The HPLC system for organic acids determination was the same
as for that of free amino acids and 50-nucleotides. The assay was
performed on a Zorbax Eclipse XDB C18 column (250 4.6 mm,
5 lm, Agilent) with injection volume of 20 lL, the mobile
phase was KH2PO4 (0.01 M, pH = 2.55)/methanol (95/5, v/v) at a
flow rate of 0.7 mL/min, and the organic acids were detected
by UV at 210 nm. Each organic acid was identified using an
authentic standard (Aladdin Reagent (Shanghai) Co., Ltd,
Shanghai, China) and quantified by the calibration curve of the
authentic compound relative to external standards. The HPLC
chromatogram of