Stabilization of NADPH or NADH in ammonia detection assays

A method to provide an assay kit for quantitatively determining the concentration of ammonia in an aqueous liquid sample, the kit comprising two different aqueous reagents, the reagents containing in aqueous solution 2-oxoglutarate, glutamate dehydrogenase (GLDH) capable of reacting nicotinamide adenine dinucleotide phosphate (NADPH) and/or nicotinamide adenine dinucleotide (NADH) as a co-substrate, and NADPH and/or NADH (NAD(P)H),

• the method comprising the steps of
• preparing a first reagent by dissolving GLDH in an aqueous solution with a pH capable of maintaining enzymatic activity of GLDH,
• preparing a second reagent by dissolving in an aqueous solution 2-oxoglutarate, GLDH, and NAD(P)H, and adjusting the pH of the second reagent to maintain enzymatic activity of GLDH in the second reagent to remove ammonia released by decomposition of NAD(P)H from the aqueous solution through GLDH enzymatic activity acting upon ammonia, NAD(P)H, and 2-oxoglutarate, thereby forming L-glutamate, NAD(P)+ and H2O in the second reagent,
• providing the first and the second reagent in separate containers, and combining the containers in a kit of parts, thereby providing an assay kit for quantitatively determining the concentration of ammonia in an aqueous liquid sample,
• wherein the concentration of ammonia in the second reagent is from about 0.01 μM to about 1.5 μM.