ISOLATION AND CHARACTERIZATION OF A NEW NONTOXIC 2-CHAIN RIBOSOME-INACTIVATING PROTEIN FROM FRUITS OF ELDER (SAMBUCUS-NIGRA L)

Sambucus (Caprifoliaceae) species contain nigrin b and ebulin I, which are two-chain ribosome-inactivating proteins (RIPs) belonging to a ne w type of RIPs which are non-toxic to mice and cultured human cells. I n this work the presence in fruits of elder (S. nigra L.) of a new non -toxic type 2 RIP (nigrin f) that co-exists with a lectin known as SNA IV is described. Nigrin f strongly inhibited protein synthesis in mam malian, but not in plant, ribosomes, promoting the depurination of sen sitive ribosomes and thus allowing the release of the RIP diagnostic R NA fragment. Nigrin f is composed of two dissimilar subunits linked by disulphide bridges with apparent M(r) values of 31 600 and 26 300. Th e N-terminal amino acid sequence revealed close homology of the cataly tic A chain with type 1 RIPs, especially those from Cucurbitaceae, and the B chain with several lectins previously isolated from Sambucus sp ecies. Nigrin f was not toxic to mice when injected intraperitoneally up to 2 mg kg(-1). In addition, NHC human cells were also insensitive to nigrin f up to 60 mu g ml(-1). Anti-nigrin b rabbit polyclonal anti bodies reacted with nigrin f, indicating that nigrin b and nigrin f ar e proteins with similar structures.