Gln is hydrolyzed to produce glutamic acid under the action of glutaminase. Glutamic acid is further catalyzed by glutamic acid dehydrogenase. Meanwhile, NAD+ is reduced to NADH, Which under the action of hydrogen transmitter, transfer electrons to WST-8 to produce the yellow product. The content of Gln can be calculated by measuring the change of absorbance value at 450 nm.