DEMONSTRATION OF RIBONUCLEASE-ACTIVITY IN THE PLANT RIBOSOME-INACTIVATING PROTEINS ALPHA-MOMORCHARINS AND BETA-MOMORCHARINS

Alpha- and beta-momorcharins, ribosome-inactivating proteins from Momo rdica charantia seeds, were utilized in this investigation. Ribonucleo lytic cleavage was observed after naked rRNA was incubated with either momorcharin. Beta-momorcharin, and to a lesser extent alpha-momorchar in, also acted on tRNA to release acid-soluble UV-absorbing products. Such activity was optimal at pH around 5.5. Using polyhomoribonucleoti des as substrate, it was found that the momorcharins preferentially ac ted on polyU, but exerted negligible effects on polyA, polyC and polyG . Chromatographic analysis of the reaction product indicated that mono and/or oligo-ribonucleotides, but not free base, were generated from polyU, suggesting that the enzymatic action involved ribonucleolytic c leavage. Similar to the results obtained with tRNA as substrate, beta- momorcharin was about 15-fold more active than alpha-momorcharin on po lyU.