PURPOSE:To highly sensitively determine the amount of the subject compound for clinical examinations, etc., by treating a specimen with a reagent containing an amino acid dehydrogenase, a thio NAD(P) compound, NAD(P) and a component forming an enzymatic cycling reaction system and subsequently measuring the amount of the produced component. CONSTITUTION:A specimen containing ammonia, an alpha-amino acid, or the corresponding alpha-keto acid is treated with a reagent containing (1) an amino acid dehydrogenase capable of causing a reverse reaction in which ammonia and alpha-keto acid corresponding to an alpha-amino acid is produced in the presence of a coenzyme selected from a thionicotinamide adenine dinucleotide compound (phosphate) [thio NAD(P)] or nicotinsmide adenine dinucleotide (phosphate) [NAD(P)] in a substrate comprising at least the amino acid and water, (2) A1 [A1 is thio NAD(P), etc.], (3) B1 [B1 is reduction type NAD(P), etc.], etc., to form a cycling reaction system of the reaction equation (A2 is the reduction type of A1; Br2 is the oxidation type of B1), and the amount of the produced A2 or B1 is measured to determine the amount of the objective compound in high sensitivity.
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