Inactivation of monoamine oxidase B by 1-phenylcyclopropylamine: mass spectral evidence for the flavin adduct

Put a ring on it: application of small aliphatic rings in medicinal chemistry

Aliphatic three- and four-membered rings including cyclopropanes, cyclobutanes, oxetanes, azetidines and bicyclo[1.1.1]pentanes have been increasingly exploited in medicinal chemistry for their beneficial physicochemical properties and applications as functional group bioisosteres. This review provides a historical perspective and comparative up to date overview of commonly applied small rings, exemplifying key principles with recent literature examples. In addition to describing the merits and advantages of each ring system, potential hazards and liabilities are also illustrated and explained, including any significant chemical or metabolic stability and toxicity risks.

Molecular aspects of monoamine oxidase B

Monoamine oxidases (MAO) influence the monoamine levels in brain by virtue of their role in neurotransmitter breakdown. MAO B is the predominant form in glial cells and in platelets. MAO B structure, function and kinetics are described as a background for the effect of alterations in its activity on behavior. The need to inhibit MAO B to combat decreased brain amines continues to drive the search for new drugs. Reversible and irreversible inhibitors are now designed using data-mining, computational screening, docking and molecular dynamics. Multi-target ligands designed to combat the elevated activity of MAO B in Alzheimer's and Parkinson's Diseases incorporate MAO inhibition (usually irreversible) as well as iron chelation, antioxidant or neuroprotective properties. The main focus of drug design is the catalytic activity of MAO, but the imidazoline I2 site in the entrance cavity of MAO B is also a pharmacological target. Endogenous regulation of MAO B expression is discussed briefly in light of new studies measuring mRNA, protein, or activity in healthy and degenerative samples, including the effect of DNA methylation on the expression. Overall, this review focuses on examples of recent research on the molecular aspects of the expression, activity, and inhibition of MAO B.

Toward an "omic" physiopathology of reactive chemicals: thirty years of mass spectrometric study of the protein adducts with endogenous and xenobiotic compounds

Cancer and degenerative diseases are major causes of morbidity and death, derived from the permanent modification of key biopolymers such as DNA and regulatory proteins by usually smaller, reactive molecules, present in the environment or generated from endogenous and xenobiotic components by the body's own biochemical mechanisms (molecular adducts). In particular, protein adducts with organic electrophiles have been studied for more than 30 [see, e.g., Calleman et al., 1978] years essentially for three purposes: (a) as passive monitors of the mean level of individual exposure to specific chemicals, either endogenously present in the human body or to which the subject is exposed through food or environmental contamination; (b) as quantitative indicators of the mean extent of the individual metabolic processing which converts a non-reactive chemical substance into its toxic products able to damage DNA (en route to cancer induction through genotoxic mechanisms) or key proteins (as in the case of several drugs, pesticides or otherwise biologically active substances); (c) to relate the extent of protein modification to that of biological function impairment (such as enzyme inhibition) finally causing the specific health damage. This review describes the role that contemporary mass spectrometry-based approaches employed in the qualitative and quantitative study of protein-electrophile adducts play in the discovery of the (bio)chemical mechanisms of toxic substances and highlights the future directions of research in this field. A particular emphasis is given to the measurement of often high levels of the protein adducts of several industrial and environmental pollutants in unexposed human populations, a phenomenon which highlights the possibility that a number of small organic molecules are generated in the human organism through minor metabolic processes, the imbalance of which may be the cause of "spontaneous" cases of cancer and of other degenerative diseases of still uncharacterized etiology. With all this in mind, it is foreseen that a holistic description of cellular functions will take advantage of new analytical methods based on time-integrated metabolomic measurements of a new biological compartment, the "adductome," aimed at better understanding integrated organism response to environmental and endogenous stressors.

Type II isopentenyl diphosphate isomerase: irreversible inactivation by covalent modification of flavin

Isopentenyl diphosphate isomerase (IDI) catalyzes the interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the basic building blocks of isoprenoid molecules. Two structurally unrelated classes of IDI are known. Type I IPP isomerase (IDI-1) utilizes a divalent metal in a protonation-deprotonation reaction; whereas, the type II enzyme (IDI-2) requires reduced flavin. Epoxy, diene, and fluorinated substrate analogues, irreversible inhibitors of IDI-1, were analyzed as mechanistic probes for IDI-2. 3,4-Oxido-3-methyl-1-butyl diphosphate (eIPP), 3-methylene-4-penten-1-yl diphosphate (vIPP), and 3-(fluoromethyl)-3-buten-1-yl diphosphate (fmIPP) inactivate IDI-2 through formation of covalent adducts with the reduced flavin. UV-visible spectra of the inactivated complexes are consistent with modification of the isoalloxazine ring at position N5. vIPP and fmIPP are also alternate substrates with isomerization competing with alkylation of the flavin cofactor. (Z)-3-(Fluoromethyl)-2-buten-1-yl diphosphate ((Z)-fmDMAPP) and (Z)-3-(difluoromethyl)-2-buten-1-yl diphosphate ((Z)-dfmDMAPP) are alternate substrates, which are isomerized to the corresponding IPP derivatives. The rates of isomerization of fmIPP and (Z)-fmDMAPP are approximately 50-fold less than IPP and DMAPP, respectively. dfmIPP is not an irreversible inhibitor. These studies indicate that the irreversible inhibitors inactivate the reduced flavin required for catalysis by electrophilic alkylation and are consistent with a protonation-deprotonation mechanism for the isomerization catalyzed by IDI-2.

A computational study on the amine-oxidation mechanism of monoamine oxidase: Insight into the polar nucleophilic mechanism

The proposed polar nucleophilic mechanism of MAO was investigated using quantum chemical calculations employing the semi-empirical PM3 method. In order to mimic the reaction at the enzyme's active site, the reactions between the flavin and the p-substituted benzylamine substrate analogs were modeled. Activation energies and rate constants of all the reactions were calculated and compared with the published experimental data. The results showed that electron-withdrawing groups at the para position of benzylamine increase the reaction rate. A good correlation between the log of the calculated rate constants and the electronic parameter (sigma) of the substituent was obtained. These results agree with the previous kinetic experiments on the effect of p-substituents on the reduction of MAO-A by benzylamine analogs. In addition, the calculated rate constants showed a correlation with the rate of reduction of the flavin in MAO-A. In order to verify the results obtained from the PM3 method single-point B3LYP/6-31G*//PM3 calculations were performed. These results demonstrated a strong reduction in the activation energy for the reaction of benzylamine derivatives having electron-withdrawing substituents, which is in agreement with the PM3 calculations and the previous experimental QSAR study. PM3 and B3LYP/6-31G* energy surfaces were obtained for the overall reaction of benzylamine with flavin. Results suggest that PM3 is a reasonable method for studying this kind of reaction. These theoretical findings support the proposed polar nucleophilic mechanism for MAO-A.

Fluorinated phenylcyclopropylamines. Part 4: effects of aryl substituents and stereochemistry on the inhibition of monoamine oxidases by 1-aryl-2-fluoro-cyclopropylamines

A series of para-ring-substituted (E)- and (Z)-1-aryl-2-fluorocyclopropylamines were examined as inhibitors of recombinant human liver monoamine oxidase A (MAO A) and B (MAO B). Unlike the parent 1-phenylcyclopropylamine, which is a selective inhibitor of MAO B, both (E)- and (Z)-diastereomers of derivatives having fluorine at the 2-position of the cyclopropane ring were potent and selective irreversible inhibitors of MAO A. Both electron releasing groups (Me, OMe) and electron attracting groups (Cl, F) substituted in the para-position caused a modest increase in activity. Geminal difluoro-substitution caused a loss of potency of 100-fold compared to either (E)- or (Z)-monofluorinated analogue. Surprisingly, (1S,2R)-2-fluoro-1-phenylcyclopropylamine and the (1R,2S)-enantiomer were essential equally potent as inhibitors of MAO A and MAO B. None of the tested 1-aryl-2-fluorocyclopropylamines exhibited significant inhibition of tyramine oxidase.

Minimising the potential for metabolic activation in drug discovery

Investigations into the role of bioactivation in the pathogenesis of xenobiotic-induced toxicity have been a major area of research since the link between reactive metabolites and carcinogenesis was first reported in the 1930s. Circumstantial evidence suggests that bioactivation of relatively inert functional groups to reactive metabolites may contribute towards certain drug-induced adverse reactions. Reactive metabolites, if not detoxified, can covalently modify essential cellular targets. The identity of the susceptible biomacromolecule(s), and the physiological consequence of its covalent modification, will dictate the resulting toxicological response (e.g., covalent modification of DNA by reactive intermediates derived from procarcinogens that potentially leads to carcinogenesis). The formation of drug-protein adducts often carries a potential risk of clinical toxicities that may not be predicted from preclinical safety studies. Animal models used to reliably predict idiosyncratic drug toxicity are unavailable at present. Furthermore, considering that the frequency of occurrence of idiosyncratic adverse drug reactions (IADRs) is fairly rare (1 in 1000 to 1 in 10,000), it is impossible to detect such phenomena in early clinical trials. Thus, the occurrence of IADRs during late clinical trials or after a drug has been released can lead to an unanticipated restriction in its use and even in its withdrawal. Major themes explored in this review include a comprehensive cataloguing of bioactivation pathways of functional groups commonly utilised in drug design efforts with appropriate strategies towards detection of corresponding reactive intermediates. Several instances wherein replacement of putative structural alerts in drugs associated with IADRs with a latent functionality eliminates the underlying liability are also presented. Examples of where bioactivation phenomenon in drug candidates can be successfully abrogated via iterative chemical interventions are also discussed. Finally, appropriate strategies that aid in potentially mitigating the risk of IADRs are explored, especially in circumstances in which the structural alert is also responsible for the primary pharmacology of the drug candidate and cannot be replaced.

Ring scission of diastereomeric 4-butylspiropentylcarbinyl radicals as a chemical model for identifying enzyme-catalyzed FAD adducts resulting from spiropentylacetyl-CoA

Both diastereomeric 4-butylspiropentylcarbinyl bromides (14a and 14b) were synthesized in seven steps starting from 1-heptyne, and the stereochemical assignments based upon NOE experiments were confirmed by converting their immediate alcohol precursors (13a and 13b) to 1,4-dibutylspiropentanes (17a and 17b) with C(1) and C(2) symmetry. Each bromide was used to generate its corresponding spiropentylcarbinyl radical (18a and 18b) via its AIBN-initiated tri-n-butyltin hydride reduction. The radical-trapped products are identified, the preferred ring scission mode is identified (C1[bond] C2 bond cleavage), and the estimated rates for the ring opening of 4-butylspiropentylcarbinyl radical (18, k(25) degrees C > or = approximately 5 x 10(9) s(-1)) and 2-butyl-1-vinylcyclopropylcarbinyl radical (33, k(25) degrees C approximately 5 x 10(8) s(-1)) are reported. High-level ab initio calculations addressing the ring-opening isomerizations of cyclopropylcarbinyl and spiropentylcarbinyl radicals also are presented. These results in conjunction with a previous study enable us to propose two structures for the enzyme-catalyzed FAD adducts resulting from spiropentylacetic acid-CoA, a synthetic byproduct of fatty acid metabolism.