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-SYNTHESISINHIBITORY ACTIVITY

Effects of chemical modifications on the protein-synthesis inhibitory (PSI) activities of momordin-a and luffin-a were investigated. Treatme nt with a 50-fold excess of diethylpyrocarbonate at pH 6.5 modified on e histidine residue in momordin-a and luffin-a and reduced their PSI a ctivities to 10% and 8.3%, respectively. Modifications with a 20-fold excess of KI3 at pH 7.0 at 0 degrees C greatly reduced their PSI activ ities to 10% by iodination of nearly one tyrosine residue. The PSI act ivity 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 fro m the modified momordin-a and luffin-a, the modified residues were ide ntified as His140, Tyr165, and Lys231, Furthermore, the amounts of thr ee 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 e xposed on the surface of momordin-a and luffin-a molecules and are inv olved in their PSI activities, probably by binding to ribosomes.