Comparative studies on kinetics of inhibition of protein synthesis in intact cells by ricin and conjugate of ricin B-chain with momordin

Ribosome inactivating proteins from plants have been widely used for the pr eparation of immunotoxins and hormonotoxins, which have potential applicati on in the therapy of diseases such as cancer. However, these hybrid toxins have been found to be less cytotoxic than native ribosome inactivating prot eins. Therefore, it is important to understand the factors that control the intrinsic toxicity of RIPs and the hybrid toxins prepared using them. Here , a hybrid toxin has been prepared by coupling ricin B-chain to momordin an d the cytotoxicity of this hybrid toxin has been compared to that observed in case of native ricin. In the two cell types used here, thymocytes and ma crophages, the conjugate was found to be about 40 fold less toxic than nati ve ricin. Kinetics of inhibition of protein synthesis showed that prior to onset of inhibition the conjugate exhibits a longer lag phase than native r icin. The rates of inhibition of protein synthesis by the conjugate were al so found to be slower than ricin. Analysis of the results suggests that in addition to cell surface binding, the B-chain of ricin facilitates another step in the transmembrane translocation of ricin A-chain to the cytosol.