Abstract: An aqueous coenzyme reagent composition contains NADH, a carbonate/bicarbonate buffer (with a pH of about 9.5-11) and water. Both NADH and the buffer are at low concentrations, e.g., about 2-5 mM for NADH and about 2-15 mM for carbonate/bicarbonate. When this coenzyme reagent is mixed with an enzyme reagent, the mixture achieves a neutral pH. The combination of high pH, low NADH concentration and low buffer concentration permit the aqueous reagent to be stored for extended periods at low temperatures without NADH degrading to form impurities which interfere with or inhibit enzyme activity, such as the activity of lactate dehydrogenase or malate dehydrogenase in an assay for ALT or AST.