1,5-Diamino-2-pentyne is both a substrate and inactivator of plant copper amine oxidases

Mechanism-based inhibitors of cytokinin oxidase/dehydrogenase attack FAD cofactor

Cytokinin oxidases/dehydrogenases (CKOs) mediate catabolic regulation of cytokinin levels in plants. Several substrate analogs containing an unsaturated side chain were studied for their possible inhibitory effect on maize CKO (ZmCKO1) by use of various bioanalytical methods. Two allenic derivatives, N(6)-(buta-2,3-dienyl)adenine (HA-8) and N(6)-(penta-2,3-dienyl)adenine (HA-1), were identified as strong mechanism-based inhibitors of the enzyme. Despite exhaustive dialysis, the enzyme remained inhibited. Conversely, substrate analogs with a triple bond in the side chain were much weaker inactivators. The crystal structures of recombinant ZmCKO1 complexed with HA-1 or HA-8 were solved to 1.95 A resolution. Together with Raman spectra of the inactivated enzyme, it was revealed that reactive imine intermediates generated by oxidation of the allenic inhibitors covalently bind to the flavin adenine dinucleotide (FAD) cofactor. The binding occurs at the C4a atom of the isoalloxazine ring of FAD, the planarity of which is consequently disrupted. All the compounds under study were also analyzed for binding to the Arabidopsis cytokinin receptors AHK3 and AHK4 in a bacterial receptor assay and for cytokinin activity in the Amaranthus bioassay. HA-1 and HA-8 were found to be good receptor ligands with a significant cytokinin activity. Nevertheless, due to their ability to inactivate CKO in the desired time intervals or developmental stages, they both represent attractive compounds for physiological studies, as the inhibition mechanism of HA-1 and HA-8 is mainly FAD dependent.

An important lysine residue in copper/quinone-containing amine oxidases

The interaction of xenon with copper/6-hydroxydopa (2,4,5-trihydroxyphenethylamine) quinone (TPQ) amine oxidases from the plant pulses lentil (Lens esculenta) and pea (Pisum sativum) (seedlings), the perennial Mediterranean shrub Euphorbia characias (latex), and the mammals cattle (serum) and pigs (kidney), were investigated by NMR and optical spectroscopy of the aqueous solutions of the enzymes. (129)Xe chemical shift provided evidence of xenon binding to one or more cavities of all these enzymes, and optical spectroscopy showed that under 10 atm of xenon gas, and in the absence of a substrate, the plant enzyme cofactor (TPQ), is converted into its reduced semiquinolamine radical. The kinetic parameters of the analyzed plant amine oxidases showed that the k(c) value of the xenon-treated enzymes was reduced by 40%. Moreover, whereas the measured K(m) value for oxygen and for the aromatic monoamine benzylamine was shown to be unchanged, the K(m) value for the diamine putrescine increased remarkably after the addition of xenon. Under the same experimental conditions, the TPQ of bovine serum amine oxidase maintained its oxidized form, whereas in pig kidney, the reduced aminoquinol species was formed without the radical species. Moreover the k(c) value of the xenon-treated pig enzyme in the presence of both benzylamine and cadaverine was shown to be dramatically reduced. It is proposed that the lysine residue at the active site of amine oxidase could be involved both in the formation of the reduced TPQ and in controlling catalytic activity.

Inhibition of diamine oxidases and polyamine oxidases by diamine-based compounds

This review reports on inhibitors of copper-containing amine oxidases and flavoprotein polyamine oxidases, which are structurally based on diamines. In the introduction, basic characteristics and classification of amine oxidases are described together with the significance of their synthetic inhibitors. The following text is divided into several chapters, which deal with diaminoketones, aza-diamines, unsaturated diamine analogs and diamines with heterocyclic substituents. Then it continues with diamine- and agmatine-based inhibitors of polyamine oxidases. Each chapter gives detailed information on the inhibition mode, potency and structural relationships. The conclusion points out possible roles of mechanism-based inhibitors of amine oxidases in physiological and medicinal research.

Mechanism-based inactivators of plant copper/quinone containing amine oxidases

Copper/quinone amine oxidases contain Cu(II) and the quinone of 2,4,5-trihydroxyphenylalanine (topaquinone; TPQ) as cofactors. TPQ is derived by post-translational modification of a conserved tyrosine residue in the protein chain. Major advances have been made during the last decade toward understanding the structure/function relationships of the active site in Cu/TPQ amine oxidases using specific inhibitors. Mechanism-based inactivators are substrate analogues that bind to the active site of an enzyme being accepted and processed by the normal catalytic mechanism of the enzyme. During the reaction a covalent modification of the enzyme occurs leading to irreversible inactivation. In this review mechanism-based inactivators of plant Cu/TPQ amine oxidases from the pulses lentil (Lens esculenta), pea (Pisum sativum), grass pea (Lathyrus sativus) and sainfoin (Onobrychis viciifolia,) are described. Substrates forming, in aerobiotic and in anaerobiotic conditions, killer products that covalently bound to the quinone cofactor or to a specific amino acid residue of the target enzyme are all reviewed.