Catalyzes the decarboxylation of carboxynorspermidine and carboxyspermidine. carboxynorspermidine + H(+) = CO2 + norspermidine carboxyspermidine + H(+) = CO2 + spermidine Homodimer. By arsenic. Belongs to the Orn/Lys/Arg decarboxylase class-II family. NspC subfamily.