Some ribosome-inactivating proteins depurinate ribosomal RNA at multiple sites

Saporin-S6, a ribosome-inactivating protein (RIP) from Saponaria officinalis released more than 1 mol of adenine/mol of ribosomes from house fly (Musca domestica) larvae and from rat liver. The release of adenine from rat liver ribosomes by several RIPs (plant enzymes with RNA N-glycosidase activity) was examined. Saporins, pokeweed antiviral protein from roots of Phytolacca americana (PAP-R), and trichokirin from Trichosanthes kirilowii seeds depurinated rat liver ribosomes at more than one site. Up to 33 mol of adenine were released from 1 mol of ribosomes. This property is not common to all RIPS.