2.4.1. Protein Tyrosine Phosphate 1B (PTP1B) Inhibitory Assay

pNPP was used to test the inhibitory function of PTP1B. In a plate with or without a sample, a recombinant PTP1B enzyme (0.5 units diluted in PTP1B reaction buffer) was added. The plate was pre-incubated for 10 minutes at 37 °C before adding the substrate (2 mMpNPP). The enzymatic reaction was stopped by adding 10 M NaOH after 15 minutes of incubation at 37 °C. A microplate spectrophotometer was used to calculate the absorbance at 405 nm (Molecular Devices, Sunnyvale, CA, USA). The reference compound was ursolic acid [25]. The results were expressed as mean ± S.E.M. of three independent experiments performed in duplicate.