α-Kirilowin, a novel ribosome-inactivating protein from seeds of Trichosanthes kirilowii (family Cucurbitaceae): A comparison with β-kirilowin and other related proteins

A novel ribosome-inactivating protein (RIP) designated α-kirilowin was isolated from the seeds of Trichosanthes kirilowii. The molecular weight of α-kirilowin was estimated by SDS-polyacrylamide gel electrophoresis to be 28 800 Da, which is slightly larger than another previously characterized ribosome-inactivating protein, β-kirilowin. The amino-acid composition of α-kirilowin grossly resembled β-kirilowin and other ribosome-inactivating proteins isolated from T. kirilowii tissues, including trichokirin, trichosanthin and karasurin. Intense immunological cross-reactivity between the two kirilowins was detected by immunodiffusion. The N-terminal sequence of α-kirilowin was identical to that of β-kirilowin, at least in the first ten residues. Peptide fingerprinting indicated both kirilowins were closely related. Biological activities as determined by inhibition of protein synthesis in a cell-free system, suppression of [³H]-thymidine incorporation into mouse melanoma cells and induction of abortion in mice were very similar for both kirilowins. We propose that the size difference between α-and β-kirilowin is either due to a C-terminal extension in α-kirilowin or differences in glycosylation, or a combination of both.