The present invention relates to compositions and methods for assaying homocysteine (Hcy) and thus related moieties, e.g., S-adenosylhomocysteine (SAH) or adenosine. More particularly, assay methods that employ, mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, are provided. The modified enzymes and fusion proteins containing the modified enzymes are also provided.