The effect of pH on the kinetic parameters and mechanism of beef liver monoamine oxidase

90 years of monoamine oxidase: some progress and some confusion

It would not be practical to attempt to deal with all the advances that have informed our understanding of the behavior and functions of this enzyme over the past 90 years. This account concentrates key advances that explain why the monoamine oxidases remain of pharmacological and biochemical interest and on some areas of continuing uncertainty. Some issues that remain to be understood or are in need of further clarification are highlighted.

Kinetic behavior and reversible inhibition of monoamine oxidases--enzymes that many want dead

Monoamine oxidase (MAO) inhibitors have proven to be valuable tools in pharmacology and therapeutics. This account concerns the behavior of the different types of reversible inhibitor and how an understanding of the kinetic mechanisms of MAO may help in their design.

Variations in activity and inhibition with pH: the protonated amine is the substrate for monoamine oxidase, but uncharged inhibitors bind better

It has been accepted that, as required mechanistically, the neutral form of the amine is the substrate for monoamine oxidase, despite the amine pK (a) of above 9.5. The pH dependence of the kinetic parameters for kynuramine oxidation by purified human MAO-A and for phenylethylamine oxidation by MAO-B in granulocytes at pH values from 5 to 10 was consistent with the protonated amine being used. Deprotonation of a group of pK (a) = 7.1 in MAO-B and pK (a) = 7.5 +/- 0.1 (n = 4) in MAO-A was important for efficient catalysis. The K(i) values for two oxazolidinone inhibitors of MAO-A gave opposite pH-dependence indicating that the uncharged form of each inhibitor bound better than the charged form. Decreased pH induced a blue shift in the spectral maximum of MAO-A indicative of a more hydrophobic environment around the flavin, and also influenced the redox properties of the flavin.

Stereospecificity and kinetic mechanism of human prenylcysteine lyase, an unusual thioether oxidase

Prenylated proteins contain either a 15-carbon farnesyl or a 20-carbon geranylgeranyl isoprenoid covalently attached to cysteine residues at or near their C terminus. The cellular abundance of prenylated proteins, as well as the stability of the thioether bond, poses a metabolic challenge to cells. A lysosomal enzyme termed prenylcysteine lyase has been identified that degrades a variety of prenylcysteines. Prenylcysteine lyase is a FAD-dependent thioether oxidase that produces free cysteine, an isoprenoid aldehyde, and hydrogen peroxide as products of the reaction. Here we report initial studies of the kinetic mechanism and stereospecificity of this unusual enzyme. We utilized product and dead end inhibitors of prenylcysteine lyase to probe the kinetic mechanism of the multistep reaction. The results with these inhibitors, together with those of other experiments, suggest that the reaction catalyzed by prenylcysteine lyase proceeds through a sequential mechanism. The reaction catalyzed by the enzyme is stereospecific, in that the pro-S hydride of the farnesylcysteine is transferred to FAD to initiate the reaction. With (2R,1'S)-[1'-(2)H(1)]farnesylcysteine as a substrate, a primary deuterium isotope effect of 2 was observed on the steady state rate. However, the absence of an isotope effect on an observed pre-steady-state burst of hydrogen peroxide formation implicates a partially rate-determining proton transfer after a relatively fast C-H (C-D) bond cleavage step. Furthermore, no pre-steady-state burst of cysteine was observed. The finding that the rate of cysteine formation was within 2-fold of the steady-state k(cat) value indicates that cysteine production is one of the primary rate-limiting steps in the reaction. These results provide substantial new information on the catalytic mechanism of prenylcysteine lyase.

Interaction of spin-labeled tryptamine with monoamine oxidase: probing the microenvironment of the active site by spin probe-spin label techniques

The spin-labeled substrate, tryptamine, was used as a structural probe of the active site of bovine liver monoamine oxidase B (amine:oxygen oxidoreductase (deaminating) (flavin-containing), EC 1.4.3.4). When the reaction was monitored by electron spin resonance (ESR), line broadening effects indicative of binding with an apparent relation to substrate specificity of the highly purified enzyme were observed. The spectrum indicated that the bound tryptamine was 'partially immobilized' with a dissociation constant of 39 microM and 2.2 mol bound per enzyme dimer. The correlation time, reflecting the environment of the tryptamine binding site, was determined to be 6.2 ns. The topology of the active site was investigated by using dual spin-label methodology in which the spin-labeled substrate, tryptamine, and the 15N-substituted and deuterated maleimide spin label covalently bound to the essential sulfhydryl groups were used. The ESR spectral data suggested that the essential sulfhydryl groups are at least 14 A away from the tryptamine-binding site. The environment surrounding both spin-labeled substrates, tryptamine and [2H,15N]MSL, and the motional properties of the enzyme are discussed.

The catalytic behaviour of monoamine oxidase

Evidence concerning the kinetic mechanism of the reaction catalyzed by monoamine oxidase is reviewed with particular reference to the possibility that the double-displacement mechanism followed by other substrates is not operative with benzylamine. The requirement for only one of the two products of the first half-reaction to be released in a double-displacement mechanism indicates that the available evidence does not exclude such a mechanism with benzylamine as the substrate. Cases in which substrates also act as time-dependent inhibitors are considered. The mechanism that can describe the inhibition and product formation is similar for the compounds MD 780236 and MPTP whereas that describing the effects of high concentrations of 2-phenethylamine is best described by a scheme involving inhibition occurring via an abortive complex.

Human brain monoamine oxidase type B: mechanism of deamination as probed by steady-state methods

Recently, evidence has been published which suggests that [Husain, M., Edmondson, D. E., & Singer, T.P. (1982) Biochemistry 21, 595-600] monoamine oxidase [amine:oxygen oxidoreductase (MAO), EC 1.4.3.4] deaminates phenylethylamine and benzylamine via two distinct kinetic pathways which involve either binary or ternary complex formation, respectively. These conclusions were drawn largely from stopped-flow kinetic analysis performed on purified enzyme removed from its native membrane and in the presence of the inhibitory detergent Triton X-100. In this study, d-amphetamine and alternative substrates were used as steady-state probes of the kinetics of deamination by the B form of human brain MAO using native membrane-bound enzyme. Initial velocity studies showed mixed-type patterns for amphetamine inhibition of phenylethylamine, tryptamine, and tyramine when either amine or oxygen was the varied substrate. Slope and intercept vs. amphetamine concentration replots were linear in all cases except for phenylethylamine (hyperbolic); Ki values obtained from linear replots of slope or intercept values were comparable. In contrast, amphetamine was a competitive inhibitor of benzylamine deamination when amine concentration was varied and uncompetitive when oxygen concentration was varied; slope and intercept replots were linear for both. When benzylamine was the alternative substrate inhibitor and tyramine and tryptamine deamination was measured, mixed-type inhibition patterns were obtained when either amine or oxygen concentration was varied; replots of slope and intercept were linear in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)

The reaction of sulfhydryl reagents with bovine hepatic monoamine oxidase. Evidence for the presence of two cysteine residues essential for activity

The bovine liver monoamine oxidase (EC 1.4.3.4) was found to be inactivated by various well-known sulfhydryl reagents like p-mercuribenzoate, methylmercuric iodide and 5,5'-dithiobis-(2-nitro benzoic acid). The present investigation shows that the inactivation of the enzyme results from reactions of these reagents with 2 out of 8 titratable sulfhydryl groups per 10(5) g of the enzyme. The substrate, benzylamine, and competitive inhibitors like benzaldehyde, p-nitrobenzaldehyde, benzyl alcohol protected the enzyme from inactivation by the mercurials or the Ellman reagent. The inactivation experiments with these sulfhydryl reagents, the protection experiments, and the kinetics as well as physicochemical observations suggest that there are only two cysteine residues that are required for activity of the enzyme. It is possible that these two residues may be active-center residues.

Evidence for essential histidine residues in bovine-liver mitochondrial monoamine oxidase

Ethoxyformic acid anhydride, amino-1H-tetrazole, and photooxidation in the presence of rose bengal, which are reagents known to react with histidine residues of proteins, were shown to inactivate monoamine oxidase. Ethoxyformic acid anhydride reacted with about 6 histidine residues per 100 000 g of protein under the experimental conditions adopted and completely inactivated the enzyme. However, NH2OH reactivated the ethaxyformic acid derivative t only. Since NH2OH specifically deacylates N-ethoxyformylimidazole, it was shown that at least some of the histidine residues are essential for activity. In addition, photooxidation experiments in the presence of 0.01% rose bengal confirmed that only histidine residues of bovine hepatic monoamine oxidase are destroyed under the designated experimental conditions. About 9 histidine residues per 100 000 g of protein were destroyed during the photooxidation experiments. In the presence of substrate, kynuramine or benzylamine, only 7 histidine residues were destroyed, which indicates that 2 histidine residues per 100 000 g of protein are essential for activity.

The reaction pathway of membrane-bound rat liver mitochondrial monoamine oxidase

1. A preparation of a partly purified mitochondrial outer-membrane fraction suitable for kinetic investigations of monoamine oxidase is described. 2. An apparatus suitable for varying the O(2) concentration in a spectrophotometer cuvette is described. 3. The reaction catalysed by the membrane-bound enzyme is shown to proceed by a double-displacement (Ping Pong) mechanism, and a formal mechanism is proposed. 4. KCN, NaN(3), benzyl cyanide and 4-cyanophenol are shown to be reversible inhibitors of the enzyme. 5. The non-linear reciprocal plot obtained with impure preparations of benzylamine, which is typical of high substrate inhibition, is shown to be due to aldehyde contamination of the substrate.

Kinetics of the diamine oxidase reaction

1. The oxidation of p-dimethylaminomethylbenzylamine was followed spectrophotometrically by measuring the change in E(250) caused by the p-dimethylaminomethylbenzaldehyde produced under a wide variety of experimental conditions. 2. The effect of variations in concentrations of both substrates (amine and oxygen) and all products (aminoaldehyde, hydrogen peroxide and ammonia) on this reaction was studied and the results used to develop a formal mechanism. 3. The nature of the rate-limiting step was elucidated by studying the effects of alterations in ionic strength, dielectric constant and deuterium substitution on the velocity of the forward reaction. 4. Thermodynamic activation energy parameters were obtained at several pH values from the effects of temperature on the reaction.

Inhibition of pig kidney diamine oxidase by substrate analogues

1. The oxidation of p-dimethylaminomethylbenzylamine was followed spectrophotometrically by measuring the change in E(250) caused by the p-dimethylaminomethylbenzaldehyde produced. 2. This reaction was inhibited by substrate analogues such as isothiouronium, guanidino, dimethylsulphonium and trimethylammonium compounds. 3. The inhibition by both mono- and bis-onium compounds has been studied and a comprehensive theory is developed to explain both the type and degree of inhibition produced.