Catalyzes the irreversible cleavage of the glycosidic bond in both 5'-methylthioadenosine (MTA) and S-adenosylhomocysteine (SAH/AdoHcy) to adenine and the corresponding thioribose, 5'-methylthioribose and S-ribosylhomocysteine, respectively (PubMed:3911944, PubMed:16101288). Also cleaves 5'-deoxyadenosine, a toxic by-product of radical S-adenosylmethionine (SAM) enzymes, into 5-deoxyribose and adenine. Thus, is required for in vivo function of the radical SAM enzymes biotin synthase and lipoic acid synthase, that are inhibited by 5'-deoxyadenosine accumulation (PubMed:15911379). Can also use 5'-isobutylthioadenosine, 5'-n-butylthioadenosine, S-adenosyl-D-homocysteine, decarboxylated adenosylhomocysteine, deaminated adenosylhomocysteine and S-2-aza-adenosylhomocysteine as substrates in vitro (PubMed:3911944). H2O + S-adenosyl-L-homocysteine = adenine + S-(5-deoxy-D-ribos-5-yl)-L-homocysteine H2O + S-methyl-5'-thioadenosine = 5-(methylsulfanyl)-D-ribose + adenine 5'-deoxyadenosine + H2O = 5-deoxy-D-ribose + adenine Strongly inhibited by aryl-substituted MTA analogs, alkyl-substituted MTA analogs, 5'-methylthioformycin, 5'-chloroformycin, S-formycinylhomocysteine, 5'-methylthiotubercidin, S-tubercidinylhomocysteine and S-8-aza-adenosylhomocysteine. Poorly inhibited by 5'-isobutylthioinosine, 5'-n-butylthioinosine, 5'-methylthio-3-deaza-adenosine, 5'-isobutylthio-3-deaza-adenosine, S-n-6-methyl-3-deaza-adenosylhomocysteine, S-adenosylhomocysteine sulphoxide and Sinefungin. kcat is 3.0 sec(-1) and 2.6 sec(-1) with 5'-methylthioadenosine and S-adenosylhomocysteine as substrate, respectively. Exhibits activity across a broad pH range. Enzyme activity is moderately improved under acidic conditions. Optimum temperature is 37-45 degrees Celsius. Rapidly inactivated after exposure for 10 minutes at 55 degrees Celsius. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; S-methyl-5-thio-alpha-D-ribose 1-phosphate from S-methyl-5'-thioadenosine (hydrolase route): step 1/2. Homodimer. Cells lacking this gene are deficient in biotin synthase (BioB) activity in vivo due to accumulation of 5'-deoxyadenosine. Belongs to the PNP/UDP phosphorylase family. MtnN subfamily.