Purple acid phosphatases (PAPs) from sweet potato (sp) have been classi-fied on the basis of their primary structure and the dinuclear metal center into isoforms spPAP1 [Fe(III)–Zn(II)] and spPAP2 [Fe(III)–Mn(II)]; for spPAP3 only the cDNA is known. With the aim of unraveling the charac-ter of the dinuclear metal center we report here the characterization of this isoform at the protein level.