Fig. 3

The standard curve established in this study to convert optical absorbance at 440 nm to NPEA depletion. Pyruvate with concentrations from 0.1 to 1.0 mM was used as the amino acceptor, and 10 mM NPEA was used as the amino donor. The microtiter plate was sealed with transparent and adhesive film and was incubated at 37 °C and 600 rpm for 24 h with the film side facing down. The absorbance of reaction wells was measured at 440 nm using a spectrophotometer, and depletion of pyruvate in all reactions was confirmed using HPLC-RI (Additional file 1: Fig. S4) (n = 4, error bars indicate standard errors)