Treatment of purified human placental lysyl oxidase with 2,4-dinitrophenylhydrazine (DNPH) resulted in a large spectral change and inhibition of enzyme activity. Proteolytic degradation of the derivatized enzyme yielded only one single coloured product, which was spectrally and chromatographically identical with the C-5 hydrazone of PQQ (pyrroloquinoline quinone) and DNPH. Since this represents the first example of a PQQ-containing enzyme in man, possible implications of the finding are discussed.
