The function of Mn(III) in plant acid phosphatase has been investigated by a metal-substitution study, and some properties of the Fe(III)-substituted enzyme were compared with those of the native Mn(III) enzyme and mammalian Fe(III)-containing acid phosphatases. 19F nuclear magnetic resonance (NMR) and proton relaxation rate measurements showed that inhibitors such as F- and nitrilotriacetic acid interact with paramagnetic Mn(III) active site. The 31P-NMR signal of the enzyme-phosphate complex was also broadened by the paramagnetic effect of Mn(III). In the metal-substitution experiments of the Mn(III)-acid phosphatase with Fe(III), Zn(II) and Cu(II), only the iron gave satisfactory substitution. The Fe(III)-substituted plant acid phosphatase exhibited an absorption maximum at 525 nm (epsilon = 3000), typical high-spin ferric ESR signal at g = 4.39, and lower pH optimum (pH 4.8) than the native Mn(III)-enzyme (pH 5.8). The phosphatase activity of the Fe(III)-substituted enzyme was reduced to about 53% of that of the native enzyme. The substrate specificities of both metallophosphatases were remarkably similar, but different from that of the Fe(III)-containing uteroferrin. The present results indicate that Mn(III) and Fe(III) in the acid phosphatase play an important role on effective binding of phosphate and acceleration of hydrolysis of phosphomonoesters at pH 4-6.