ABSTRACT
This chapter describes the toxins are proteins produced by plants or microrganisms, and they share the property of inhibiting protein synthesis by acting catalytic ally, although the mechanisms sometimes differ. The native molecule is higly toxic for animals and cultured cells. It catalytically inhibits protein synthesis in cultured cells, but has no enzymatic activity in a cell-free system. The protein is produced as a single polypeptide chain, devoid of enzymatic activity in a cell-free system. Activation occurs with mild denaturating agents that are able to unfold the protein chain, or as a result of limited proteolysis and thiol reduction as for diphtheria toxin. A single-chain protein with antiviral properties isolated from the leaves of Phytolacca americana and designated “pokeweed antiviral protein” inhibits protein synthesis by a mechanism identical to that of ricin A-chain. Ricin and related toxins inhibit protein synthesis in both intact animal cells and cell-free systems.
