In this paper, the measurement results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF/MS) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS) demonstrated that the complex of horseradish peroxidase (HRP) and 12(III) (La-HRP) can be formed in horseradish in vivo after horseradish is treated with 80 mu M La(III). The electrochemical measurements further demonstrated that the direct electrochemical performance and the bioelectrocatalytic activity of the La-HRP complex are worse than that of HRP. This means that when HRP is treated with the high concentration of La(III), the natural function of HRP is inhibited due to the change in the structure of HRP. Thus, it would be harmful for horseradish and other organisms. This result would provide some references for better understanding the toxicity mechanism of rare earth elements and heavy metals in the organisms. (C) 2010 Elsevier B.V. All rights reserved.