The well known enzyme involved in oxidation of amines is monoamine oxidase (MAO, EC 1.1.3.4). The reason for this familiarity is the fact that this flavoprotein oxidizes amine hormones and neurotransmitters, compounds regulating the behaviour of mammalian organisms. So far, less well known mammalian amine oxidases (with respect to physiological role and mechanism of action) were: histaminase, diamine oxidase, semicarbazide-sensitive amine oxidase, etc., enzymes rubricated under EC 1.4.3.6. Since it was only recently established for some of the latter that they have topaquinone (TPQ, Fig. 1) as cofactor [1], it can be expected that knowledge on these enzymes will rapidly increase the coming years. A quinone like TPQ appears to be well suited as participant in the conversion of an amine by an oxidase since protein-lysine 6-oxidase (EC 1.4.3.13), oxidizing the lysyl residues in collagen so that cross-links can be formed, has the related LTQ (Fig. 1) as cofactor [2].