Molecular function

Catalysis of the reaction: ADP + ATP = phosphate + P(1),P(4)-bis(5'-adenosyl)tetraphosphate.

AP-4-A phosphorylase activity, adenine triphosphate adenylyltransferase activity, bis(5'-nucleosyl)-tetraphosphate phosphorylase (NDP-forming) activity, diadenosine 5',5'''-P(1),P(4)-tetraphosphate phosphorylase activity, diadenosine 5',5'''-P1,P4-tetraphosphate alphabeta-phosphorylase (ADP-forming), diadenosine 5',5'''-P1,P4-tetraphosphate phosphorylase activity, diadenosinetetraphosphate alpha-beta-phosphorylase activity, diadenosinetetraphosphate alphabeta-phosphorylase activity, dinucleoside oligophosphate alphabeta-phosphorylase activity