Fig 1.
Structure of pyrroloquinoline quinone (PQQ).
Fig 2.
(a) The measurement mechanism of enzymatic activity. The GDH activity was determined by measuring the rate of discoloration of DCIP blue at 600 nm with Infinite 200 PRO, and ΔABS was calculated by (ABS 1 min, sample − ABS 21 min, sample) − (ABS 1 min, blank − ABS 21 min, blank). (b) Standard curve for PQQ determination by the enzymatic method. The reconstitution mixture contained 20 mM MOPS buffer, pH 7.0, 0.1% TritonX-100, 1 mM CaCl2, 0.1% bovine serum albumin, variable concentrations of PQQ, and 0.4 ng/mL GDH. The incubation period was 30 min. Detailed analytical procedures are described in Materials and methods section.
Fig 3.
(a) Product ion mass spectrum and ESI chromatograms of PQQ and 13C-PQQ. PQQ and 13C-PQQ standards (10 μM in 30% (v/v) acetonitrile/10 mM DBAA aqueous solution) were each infused directly into the MS/MS apparatus at a flow rate of 5 μL/min. A mixture of standards (2 pmol each) was analyzed by LC-MS/MS. (b) The proposed fragmentation pattern of PQQ and [U-13]C-PQQ. (c) Calibration curve of PQQ by LC-MS/MS. Different amounts of PQQ (15–650 ng) were analyzed in the ESI mode. Detailed analytical procedures are described in Materials and methods section.
Fig 4.
(a) The scheme of PQQ extraction. (b) Chromatogram peaks of PQQ and 13C-PQQ (internal standard) from medium extraction. Detailed analytical procedures are described in Materials and methods section.
Table 1.
Concentration of PQQ in food samples determined by the enzymatic method.
Fig 5.
Chromatogram peaks PQQ and 13C-PQQ (internal standard) from extraction of vinegar F (a) and H (b).
Detailed analytical procedures are described in Materials and methods section.