Ribosome-inactivating proteins: progress and problems.

Ribosome-inactivating proteins (RIPs), mostly from plants, are enzymes which depurinate rRNA, thus inhibiting protein synthesis, and depurinate also other polynucleotide substrates. The biological activity of RIPs is not completely clarified, sometimes being independent of the inhibition of protein synthesis. There are differences in the cytotoxicity of RIPs, and consequently in their toxicity to animals. Some RIPs are potent toxins, the best known being ricin, a potential biological weapon. Recently, some new toxins were identified. RIPs cause apoptotic and necrotic lesions, and induce production of cytokines causing inflammation. RIPs are potentially useful in agriculture and medicine because (i) they have antiviral activity and (ii) they are used for the preparation of conjugates with antibodies (“immunotoxins”) or other carriers, specifically toxic to the cells target of the carrier, which may be helpful in therapy. The RIPs distribution, mechanism of action and role in Nature are not completely understood, and their practical applications may be the subject matter of several future developments.