A divalent cation is essential for activity. Mn2+?(2-6 mM) is most effective. The enzyme controls intracellular levels of?P1,P5-bis(5′-adenosyl)pentaphosphate and?P1,P6-bis(5′-adenosyl)hexaphosphate. Weak activity with?P1,P4-bis(5′-adenosyl)tetraphosphate. Marked preference for adenine over guanine nucleotides.