Purification and characterization of trichosanthin. Homology to the ricin A chain and implications as to mechanism of abortifacient activity.

Trichosanthin, a protein from the Chinese medicinal herb Trichosanthes kirilowii, was purified in two essentially quantitative steps involving CM-Sephadex chromatography and reverse-phase high performance liquid chromatography. The protein was found to have a molecular mass of 25-26 kDa, to contain no cysteine, and to contain no glycosidic linkages. Pure trichosanthin was found to have potent abortifacient activity in pregnant mice. In order to understand the molecular basis of this unique biological activity, we have examined the amino acid sequence of the protein. As purified, trichosanthin was found to contain two amino-terminal sequences which differed only in the absence or presence of a tyrosine at residue 1. Sequence analysis of trichosanthin has allowed for determination of the NH2-terminal 38-amino acid residues. Comparison of this sequence to those present in a data base revealed homology with the ricin A-chain. Consistent with this structural homology, we have found that trichosanthin is a potent inhibitor of protein synthesis in a reticulocyte lysate system.