THE ROLE OF CALCIUM-IONS FOR THE EXPRESSION OF RICIN TOXICITY IN CULTURED MACROPHAGES

Ricin toxin, which consists of two distinct polypeptide moieties, A and B chains, is cytotoxic to the cultured macrophage cell line, J774A.1. Ricin is a protein synthesis inhibitor, and incubating macrophages for 4 hours with ricin (1 pM to 10 nM) in standard medium containing calcium and magnesium inhibited H-3-leucine incorporation into protein (97%, at 1 nM ricin). However, in Ca2+-free medium, protein synthesis was inhibited only 19%. EGTA pretreatment (to deplete intracellular calcium) also partly protected cells from protein synthesis inhibition, in spite of added calcium (2 mM) in the incubation medium. Decreased toxicity in the absence of extracellular calcium resulted from decreased toxin binding. Adding or deleting Mg2+ did not affect protein synthesis or binding of I-125-ricin in cultured macrophages. We conclude that calcium is required for ricin to exert its inhibitory effect on protein synthesis in cultured macrophages.