Canatoxin, a toxic protein from jack beans (Canavalia ensiformis), is a variant form of urease (EC 3.5.1.5): biological effects of urease independent of its ureolytic activity

摘要

pCanatoxin is a toxic protein from iCanavalia ensiformis/i seeds, lethal to mice (LDsub50/sub = 2mg/kg) and insects. Further characterization of canatoxin showed that its main native form (184kDa) is a non-covalently linked dimer of a 95kDa polypeptide containing zinc and nickel. Partial sequencing of internal peptides indicated homology with urease (EC 3.5.1.5) from the same seed. Canatoxin has approx. 30% of urease9s activity for urea, and iK/isubm/sub of 2–7mM. The proteins differ in their affinities for metal ions and were separated by affinity chromatography on a Znsup2+/sup matrix. Similar to canatoxin, urease activates blood platelets and interacts with glycoconjugates. In contrast with canatoxin, no lethality was seen in mice injected with urease (10mg/kg). Pretreatment with ip/i-hydroxymercuribenzoate irreversibly abolished the ureolytic activity of both proteins. On the other hand, ip/i-hydroxymercuribenzoate-treated canatoxin was still lethal to mice, and both treated proteins were fully active in promoting platelet aggregation and binding to glycoconjugates. Taken together, our data indicate that canatoxin is a variant form of urease. Moreover, we show for the first time that these proteins display several biological effects that are unrelated to their enzymic activity for urea./p