Synthesis of SP-CTA. A. Schematic representation of the procedure used to synthesize SP-CTA. B. Western blots of final SP-CTA product. SP-CTA and the filtrate from the Centricon Plus-20 concentrating tubes (Wash) were run on western blots and probed with antibodies to substance P and the catalytic subunit of cholera toxin (CTA). The SP-CTA product reacted with both antibodies.