Resistance of exotoxin A to purified Pseudomonas proteolytic enzymes

Pseudomonas toxin is produced as a proenzyme which is cytotoxic for cells in culture but must be activated to express full enzymatic activity. The ability of purified pseudomonas alkaline protease and elastase or of culture filtrates of two strains of Pseudomonas aeruginosa to modify the activity of pseudomonas toxin was examined. Two parameters of toxin activity were followed: enzymatic activity, i.e., the adenosine diphosphate (ADP) ribosylation of elongation factor 2, and biological activity, i.e., inhibition of protein synthesis in cultured mouse fibroblasts. Biological activity of toxin depends upon an intact toxin molecule, whereas enzyme activity requires only a functional A region. Incubation with purified pseudomonas proteolytic enzymes did not alter either enzymatic or biological activity. The toxin is not refractory to the action of all proteolytic enzymes, since thermolysin rapidly destroyed the toxin molecule. Treatment of toxin with culture filtrates of P. aeruginosa reduced ADP ribosylation activity, but increased the ability of toxin to inhibit protein synthesis in cell monolayers. Incubation of culture filtrates with one of the protease inhibitors alpha-2-macroglobulin or phosphoramidon did not alter the effect of the filtrates on biological activity. Alpha-2-macroglobulin, however, caused a fourfold stimulation of ADP ribosylation activity of the toxin. We conclude that pseudomonas alkaline protease and elastase are not responsible for the modifications in toxin activity induced by culture filtrates of P. aeruginosa; the factors responsible have not yet been identified, but are not inactivated by phosphoramidon or alpha-2-macroglobulin.