Lysine decarboxylase (EC 4.1.1.18) was purified 364-fold from 2-day-old soybean (Glycine max) axes. The enzyme was a monomeric protein having a molecular mass of 95,000 Da and an isoelectric point of 4.0. The K(m) for L-lysine was 1.17 mM. The optimal temperature and pH of the enzyme were 37 degrees C and 7.5, respectively. Storage of the enzyme at temperature ranging from 0 to 4 degrees C caused a 50% loss of the activity in 24 h. The enzyme was competitively inhibited by Cl- with a Ki value of 1.46 mM. However, the activity of the purified enzyme was not inhibited by F-, Br-, I-, H2PO4-, HPO4(2-), or SO4(2-). Cadaverine at 1 mM inhibited the enzyme activity by 35%.