Extracellular heme peroxidases in actinomycetes: a case of mistaken identity

Actinomycetes secrete into their surroundings a suite of enzymes involved i n the biodegradation of plant lignocellulose; these have been reported to i nclude both hydrolytic and oxidative enzymes, including peroxidases. Report s of secreted peroxidases have been based upon observations of peroxidase-l ike activity associated with fractions that exhibit optical spectra reminis cent of heme peroxidases, such as the lignin peroxidases of wood-rotting fu ngi. Here we show that the appearance of the secreted pseudoperoxidase of t he thermophilic actinomycete Thermomonospora fusca BD25 is also associated with the appearance of a heme-like spectrum. The species responsible for th is spectrum is a metalloporphyrin; however, we show that this metalloporphy rin is not heme but zinc coproporphyrin. The same porphyrin was found in th e growth medium of the actinomycete Streptomyces viridosporus T7A. We there fore propose that earlier reports of heme peroxidases secreted by actinomyc etes were due to the incorrect assignment of optical spectra to heme groups rather than to noniron-containing porphyrins and that lignin-degrading hem e peroxidases are not secreted by actinomycetes. The porphyrin, an excretor y product, is degraded during peroxidase assays. The low levels of secreted peroxidase activity are associated with a nonheme protein fraction previou sly shown to contain copper. We suggest that the role of the secreted coppe r-containing protein may be to bind and detoxify metals that can cause inhi bition of heme biosynthesis and thus stimulate porphyrin excretion.