Glutaraldehyde cross-linking alters the environment around Trp(29) of cobrotoxin and the pathway for regaining its fine structure during refolding.

Abstract

Cobrotoxin, purified from the venom of Naja naja atra (Taiwan cobra), was subjected to modification with glutaraldehyde in order to prepare intra- and intermolecule cross-linked derivatives. Monomeric and dimeric derivatives were separated from polymeric derivatives by gel filtration. The results of amino acid analysis and sequence determination revealed that only Lys residues were selectively modified by glutaraldehyde. Glutaraldehyde cross-linking was accompanied by a change in the gross conformation of cobrotoxin as revealed by circular dichroism spectra of the modified derivatives. Compared with cobrotoxin, Trp(29) of monomeric and dimeric derivatives was in an apolar microenvironment. This was in agreement with acrylamide quenching studies showing that the spatial position of the Trp indole ring became buried in the interior of the molecule after glutaraldehyde cross-linking. Moreover, the Trp of modified derivatives was less accessible for iodide than that observed with cobrotoxin. Notably, disulfide reduction could not completely unfold the structure of glutaraldehyde-modified derivatives as evidenced by the results of acrylamide quenching studies and enzyme-linked immunoassay. Study of the characteristic changes in Trp fluorescence after the initiation of refolding suggested that the fine structure around Trp(29) of cobrotoxin and glutaraldehyde-modified derivatives was formed differently. These results suggest that glutaraldehyde cross-linking leads to a change in the microenvironment of cobrotoxin Trp(29) and alters the pathway of its fine structure formation during the refolding of cobrotoxin.