Catalyzes a salvage reaction resulting in the formation of AMP, that is energically less costly than de novo synthesis. May contribute to the recycling of adenine into adenylate nucleotides and the inactivation of cytokinins by phosphoribosylation. Possesses low activity toward adenine and cytokinins. AMP + diphosphate = 5-phospho-alpha-D-ribose 1-diphosphate + adenine Optimum pH is 7.4-7.5. Purine metabolism; AMP biosynthesis via salvage pathway; AMP from adenine: step 1/1. Homodimer. Belongs to the purine/pyrimidine phosphoribosyltransferase family.