Ap4A phosphorylase catalyzes the phosphorolytic degradation of bis(5'-adenosyl) tetraphosphate (Ap4A) into ADP and ATP. Can also use other Np4N' nucleotides (where N and N' stand for A,C,G or U) as substrates with equal efficiency. Cannot catalyze the reverse reaction. Additionally, this enzyme can also catalyze the phosphorolytic degradation of adenosine 5'-phosphosulfate (AMPS) into ADP and sulfate, the reversible exchange reaction between inorganic phosphate and the beta-phosphate of a nucleoside diphosphate (NDP), and the synthesis of Ap4A from AMPS plus ATP. ADP + ATP + H(+) = P(1),P(4)-bis(5'-adenosyl) tetraphosphate + phosphate ADP + H(+) + sulfate = adenosine 5'-phosphosulfate + phosphate kcat is 81.2 sec(-1) with Ap4A as substrate. Optimum pH is 7.5. Monomer. The N-terminus is blocked. Inactivation of both APA1 and APA2 promotes a great increase in the cellular concentration of bis(5'-nuceleosidyl) tetraphosphate nucleotides. Present with 19600 molecules/cell in log phase SD medium. Belongs to the ATP adenylyltransferase family.