Protein A vectorized toxins--I. Preparation and properties of protein A-ricin toxin conjugates

Protein A of Staphylococcus aureus (mol. wt 43,000) was covalently bound to ricin toxin (mol. wt 60,000) by N-succinimidyl 3-(2-pyridyldithio) propionate. The conjugate consisting of one molecule of protein A bound to one molecule of ricin toxin (mol. wt 100,000) was purified by successive affinity chromatographies on IgG-Sepharose 4B and ConA-Sepharose 4B. The purified protein A-ricin toxin conjugate was able to bind and kill IgG antibody coated leukemia EL4 cells leaving unaffected EL4 cells not coated with antibody. The cytotoxic efficacy of the conjugate was comparable to that of nonconjugated ricin toxin. The results recommend the use of protein A-ricin toxin conjugate as a "universal" specific toxin for the "in vitro" killing of various antibody-coated target cells.