Adenosylhomocysteine is a competitive inhibitor of S-adenosyl-L-methionine-dependent methyl transferase reactions; therefore adenosylhomocysteinase may play a key role in the control of methylations via regulation of the intracellular concentration of adenosylhomocysteine. H2O + S-adenosyl-L-homocysteine = adenosine + L-homocysteine Binds 1 NAD(+) per subunit. Amino-acid biosynthesis; L-homocysteine biosynthesis; L-homocysteine from S-adenosyl-L-homocysteine: step 1/1. Homotetramer. Belongs to the adenosylhomocysteinase family.