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. ©Munksgaard 1996.