Measurements of the reaction of sodium salt of pyrroloquinoline quinone (PQQNa(2)) with vitamin C (Vit C) were performed in phosphate-buffered solution (pH 7.4) at 25 degrees C under nitrogen atmosphere, using UV-vis spectrophotometry. The absorption spectrum of PQQNa(2) decreased in intensity due to the reaction with Vit C and was changed to that of pyrroloquinoline quinol (PQQH(2), a reduced form of PQQ). One molecule of PQQ was reduced by two molecules of Vit C producing a molecule of PQQH(2) in the buffer solution. PQQH(2), thus produced, was recycled to PQQ due to air oxidation. PQQ and Vit C coexist in many biological systems, such as vegetables, fruits, as well as in human tissues. The results obtained suggest that PQQ is reduced by Vit C and functions as an antioxidant in biological systems, because it has been reported that PQQH(2) shows very high free-radical scavenging and singlet-oxygen quenching activities in buffer solutions.