Ureases (urea amidohydrolase;EC 3.5.1.5) are nickel dependent enzymes that catalyze the hydrolysis of urea to ammonia and carbon dioxide. Ureases have been isolated from a wide variety of organisms including plants, fungi and bacteria. In plants, ureases are probably involved in the defense against pathogens.  In Canavalia ensiformis (Leguminosae) two urease isoforms were isolated and both showed several biological effects independent from the ureolytic activity, as platelet aggregation, hemagglutination and toxicity to mouse (i.p.) and some insect orders (oral route). The insecticidal activity of urease is due a 10kDa peptide released by insect digestive enzymes. The chemical modification of aminoacid residues can provide essential information about the protein structure and function. This work aimed to characterize the influence of lysine, aspartic and glutamic acid residues on urease structure and function. Lysine modification of the native urease was performed with acetic anhidride. Both glutamic and aspartic acids were modified with 1-ethyl-3-[3-(dimethylamine) propyl] carbodiimide (EDC) and Ethylenediamine. Derivatized ureases were then bioassayed in the cotton stainer bug (Dysdercus peruvianus) for insecticidal activity.  We observed that any of modified residues affected the enzymatic activity, but all of them interfere in the insecticidal activity.  We are now evaluating if these modifications altered the release of the entomotoxic peptide by insect cathepsins. Preliminary data indicate that lysine modification did not alter the urease hydrolysis by insect digestive enzymes suggesting that these residues are essential for entomotoxic activity of the peptide, but not for its release.