Thermostable extracellular peroxidases from Streptomyces thermoviolaceus
Streptomyces thermoviolaceus is a thermophilic actinomycete that was found to produce relatively large amounts of extracellular peroxidase activity when grown on xylan as primary carbon source. The activity was due to multiple isoforms of peroxidase, of which two, designated P-3 and P-5, were predominant. The two proteins were purified to homogeneity by a combination of ultrafiltration, ammonium sulphate precipitation, anion-exchange chromatography, gel filtration and preparative gel electrophoresis. The peroxidases were found to be haemoproteins that catalysed the oxidation of a range of substrates in the presence of hydrogen peroxide. Both are monomeric acidic proteins (P-3: 82 kDa, pl 5.0; P-5: 60 kDa, pl 4.75) but with some differences in substrate specificity, P-3 exhibiting the broader substrate range. Peroxidase activity was optimal at ph values close to neutrality, and both enzymes were robust, exhibiting activity at elevated temperatures in the presence of denaturing agents such as SDS or 8 M urea. Peroxidase P-3 was stable at 50 degrees C for more than 24 h and had a half-life of 70 min at 70 degrees C. Polyclonal antibodies prepared against each isoform cross-reacted, indicating that the proteins were antigenically related. No cross-reactions were detected against horseradish peroxidase or crude peroxidase preparations from two other thermophilic streptomycetes.