Importance of lysine and arginine residues to the biological activity of trichosanthin, a ribosome-inactivating protein from Trichosanthes kirilowii tubers.

Lysine and arginine residues in trichosanthin were modified with ethyl acetimidate and phenylglyoxal, respectively. The effects of these chemical modifications on the cell-free protein-synthesis-inhibitory activity and the antigen-antibody (Ag-Ab) interaction between trichosanthin and its antibody were examined. Ethyl acetimidate modification abolished the protein-synthesis-inhibitory activity of trichosanthin. The inactivation process followed simple first-order kinetics with an inactivation half-life of about 18.5 min. Modification of one lysine residue seemed to be responsible for such inactivation. Arginine modification also inactivated trichosanthin. The inactivation process, however, was biphasic with inactivation half-lives of approximately 15 and 540 min. Arginine modification of trichosanthin had little or no effect on Ag-Ab interaction, whereas lysine modification slightly but significantly weakened Ag-Ab binding.