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Ascorbate peroxidase (Apx) plays important roles both as a reductant and as a H2O2 scavenger via ascorbate (AsA). In this paper, we discuss how a ClApx cDNA (1,068 bp, GQ465430) encoding a putative Apx was cloned from lemon (Citrus limon). The deduced amino acid sequence is similar to the Apxes from other plant species. A 3-D structural model of ClApx was constructed based on the crystal structure of Pisum sativum Apx (PDB code 1APX). To characterize the ClApx protein, the coding region was subcloned into an expression vector pYEX-S1 and transformed into Saccharomyces cerevisiae. The recombinant His6-tagged ClApx was overexpressed and purified by Ni2+-nitrilotriacetic acid Sepharose. The purified enzyme showed two prominent bands on 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The Michaelis constant (KM) values of the recombinant enzyme for AsA and H2O2 were 0.40 and 0.11 mM, respectively. The enzyme was active from pH range 6 to 8. The thermal inactivation of the enzyme showed a half-life of 6.5 min at 45°C, and its inactivation rate constant Ki was 1.1 × 10-1 min-1. The enzyme retained 35% activity after chymotrypsin digestion at pH 8 and 37°C for 40 min. [ABSTRACT FROM AUTHOR]