Catalyzes the biosynthesis of agmatine from arginine. H(+) + L-arginine = agmatine + CO2 Down-regulated by polyamine end products putrescine and spermidine. Optimum pH is 8.4. Optimum temperature is 50 degrees Celsius. Thermostable. Active from 20 to 80 degrees Celsius. Amine and polyamine biosynthesis; agmatine biosynthesis; agmatine from L-arginine: step 1/1. Homotetramer. By growth in an acidic enriched medium containing arginine (biodegradative form), by growth in minimal media at neutral pH (biosynthetic). Putrescine and spermidine repress the speA gene and feedback inhibit ADC. Processed post-translationally to a 70 kDa mature form. The N-terminus is blocked. ADC can be found in two forms: biodegradative and biosynthetic. The biodegradative form may play a role in regulating pH by consuming proteins. Belongs to the Orn/Lys/Arg decarboxylase class-II family. SpeA subfamily.