PqqC/D converts the biosynthetic intermediate purified from a pqqC mutant to pyrroloquinoline quinone (PQQ), and both NAD(P)H and cytosolic fraction, named as activating factor (ActF), are required to show its higher production. Dithiothreitol alone, as well as ActF plus NAD(P)H, enhanced the PQQ production by PqqC/D. Thioredoxin-thioredoxin reductase system with NADPH showed similar effect. PqqC/D made a tight complex with PQQ, however, in the presence of dithiothreitol, PQQ was dissociated from the protein. ActF showed NADPH oxidase activity which was enhanced by the addition of PQQ. These data suggest that PqqC/D produces the reduced PQQ from the intermediate in vivo, but in vitro, it is further oxidized by molecular oxygen and then the oxidized PQQ is trapped in PqqC/D to show product inhibition. (c) 2007 Elsevier Inc. All rights reserved.