Abstract: A method of quantifying a substrate, by which the substrate contained in various samples can be conveniently and quickly quantified without resort to any troublesome pretreatments, and a biosensor. More particularly speaking, a method of quantifying a substrate in a sample by using an electrode system made of electrically conductive materials and a reaction reagent comprising at least a dehydrogenase, a coenzyme an electron mediator and a tetrazolium salt, which comprises performing an enzyme reaction and a redox reaction between the reaction reagent and the substrate in the sample, and detecting a formazan formed as the final reaction product by using the electrode system and a if biosensor with the use of the same are provided.

Abstract: An L-phenylalanine sensor wherein all of the reagents required in determining L-phenylalanine are integrated together, thereby enabling quick and convenient quantification without resort to any special equipment, devices or techniques. This sensor is fabricated by integrating L-phenylalanine dehydrogenase, oxidized nicotinamide adenine dinucleotide (NAD+) or oxidized nicotinamide adenine dinucleotide phosphate (NADP+) as a coenzyme and an electron mediator, which are employed as reagents, with an electrode system comprising at least a working electrode and a counter electrode. Also, a method of determining L-phenylalanine which comprises adding a sample containing L-phenylalanine to the above-described sensor and electrochemically quantitating L-phenylalanine is provided.