Purine nucleoside enzyme that catalyzes the phosphorolysis of adenosine and inosine nucleosides, yielding D-ribose 1-phosphate and the respective free bases, adenine and hypoxanthine. Also catalyzes the phosphorolysis of S-methyl-5'-thioadenosine into adenine and S-methyl-5-thio-alpha-D-ribose 1-phosphate. Also has adenosine deaminase activity. adenosine + phosphate = adenine + alpha-D-ribose 1-phosphate phosphate + S-methyl-5'-thioadenosine = adenine + S-methyl-5-thio-alpha-D-ribose 1-phosphate inosine + phosphate = alpha-D-ribose 1-phosphate + hypoxanthine adenosine + H(+) + H2O = inosine + NH4(+) kcat is 13000 min(-1) with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid as substrate. kcat is 33300 min(-1) with syringaldazine as substrate (at pH 4.5 and 40 degrees Celsius). Optimum pH is 4.5-6.0. Maintains 80% activity at pH 4.0-7.5. Optimum temperature is 52 degrees Celsius. Maintains more than 80% activity at 55 degrees Celsius. Homodimer. Belongs to the purine nucleoside phosphorylase YfiH/LACC1 family.