Reassessment of the active site quino-cofactor proposed to occur in the Aspergillus niger amine oxidase AO-I from the properties of model compounds

Quino-cofactors have been found in a wide variety of prokaryotic and eukary otic organisms. Two variants have, thus far, been demonstrated to derive fr om tyrosine precursors: these are the 2,4,5-trihydroxyphenylalanine quinone (topa quinone or TPQ) [Janes, S. M., et al. (1990) Science 248, 98] and an o-quinone analogue containing the side chain of a lysine residue (lysyltyr osine quinone or LTQ) [Wang, S. Z., et al, (1996) Science 273, 1078]. Addit ionally, a third variant of the family of tyrosine-derived cofactors has be en reported to exist in an Aspergillus niger amine oxidase AO-I. This was d escribed as an o-quinone cross-linked to the side chain of a glutamate resi due [Frebort, I. (1996) Biochim. Biophys. Acta 1295, 59]. We have synthesiz ed model compounds related to the proposed structure. Characterization of t he redox properties for the model compound and spectral properties of its 4 -nitrophenylhydrazine derivative lead us to conclude that the cofactor in A . niger amine oxidase AO-I has been misidentified. A TPQ carboxylate ester is considered an unlikely candidate for a biologically functional quino-cof actor.