Proteolytic processing of ricin A chain is not required for cytotoxicity

[125I]-labeled ricin A chain was endocytosed by macrophages and Vero cells either on its own or as part of the ricin holotoxin. Subsequently [125I]-ricin A chain was recovered from intoxicated cells by immunoprecipitation or acetone precipitation. The recovered protein was shown to have the same electrophoretic mobility as the material supplied to the cells. These data challenge an earlier suggestion that limited intracellular processing of ricin A chain by proteolytic enzymes is required for maximal toxicity.