Role of Calcium in the Reaction between Pyrroloquinoline Quinone and Pyridine Nucleotides Monomers and Dimers

Redox reactions were carried out in aerobiosis and anaerobiosis between NAD(P) dimers or NAD(P)H and pyrroloquinoline quinone (PQQ) in different buffers. The buffer system and pH significantly affected the oxidation rates of nucleotides and the ESR signal intensity of the PQQ• radical formed in anaerobiosis by comproportion between the quinone and quinol forms. The relative reactivity of the four nucleotides toward PQQ was affected by pH and buffer nature. PQQ, which behaves as an electron shuttle from nucleotides to oxygen, was first converted to PQQH2 and then rapidly reoxidized by oxygen, with formation of hydrogen peroxide. Both NAD(P) dimers and NAD(P)H consumed 1 mol of oxygen per mole of reacted molecule of pyridine nucleotide, yielding 1 or 2 mol of NAD(P)+ from NAD(P)H or from NAD(P) dimers, respectively. Chelating agents such as EDTA and phytate strongly decreased the reaction rate and the PQQ• radical signal intensity. Kinetics carried out in the presence of metal ions showed instead an increased reaction rate in the order Ca2+ ⪢ Mg2+ > Na+ ⪢ K+. Spectrofluorimetric measurements of PQQ with increasing concentrations of Ca2+ showed a fluorescence quenching and shift of the maximum emission toward lower wavelengths, while other metal ions showed minor effects, if any. Therefore, it is demonstrated that Ca2+ binds to PQQ, probably forming a complex which is more reactive with both one-electron (NAD(P) dimers) or two-electron donors (NAD(P)H) in nonenzymic reactions. It is important to recall that Ca2+ was already found to play active role in PQQ-containing enzymes.