Electrochemiluminescent enzymes, their preparation and use as biosensors are disclosed. Specifically, two appendages are covalently attached to a desired dehydrogenase enzyme; (1) a nicotinamide adenine cofactor or analog thereof, and (2) a luminescent ruthenium complex. For example, glucose concentration is the following way. A doubly-modified glucose dehydrogenase could oxidize glucose with concomitant reduction of the attached NAD to NADH. Because NADH, but not NAD , is able to interact with surface ruthenium to promote ECL, only enzyme molecules that have reacted with glucose will emit light from their ruthenium label in an ECL instrument. The relative close proximity of NADH and ruthenium on the enzyme surface enhances light emission as compared to the same concentrations in free solution. When NADH reduces ruthenium, it returns to become NAD , permitting multiple cycles of ECL light emission from a single enzyme molecule. Such biosensors can be used in solution or bound to a solid surface. Assays employing the biosensor molecules can be performed on an IGEN Origen3 Analyzer.