Chemical modifications of momordin-a and luffin-a, ribosome-inactivating proteins from the seeds of Momordica charantia and Luffa cylindrica: involvement of His140, Tyr165, and Lys231 in the protein-synthesis inhibitory activity.

Effects of chemical modifications on the protein-synthesis inhibitory (PSI) activities of momordin-a and luffin-a were investigated. Treatment with a 50-fold excess of diethylpyrocarbonate at pH 6.5 modified one histidine residue in momordin-a and luffin-a and reduced their PSI activities to 10% and 8.3%, respectively. Modifications with a 20-fold excess of KI3 at pH 7.0 at 0 degree C greatly reduced their PSI activities to 10% by iodination of nearly one tyrosine residue. The PSI activity of momordin-a was rapidly reduced to 6.4% by the modification of one lysine residue with trinitrobenzensulfonic acid as in the case of luffin-a reported previously. By analyses of the tryptic peptides from the modified momordin-a and luffin-a, the modified residues were identified as His140, Tyr165, and Lys231. Furthermore, the amounts of three modified momordin-a binding to rat liver ribosomes were reduced to about half or less than half of that of native momordin-a. From these results, it was suggested that His140, Tyr165, and Lys231 are highly exposed on the surface of momordin-a and luffin-a molecules and are involved in their PSI activities, probably by binding to ribosomes.