Isolation and characterization of a new non-toxic two-chain ribosome-inactivating protein from fruits of elder (Sambucus nigraL.)

Sambucus (Caprifoliaceae) species contain nigrin b and ebulin I, which are two-chain ribosome-inactivating proteins (RIPs) belonging to a new type of RIPs which are non-toxic to mice and cultured human cells. In 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 mammalian, but not in plant, ribosomes, promoting the depurination of sensitive ribosomes and thus allowing the release of the RIP diagnostic RNA fragment. Nigrin f is composed of two dissimilar subunits linked by disulphide bridges with apparent M r values of 31 600 and 26 300. The N-terminal amino acid sequence revealed close homology of the catalytic A chain with type 1 RIPs, especially those from Cucurbitaceae, and the B chain with several lectins previously isolated from Sambucus species. 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 μg ml -1 . Anti-nigrin b rabbit polyclonal antibodies reacted with nigrin f, indicating that nigrin b and nigrin f are proteins with similar structures.