Degradation of ricin A chain by endosomal and lysosomal enzymes--the protective role of ricin B chain.

We investigated the role of intracellular processing of ricin A chain (RTA) by proteolytic enzymes on the expression of its ribosome inhibitory activity. Endosomal and lysosomal proteases extracted from Jurkat cells and purified cathepsins B, D and G were incubated with RTA, resulting in generation of a 28-kDa fragment by proteolytic cleavage. This process was reminiscent of the nicking of Pseudomonas exotoxin and Diphtheria toxin by intracellular proteases to produce functionally active toxin fragments. However, the ribosome inhibitory activity of the purified 28-kDa fragment of RTA was 11,000-fold less than that of native RTA, suggesting that such cleavage is not an essential step in the cytotoxic activity of the toxin. Addition of ricin B chain (RTB) in degradation assays resulted in the protection of RTA from proteolytic activities of lysosomes and cathepsins. However, RTB did not protect another RNA acting protein, RNAase; nor did excess amounts of unlabeled RTA or IgG protect labelled RTA from degradation, suggesting that the protective effect of RTB was specific to its interaction with RTA. Such a protective role for RTB may partially account for the higher toxicity of immunotoxins (ITs) containing whole ricin compared to ITs containing only RTA.