Inhibition of the activity of pseudomonas toxin by methylamine.

Methylamine at a concentration of 20 mM protected mouse LM cell fibroblasts from the action of pseudomonas toxin. Nearly total protection was observed when cells were pretreated with amine before the addition to toxin and amine, when amine and toxin were added simultaneously, or when amine was added up to 30 min after toxin binding. Later addition of methylamine afforded partial protection of the monolayers. Using electron microscopy, we observed that toxin initially bound diffusely to the cell surface but rapidly moved to coated pits and was internalized after cells were warmed to 37 C. Methylamine blocked the clustering of toxin into coated-pit areas of the membrane but did not alter the overall level of toxin internalization. It is suggested that pseudomonas toxin enters mammalian cells by receptor-mediated endocytosis and that methylamine alters the entry process. In those instances in which partial protection was seen when methylamine was added after toxin internalization, the primary amine may be functioning by inactivation of lysosomal processing of the toxin.