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用一种定点修饰天花粉蛋白（ｔｒｉｃｈｏｓａｎｔｈｉｎ，ＴＣＳ）的方法，将聚乙二醇（ＰＥＧ）偶联到预先选定的位点．利用ｎＴＣＳ无半胱氨酸（Ｃｙｓ）残基这一特点，通过定点突变将一个Ｃｙｓ残基引入ＴＣＳ以取代第７位的丝氨酸（Ｓｅｒ）残基．然后，与巯基反应的ＰＥＧ－ｍ ａｌｅｉｍ ｉｄｅ 即可偶联到新引入的Ｃｙｓ 残基上．经纯化得到均一的ＰＥＧ－ＴＣＳ复合物，在ＳＤＳ－ＰＡＧＥ上显示一条区带，表观分子量为３８ ｋＤ．复合物的体外致核糖体失活活性降低了６倍，但其体内引产活性与ｎＴＣＳ相同．定点ＰＥＧ修饰方法为改造ＴＣＳ提供了新途径．
A method of site-directed trichosanthin (TCS) conjugation of polyethylene glycol (PEG) to pre-selected sites. Using the feature of nTCS without cysteine (Cys) residues, a Cys residue was introduced into TCS by site-directed mutagenesis to replace the serine residue at position 7. The PEG-alem ide reacted with thiol can then be coupled to the newly introduced Cys residue. After purification, a homogeneous PEG-TCS complex was obtained, showing a band on SDS-PAGE with an apparent molecular weight of 38 kD. The in vitro ribosomal inactivation activity of the complex was reduced by a factor of 6, but its in vivo abortion activity was the same as nTCS. Site-directed PEG modification provides a new way to transform TCS.