Pyrroloquinoline quinone (PQQ) is reduced to pyrroloquinoline quinol (PQQH₂) by vitamin C, and PQQH₂ produced is recycled to PQQ by air oxidation in buffer solution at pH 7.4

Abstract

Measurements of the reaction of sodium salt of pyrroloquinoline quinone (PQQNa₂) with vitamin C (Vit C) were performed in phosphate-buffered solution (pH 7.4) at 25 °C under nitrogen atmosphere, using UV–vis spectrophotometry. The absorption spectrum of PQQNa₂ decreased in intensity due to the reaction with Vit C and was changed to that of pyrroloquinoline quinol (PQQH₂, a reduced form of PQQ). One molecule of PQQ was reduced by two molecules of Vit C producing a molecule of PQQH₂ in the buffer solution. PQQH₂, 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 PQQH₂ shows very high free-radical scavenging and singlet-oxygen quenching activities in buffer solutions.

Graphical abstract

Recycling of PQQH₂ and PQQ under the coexistence of Vit C.

Key words:

• pyrroloquinoline quinone
• pyrroloquinoline quinol
• reaction rate
• UV–vis absorption spectrum
• vitamin C