Trichosanthin (TCS) is the active component extracted from Tianhuafen, a traditional herbal medicine that has been used for abortion in China for centuries. It belongs to the type-I ribosomeinactivating protein (RIP) family and can inactivate the eukaryotic ribosome through its RNA N-glycosidase activity. Recentstudies have shown TCS to be multifunctional, its pharmacologicalproperties including immunomodulatory, anti-tumourand anti-HIV activities. The membrane-insertion property ofTCS is thought to be essential for its physiological effect, for itmust get across the membrane before it can enter the cytoplasmand exert its RIP function. In this paper, the membrane-insertionmechanism of TCS was studied. The monolayer experimentrevealed that TCS's membrane-insertion ability was dependenton low pH. Fluorescence spectroscopy using 1-anilinonaph-thalene-8-sulphonic acid as a probe showed that low pH mayinduce the conformational change of TCS that leads to thehydrophobic-site exposure, and the CD result showed that thisconformational change did not alter its secondary structure.Such conformational change leads to an intermediate state,called the molten globular state' by previous investigators. ThepH-dependent membrane insertion and conformational changewere related by the fact that the optimal membrane-surface pHneeded was the same for the two events. From these and otherresults, a membrane-insertion model was proposed.

The interaction of apolipoprotein H (Apo H) with lipid membrane has been considered to be a basic mechanism for the biological function of the protein. Previous reports have demonstratedthat Apo H can interact only with membranes containinganionic phospholipids. Here we study the membrane-inducedconformational change of Apo H by CD spectroscopy with twodifferent model systems: anionic-phospholipid-containing liposomes[such as 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol(DMPG) and cardiolipin], and the water}methanol mixtures atmoderately low pH, which mimic the micro-physicochemicalenvironment near the membrane surface. It is found that Apo Hundergoes a remarkable conformational change on interactionwith liposomes containing anionic phospholipid. To interact withliposomes containing DMPG, there is a 6.8%increase in a-helixin the secondary structures; in liposomes containing cardiolipin,however, there is a 12.6% increase in a-helix and a 9% decreasein b-sheet. The similar conformation change in Apo H can beinduced by treatment with an appropriate mixture of water}methanol. The results indicate that the association of Apo Hwith membrane is correlated with a certain conformationalchange in the secondary structure of the protein.

Many proteins exhibit both a water-soluble and a membrane-bound state. The proteins in the membrane-bound state obtain a distinct structure from that in the bulk, which exists in many important biological processes. In the present paper we would stress that the variation of the physical chemistry properties of the microenvironment adjacent to the membranesurface region play an important role in the process of the membrane-induced conformational changes of the proteins.

Apolipoprotein H (ApoH) is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. In this paper, the interaction of ApoH with phospholipid vesicles was characterized by (i) detecting the wavelength shift of the fluorescence spectrum of ApoH and (ii) measuring the fluorescence quenching extent of ApoH by the membrane resident quencher 1-palmitoyl-2-stearoyl-(5-doxyl)-sn-glycero-3-phosphocholine (DPC). The observed blue shift upon addition of DMPG vesicles indicated that the tryptophan residues of ApoH moved from a polar to a nonpolar environment. The insertion ability of ApoH into PG-containing v esicles did not depend on the PG content in a stoichiometric way as did the blue shift, indicating that the negatively charged DMPG does not serve as a specific binding site but rather provides a suitable microenvironment for ApoH interaction. The finding that the detachment effect of cations on the blue shift is remarkably different from that on the quenching extent suggests that ApoH is capable of existing in two different conformations when membrane-bound.