Catalyzes the reversible phosphorylation of S-methyl-5'-thioinosine (MTI) to hypoxanthine and 5-methylthioribose-1-phosphate. Involved in the breakdown of S-methyl-5'-thioadenosine (MTA), a major by-product of polyamine biosynthesis. Catabolism of (MTA) occurs via deamination to MTI and phosphorolysis to hypoxanthine. phosphate + S-methyl-5'-thioinosine = hypoxanthine + S-methyl-5-thio-alpha-D-ribose 1-phosphate Purine metabolism; purine nucleoside salvage. Homotrimer. Although this enzyme belongs to the family of MTA phosphorylases based on sequence homology, it has been shown that conserved amino acid substitutions in the substrate binding pocket convert the substrate specificity of this enzyme from 6-aminopurines to 6-oxopurines. Belongs to the PNP/MTAP phosphorylase family. MTAP subfamily.