Catalyzes a cyclopropane ring-opening reaction, the irreversible conversion of 1-aminocyclopropane-1-carboxylate (ACC) to ammonia and alpha-ketobutyrate. 1-aminocyclopropane-1-carboxylate + H2O = 2-oxobutanoate + NH4(+) Belongs to the ACC deaminase/D-cysteine desulfhydrase family.