Accessibility of the active site of crotoxin B in the crotoxin complex

Basic phospholipases A and the crotoxin complex isolated from Crotalus durissus terrificus venom exhibited similar initial reaction rates, time course and degree of hydrolysis of synthetic short chain lecithins in the monomeric state. Although monomeric lecithins seem to promote dissociation of crotoxin up to a certain extent, this cannot explain the high activity observed with the complex. The crotoxin complex is able to bind the non-hydrolyzable analog D-diheptanoyllecithin, as demonstrated by equilibrium gel-filtration, with a dissociation constant of 0.12 mM. This value is similar to the dissociation constant of the crotoxin B-D-diheptanoyllecithin complex (about 0.13 mM), estimated from the protection against enzyme inactivation by p-bromophenacyl bromide, which further supports the free accessibility of the substrate to the enzyme active site in the crotoxin complex. The lack of enzyme inactivation when crotoxin is treated with p-bromophenacyl bromide may be interpreted in terms of the specific requirements of the reagent to react with the enzyme rather than protection of the active site. Crotoxin B inhibition by complex formation with crotoxin A, which is not apparent on monomeric substrates, seems not to involve the active site of the enzyme.