Notexin from Notechis scutatus scutatus snake venom was subjected to Lys modification with pyridoxal 5'-phosphate (PLP), and one major modified derivative was purified on a cation-exchanger SP-8HR column. The results of amino acid analysis and sequence determination revealed that only 2 Lys residues at positions 82 and 115 out of 11 Lys residues in notexin were modified. The incorporation of PLP into the protein was accompanied by the loss of 53% lethal toxicity, but the modified notexin showed an about 1.2-fold increase in enzymatic activity. However, the secondary structure of the toxin molecule did not significantly change after modification with PLP as revealed by the CD spectra, and the antigenicity of PLP derivative remained unchanged. The modified derivative retained its affinity for Ca2+, indicating that the modified Lys residues did not participate in Ca2+ binding. These results indicate that modification of Lys residues causes a differential effect on the enzymatic activity and lethal toxicity of notexin, and suggest that notexin might possess two functional sites, one responsible for the catalytic activity and the other associated with its lethal effect.