Simultaneous existence of two types of ribosome-inactivating proteins in the seeds of Cinnamonum camphora--characterization of the enzymatic activities of these cytotoxic proteins.

Two types of ribosome-inactivating proteins (RIPs), camphorin and cinnamomin, have been purified from the seeds of Cinnamomum camphora. Camphorin is a type I RIP with a molecular mass of 23 kDa. Cinnamomin is a type II RIP having three isoforms, its molecular mass is around 61 kDa. The A- and B-chain derived from cinnamomin isoforms have similar mobilities in SDS-polyacrylamide gel. Camphorin and cinnamomin are both glycoproteins. In phylogenesis, it is particularly significant to find the simultaneous existence of type I and type II RIP as well as three isoforms of the latter in the same organ of a higher plant. The molecular mechanism of action of these two RIPs on mammalian ribosomes was demonstrated to be the specific RNA N-glycosidase activity. They modify 28S RNA among the four species of ribosomal RNAs. The cleavage site is the adenosine at position 4324 (rat liver 28S rRNA) embedded in the highly conserved 'R/S domain'. It is the first report that an intact type II RIP (cinnamomin) exhibits RNA N-glycosidase activity. Additionally, camphorin and cinnamomin can cleave supercoiled double-stranded DNA into nicked and linear forms (Ling et al. (1994) FEBS Lett. 345, 143-146).