The membrane insertion of trichosanthin is membrane-surface-pH dependent

Trichosanthin (TCS) is the active component extracted from Tianhuafen, a tr aditional herbal medicine that has been used for abortion in China for cent uries. It belongs to the type-1 ribosome-inactivating protein (RIP) family and can inactivate the eukaryotic ribosome through its RNA N-glycosidase ac tivity. Recent studies have shown TCS to be multifunctional, its pharmacolo gical properties including immunomodulatory, anti-tumour and anti-HIV activ ities. The membrane-insertion property of TCS is thought to be essential fo r its physiological effect, for it must get across the membrane before it c an enter the cytoplasm and exert its RIP function. In this paper, the membr ane-insertion mechanism of TCS was studied. The monolayer experiment reveal ed that TCS's membrane-insertion ability was dependent on low pH. Fluoresce nce spectroscopy using 1-anilinonaph-thalene-8-sulphonic acid as a probe sh owed that low pH may induce the conformational change of TCS that leads to the hydrophobic-site exposure, and the CD result showed that this conformat ional change did not alter its secondary structure. Such conformational cha nge leads to an intermediate state, called the 'molten globular state' by p revious investigators. The pH-dependent membrane insertion and conformation al change were related by the fact that the optimal membrane-surface pH nee ded was the same for the two events. From these and other results, a membra ne-insertion model was proposed.