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 buffe rs. The buffer system and pH significantly affected the oxidation rates of nucleotides and the ESR signal intensity of the PQQ(.) radical formed in an aerobiosis by comproportion between the quinone and quinol forms. The relat ive reactivity of the four nucleotides toward PQQ was affected by pH and bu ffer nature. PQQ, which behaves as an electron shuttle from nucleotides to oxygen, was first converted to PQQH(2) and then rapidly reoxidized by oxyge n, with formation of hydrogen peroxide, Both NAD(P) dimers and NAD(P)H cons umed 1 mol of oxygen per mole of reacted molecule of pyridine nucleotide, y ielding 1 or 2 mol of NAD(P)(+) from NAD(P)H or from NAD(P) dimers, respect ively. Chelating agents such as EDTA and phytate strongly decreased the rea ction rate and the PQQ(.) radical signal intensity. Kinetics carried out in the presence of metal ions showed instead an increased reaction rate in th e order Ca2+ >> Mg2+ > Na+ >> K+. Spectrofluorimetric measurements of PQQ w ith increasing concentrations of Ca2+ showed a fluorescence quenching and s hift of the maximum emission toward lower wavelengths, while other metal io ns showed minor effects, if any, Therefore, it is demonstrated that Ca2+ bi nds 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 re actions. It is important to recall that Ca2+ was already found to play acti ve role in PQQ-containing enzymes. (C) 1999 Academic Press.