Demonstration of ribonuclease activity in the plant ribosome-inactivating proteins alpha- and beta-momorcharins.

Alpha- and beta-momorcharins, ribosome-inactivating proteins from Momordica charantia seeds, were utilized in this investigation. Ribonucleolytic cleavage was observed after naked rRNA was incubated with either momorcharin. Beta-momorcharin, and to a lesser extent alpha-momorcharin, also acted on tRNA to release acid-soluble UV-absorbing products. Such activity was optimal at pH around 5.5. Using polyhomoribonucleotides as substrate, it was found that the momorcharins preferentially acted 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 cleavage. Similar to the results obtained with tRNA as substrate, beta-momorcharin was about 15-fold more active than alpha-momorcharin on polyU.