Stereochemical probes of bovine plasma amine oxidase: evidence for mirror image processing and a syn abstraction of hydrogens from C-1 and C-2 of dopamine

The chemo- enzymatic synthesis of labeled l-amino acids and some of their derivatives

This review compiles the combined chemical and enzymatic synthesis of aromatic l-amino acids (l-phenylalanine, l-tyrosine, l-DOPA, l-tryptophan, and their derivatives and precursors) specifically labeled with carbon and hydrogen isotopes, which were elaborated in our research group by the past 20 years. These compounds could be then employed to characterize the mechanisms of enzymatic reactions via kinetic and solvent isotope effects methods.

Molecular and mechanistic properties of the membrane-bound mitochondrial monoamine oxidases

The past decade has brought major advances in our knowledge of the structures and mechanisms of MAO A and MAO B, which are pharmacological targets for specific inhibitors. In both enzymes, crystallographic and biochemical data show their respective C-terminal transmembrane helices anchor the enzymes to the outer mitochondrial membrane. Pulsed EPR data show both enzymes are dimeric in their membrane-bound forms with agreement between distances measured in their crystalline forms. Distances measured between active site-directed spin-labels in membrane preparations show excellent agreement with those estimated from crystallographic data. Our knowledge of requirements for development of specific reversible MAO B inhibitors is in a fairly mature status. Less is known regarding the structural requirements for highly specific reversible MAO A inhibitors. In spite of their 70% level of sequence identity and similarities of C(alpha) folds, the two enzymes exhibit significant functional and structural differences that can be exploited in the ultimate goal of the development of highly specific inhibitors. This review summarizes the current structural and mechanistic information available that can be utilized in the development of future highly specific neuroprotectants and cardioprotectants.

Crystal Structure of Amine Oxidase from Bovine Serum

Copper-containing amine oxidase extracted from bovine serum (BSAO) was crystallized and its three-dimensional structure at 2.37A resolution is described. The biological unit of BSAO is a homodimer, formed by two monomers related to each other by a non-crystallographic 2-fold axis. Each monomer is composed of three domains, similar to those of other amine oxidases from lower species. The two monomers are structurally equivalent, despite some minor differences at the two active sites. A large funnel allows access of substrates to the active-site; another cavity, accessible to the solvent, is also present between the two monomers; this second cavity could allow the entrance of molecular oxygen necessary for the oxidative reaction. Some sugar residues, bound to Asn, were still present and visible in the electron density map, in spite of the exhaustive deglycosylation necessary to grow the crystals. The comparison of the BSAO structure with those of other resolved AO structures shows strong dissimilarities in the architecture and charge distribution of the cavities leading to the active-site, possibly explaining the differences in substrate specificity.

Mechanism of post-translational quinone formation in copper amine oxidases and its relationship to the catalytic turnover

Copper amine oxidases (CAOs) post-translationally construct a redox-active quinone from an amino acid side chain in their polypeptide chain. As such, these enzymes illustrate how nature is able to expand upon naturally-occurring side chains to create new, catalytically powerful functionalities. The active sites of the CAOs are highly unusual in their ability to catalyze two very different reactions: single-turnover, oxygen-dependent quinone formation, followed by catalytic oxidation (formally dehydrogenation) of amines. This review summarizes our current understanding of the pathway whereby the 2,4,5-trihydroxyphenylalanyl quinone (TPQ) cofactor is generated from the phenolic side chain of tyrosine. This reaction occurs spontaneously intermediates in the presence of O(2) and active site bound Cu(II), without the assistance of other proteins or cofactors. Ongoing work has focused on uncovering the details of the TPQ formation mechanism. A larger goal is to understand how a single active site is capable of supporting both quinone formation and subsequent catalytic turnover.

Evidence for alternative binding modes in the interaction of benzylamine analogues with bovine liver monoamine oxidase B

The interaction of purified bovine liver MAO B with the benzylamine analogues N,N-dimethylbenzylamine and alpha-methylbenzylamine has been investigated. Both classes of analogues are competitive inhibitors of benzylamine oxidase activity. The K(i) values were determined for nine different para-substituted N, N-dimethylbenzylamine analogues. Analysis of the binding affinities demonstrate the deprotonated forms of the tertiary amines are preferentially bound to MAO B and the affinity decreases with increasing van der Waals volume of the para-substituent. The correlation for this relation is:Log K(i)=-0.97+/-(0.28)sigma+(0. 75+/-0.11)(0.1xV(w))-4.24+/-(0.16)alpha-Methyl benzylamine analogues are also found to be competitive inhibitors of MAO B-catalyzed benzylamine oxidation. Similar K(i) values were determined using either the S or R stereoisomers. Analysis of the binding affinities of five para-substituted alpha-methylbenzylamine analogues to MAO B shows the deprotonated form also to be preferentially bound and the affinity is marginally increased with increasing van der Waals volume of the para-substituent:Log K(i)=-0.71sigma-(0.32)(0. 1xV(w))-3.50Comparison of these data with that previously published for para-substituted benzylamine binding to MAO B (Walker and Edmondson, Biochemistry 33 (1994) 7088-7098) demonstrates that these benzylamine analogues exhibit differing modes of binding to the active site of MAO B. The presence of an electronic substituent effect in the binding of these two classes of analogues compared with the lack of an observable electronic effect in the binding of benzylamine to MAO B is consistent with the proposal that orientation of the benzyl ring of the bound substrate is responsible for the absence of an electronic substituent effect on the rate of the reductive half reaction (Miller and Edmondson, Biochemistry 38 (1999) 13670-13683).

Stereochemistry and cofactor identity status of semicarbazide-sensitive amine oxidases

The relationship between the soluble copper topaquinone amine oxidases, the membrane bound semicarbazide-sensitive amine oxidases and lysyl oxidase remains unclear. The stereochemical course of substrate oxidation has been determined for each enzyme type and these studies suggest that SSAO and lysyl oxidase are closely related mechanistically, and that they are distinct from the copper amine oxidases. Both lysyl oxidase and SSAO catalyze the oxidation of tyramine with removal of the pro-S hydrogen from C-1 of this substrate. The copper amine oxidase enzymes that react with abstraction of the pro-S hydrogen from C-1 of substrates do not exhibit a solvent exchange pathway. In contrast, this exchange occurs in lysyl oxidase and SSAO reactions. The organic cofactor in all three enzyme types is a quinone; however, the spectral features of phenylhydrazine and p-nitrophenylhydrazine-derivatized SSAO differ from those reported for all known topaquinone-containing enzymes. Cofactor identification is further complicated by the lack of the characteristic topa motif, Asn-Tyr-Asp/Glu, in lysyl oxidase and the absence of any sequence information for SSAO.

Stereochemical course of tyramine oxidation by semicarbazide-sensitive amine oxidase

Two semicarbazide-sensitive amine oxidases (SSAO's) from bovine and porcine aortic tissue were partially purified and characterized, and the stereochemical course of amine oxidation was evaluated. The porcine and bovine SSAO's were membrane bound glycoproteins, with Km values for benzylamine of 8 and 16 microM, respectively. The reactivity of SSAO with semicarbazide and phenylhydrazine suggests that the cofactor is a carbonyl type molecule. The stereochemical course of the bovine and porcine aortic semicarbazide-sensitive amine oxidase reaction was investigated using chiral tyramines, deuterated at C-1 and C-2, and 1H-NMR spectroscopy to establish the loss or retention of deuterium in product p-hydroxyphenethyl alcohols. The preferred mode of tyramine oxidation was found to occur with the loss of pro-S proton at C-1, coupled with solvent exchange into C-2, a pattern which has not been observed for any copper amine oxidase examined to date. The solvent exchange reaction also occurred stereospecifically, with loss from and reprotonation to the pro-R position, suggesting that these two processes occur from the same face of the enamine double bond.

Activation of bovine plasma benzylamine oxidase (BzAO) by 1-methyl-4-(2-methylphenyl)-1,2,3,6-tetrahydropyridine

We have shown previously that certain analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are potent inhibitors of human and bovine plasma benzylamine oxidase (BzAO: EC 1.4.3.6). Inhibition was competitive, reversible and allosteric. Under certain conditions competitive inhibitors of allosteric enzymes can act as allosteric activators. In the present work, 1-methyl-4-(2-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH3MPTP) was found to activate bovine plasma BzAO at low substrate and 2'-CH3MPTP concentrations. At higher 2'-CH3MPTP concentrations, the activation was negated.

Reductive trapping of substrate to methylamine oxidase from Arthrobacter P1

Methylamine oxidase (EC 1.4.3.6) from Arthrobacter P1 was inactivated by NaCNBH3 in the presence of [14C]benzylamine, leading to the incorporation of 1 mol of radiolabeled substrate/mol of enzyme subunit at complete inactivation. By contrast, no labeling of enzyme was observed using [3H]NaCNBH3 as reductant. These results are analogous to those previously reported for the eukaryotic enzyme, bovine serum plasma amine oxidase [(1987) J. Biol. Chem. 262, 962-965]. The observed pattern of labeling is consistent with the presence of dicarbonyl cofactor at the active site of methylamine oxidase. Further, these studies suggest that our reductive trapping technique, in which the pattern of radiolabeling of an enzyme is compared using C-14 substrate vs tritiated reductant, may serve as a general assay for covalently bound dicarbonyl structures.

Spectroscopic studies of the reaction between bovine serum amine oxidase (copper-containing) and some hydrazides and hydrazines

The carbonyl cofactor of bovine serum amine oxidase, recently identified as pyrroloquinoline quinone [Ameyama, Hayashi, Matsushita, Shinagawa & Adachi (1984) Agric. Biol. Chem. 48, 561-565; Lobenstein-Verbeek, Jongejan, Frank & Duine (1984) FEBS Lett. 170, 305-309], reacts stoichiometrically and irreversibly with hydrazides of phenylacetic acid and of benzoic acid. With the phenylacetic hydrazides a reversible intermediate step was detected by competition with substrate, carbonylic reagents or phenylhydrazine, a typical inhibitor of the enzyme. All hydrazides form an intense broad band with maximum absorbance in a narrow wavelength range (350-360 nm), irrespective of the acyl group, suggesting that the transition is located on the organic cofactor. A different situation is found with some phenylhydrazines, where extended conjugation can occur between the cofactor and the phenyl pi-electron system via the azo group, as shown by the lower energy and higher intensity of the transition. In this case the transition is sensitive to substituents in the phenyl ring. The c.d. spectrum of the adducts is influenced by the type of hydrazide (derived from phenylacetic acid or benzoic acid), by pH and by NN-diethyldithiocarbamate binding to copper, probably as a result of shifts of equilibria between hydrazone-azo tautomers.