Catalyzes the deamination of three SAM-derived enzymatic products, namely 5'-deoxyadenosine, S-adenosyl-L-homocysteine, and 5'-methylthioadenosine, to produce the inosine analogs. Can also deaminate adenosine. The preferred substrate for this enzyme is 5'-deoxyadenosine, but all these substrates are efficiently deaminated. Likely functions in a S-adenosyl-L-methionine (SAM) recycling pathway from S-adenosyl-L-homocysteine (SAH) produced from SAM-dependent methylation reactions. May also be involved in the recycling of 5'-deoxyadenosine, whereupon the 5'-deoxyribose moiety of 5'-deoxyinosine is further metabolized to deoxyhexoses used for the biosynthesis of aromatic amino acids in methanogens. 5'-deoxyadenosine + H(+) + H2O = 5'-deoxyinosine + NH4(+) H(+) + H2O + S-adenosyl-L-homocysteine = NH4(+) + S-inosyl-L-homocysteine H(+) + H2O + S-methyl-5'-thioadenosine = NH4(+) + S-methyl-5'-thioinosine adenosine + H(+) + H2O = inosine + NH4(+) Binds 1 zinc ion per subunit. Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis. Homotetramer. SAH is a product of SAM methyltransferases and is known to be a feedback inhibitor of these enzymes. As a result of this inhibition, organisms have evolved efficient enzymes to metabolize SAH via different pathways. The pathway found in methanogens differs from the canonical pathway, it uses the deamination of S-adenosyl-L-homocysteine to form S-inosyl-L-homocysteine for the regeneration of SAM from S-adenosyl-L-homocysteine. 5'-deoxyadenosine is a radical SAM enzyme reaction product which strongly inhibits radical SAM enzymes. A pathway for removing this product must be present in methanogens where the MTA/SAH nucleosidase which normally metabolizes this compound is absent. Belongs to the metallo-dependent hydrolases superfamily. MTA/SAH deaminase family.