Flow injection method for the determination of thyroxine by inhibition of glutamate dehydrogenase Original Research Article

Thyroxine is an important hormone and is reported to be an inhibitor of glutamate dehydrogenase (GlDH). This inhibitory effect of thyroxine is used to develop a simple and highly reproducible flow injection method for its determination. The change in NADH absorbance at 340 nm in the presence of the enzyme and thyroxine is measured on-line and related to the percent inhibition (%I) which increases with inhibitor concentration. The relative standard deviation is 1.8% for 8×10−6 M thyroxine (n=7) and the 3σ limit of detection is 9×10−7 M. The linear equation over the range 0.2–8×10−5 M was %I=4.9+8.97×105 [thyroxine, M], and the correlation coefficient was 0.998 (n=7).