundings a suite of enzymes involved in the biodegradation of plant lignocellulose; these have been reported to include both hydrolytic and oxidative enzymes, including peroxidases. Reports of secreted peroxidases have been based upon observations of peroxidase-like activity associated with fractions that exhibit optical spectra reminiscent of heme peroxidases, such as the lignin peroxidases of wood-rotting fungi. Here we show that the appearance of the secreted pseudoperoxidase of the thermophilic actinomycete Thermomonospora fusca BD25 is also associated with the appearance of a heme-like spectrum. The species responsible for this spectrum is a metalloporphyrin; however, we show that this metalloporphyrin is not heme but zinc coproporphyrin. The same porphyrin was found in the growth medium of the actinomycete Streptomyces viridosporus T7A. We therefore propose that earlier reports of heme peroxidases secreted by actinomycetes were due to the incorrect assignment of optical spectra to heme groups rather than to noniron-containing porphyrins and that lignin-degrading heme peroxidases are not secreted by actinomycetes. The porphyrin, an excretory product, is degraded during peroxidase assays. The low levels of secreted peroxidase activity are associated with a nonheme protein fraction previously shown to contain copper. We suggest that the role of the secreted copper-containing protein may be to bind and detoxify metals that can cause inhibition of heme biosynthesis and thus stimulate porphyrin excretion.Biodegradation of lignocellulose by microorganisms plays an important role in carbon cycling, is of biotechnological interest to the paper industry, and has potential application in the field of bioremediation. Some microorganisms secrete a range of enzymes that completely degrade all the components of lignocellulose (lignin, hemicelluloses, and cellulose), while others secrete a narrower range of enzymes that only partially achieve this degradation (8,19,21). White rot fungi secrete both cellulolytic and ligninolytic enzymes; heme enzymes are major components of this ligninolytic activity and include the wellcharacterized lignin and manganese peroxidases of Phanerochaete chrysosporium (12).Some actinomycetes, including thermophilic species and streptomycetes, secrete cellulose-and hemicellulose-degrading enzymes (2, 4, 11). Since the discovery of extracellular lignindegrading heme peroxidases of wood-rotting fungi, considerable effort has been expended in searching for analogous enzymes in the cellulolytic actinomycetes (9,22,24). Indeed, some proteins secreted by the actinomycetes Streptomyces thermoviolaceus (17), Streptomyces viridosporus T7A (6, 26, 27), and Thermomonospora fusca BD25 (recently reclassified as Thermobifida fusca [35]) (3, 28) have low peroxidase activity and have been isolated and partially characterized. The peroxidase-like proteins from S. thermoviolaceus and S. viridosporus T7A were assigned as heme peroxidases on the basis of their optical spectra. H...