A peroxidase (EC 1.11.1.7) has been isolated and purified from Senna angustifolia. The enzyme was purified by ion-exchange chromatography on high Q and high S columns. SDS-PAGE electrophoresis showed that the protein has a molecular mass of approximately 70 kDa. Hydroxy-anthraquinones and hydroxy-anthracenones were evaluated as substrate of S. angustifolia and horseradish peroxidases. Both peroxidases catalyzed the oxidation of alizarin and purpurin anthraquinones to the corresponding 3,3'-bializarin and the new compound 3,3'-bipurpurin, respectively, as well as the formation of 2,2'-biquinizarin from quinizarin anthracenone. The K(Mapp) and V(max) values for alizarin and purpurin were 97 and 95 microM, and 1.5 and 2.1 microM min(-1) mg prot(-1), respectively. The results suggest that peroxidase may participate in the biogenesis of anthraquinones.