The interaction of aminogroups with pyrroloquinoline quinone as detected by the reduction of nitroblue tetrazolium

The interaction of pyrroloquinoline quinone (PQQ) with amino groups was followed by measuring the capacity of adducts to reduce nitroblue tetrazolium (NBT). Of the natural amino acids only glycine, ornithine, and lysine interacted strongly with PQQ. The reducing activity of other less reactive amino acids, but not of lysine, was increased by ammonia, primary or secondary amines. Divalent cations, in contrast inhibited development of NBT-reducing activity. PQQ also developed NBT-reactivity in the presence of serotonin and albumin. A reaction scheme is proposed which explains these findings. It is suggested that the NBT-reducing activity of plasma which is not caused by glycation of plasma proteins, arises from PQQ adducts inherent to plasma. This NBT-reducing activity corresponds to approximately 10 micrograms PQQ/ml plasma.