Maillard reaction products in tissue proteins: New products and new perspectives

Summary. The chemical modification of protein by nonenzymatic browning or Maillard reactions increases with age and in disease. Maillard products are formed by reactions of both carbohydrate- and lipid-derived intermediates with proteins, leading to formation of advanced glycation and lipoxidation end-products (AGE/ALEs). These modifications and other oxidative modifications of amino acids increase together in proteins and are indicators of tissue aging and pathology. In this review, we describe the major pathways and characteristic products of chemical modification of proteins by carbohydrates and lipids during the Maillard reactions and identify major intersections between these pathways. We also describe a new class of intracellular sulfhydryl modifications, Cys-AGE/ALEs, that may play an important role in regulatory biology and represent a primitive link between nonenzymatic and enzymatic chemistry in biological systems.     

On non-cyclooxygenase prostaglandin synthesis in the sea whip coral, Plexaura homomalla: an 8(R)-lipoxygenase pathway leads to formation of an alpha-ketol and a Racemic prostanoid

Plexaura homomalla is a rich natural source of prostaglandins and recent evidence suggest the prostaglandin biosynthesis could occur through a lipoxygenase pathway. We have investigated the metabolism of arachidonic acid in homogenates and acetone powders of the fresh frozen coral. The biosynthesis of natural prostaglandins was not detected. However, we find a prominent 8(R)-lipoxygenase pathway leading to an alpha-ketol, characterized by high pressure liquid chromatography, gas chromatography-mass spectrometry, and NMR as 8-hydroxy, 9-keto-eicosa-5Z, 11Z, 14Z-trienoic acid, and a prostaglandin A-like cyclopentenone identified as 9-oxo-[8, 12-cis]-prosta-5Z, 10, 14Z-trienoic acid. These reactions appear analogous to the transformation of linolenic acid hydroperoxide by "isomerase" and "cyclase" of corn and flaxseed. From analysis of the absolute configurations of the coral products, and from additional stable isotope labeling experiments in H218O and D2O, we deduce that both compounds arise via conversion of 8(R)-HPETE to an 8(R), 9-allene oxide, 8R,9-oxido-eicosa-5Z, 9, 11Z, 14Z-tetraenoic acid. This unstable intermediate undergoes hydrolysis to form the alpha-ketol or cyclization to give the cyclopentenone. Significantly, we find that the prostaglandin-like product is a racemic mixture of cis side chain enantiomers, pointing to its nonenzymatic origin from the allene oxide. The alpha-ketol is formed with partial racemization and inversion of configuration, also compatible with formation in a nonenzymatic reaction. We conclude that the isomerase and cyclase reactions may merely reflect nonenzymatic breakdown of the enzymatically formed allene oxide. The origin of the endogenous (chiral) prostaglandins of the coral may involve an allene oxide intermediate, although the potential for formation of racemic products presents an interesting dilemma regarding its relationship to the natural pathway of biosynthesis.     

Enantiomeric Crystallization from DL-Aspartic and DL-Glutamic Acids: Implications for Biomolecular Chirality in the Origin of Life

Amino acids in living systems consist almost exclusively of the L-enantiomer. How and when this homochiral characteristic of life came to be has been a matter of intense investigation for many years. Among the hypotheses proposed to explain theappearance of chiral homogeneity, the spontaneous resolution of conglomerates seems one of the most plausible. Racemic solids may crystallize from solution either as racemic compounds(both enantiomeric molecules in the same crystal), or lesscommonly as conglomerates (each enantiomer molecule separate indifferent enantiomeric crystals). Only conglomerates can developa spontaneous resolution (one of the enantiomeric molecule crystallizes preferentially, the other one remains in solution).Most of natural amino acids are racemic compounds at moderatetemperatures. How can we expect a hypothetical spontaneous resolution of these amino acids if they are not conglomerates?In this paper we show how DL-aspartic and DL-glutamic amino acids(racemic compounds), crystallize at ambient conditions as trueconglomerates. The experimental conditions here described,that allows this `anomalous' behaviour, could be also found innatural sedimentary environments. We suggest that these experimental procedures and its natural equivalents, have apotential interest for the investigation of the spontaneous resolution of racemic compounds comprising molecules associatedwith the origin of life.     

Synthesis of the coenzymes adenosine diphosphate glucose,guanosine diphosphate glucose,and cytidine diphosphoethanolamine under primitive earth conditions

The nonenzymatic synthesis of the coenzymes adenosine diphosphate glucose (ADPG), guanosine diphosphate glucose (GDPG), and cytidine diphosphoethanolamine (CDP-ethanolamine) has been carried out under conditions considered to have been prevalent on the early Earth. The production of these compounds was performed by allowing simple precursor molecules to react under aqueous solutions, at moderate temperatures and short periods of time, with mediation by cyanamide or urea. These two condensing agents are considered to have been present in significant amounts on the primitive Earth and have been previously used in the nonenzymatic synthesis of several other important biochemical compounds. In our experiments, ADPG was obtained by heating glucose-1-phosphate (G1P) and ATP in the presence of cyanamide for 24 h at 70 degrees C. The reaction of G1P and GTP under the same conditions yielded GDPG. The cyanamide-mediated production of CDP-ethanolamine was carried out by reacting a mixture of ethanolamine phosphate and CTP for 24 h at 70 degrees C. The separation and identification of the reaction products was carried out by paper chromatography, thin-layer chromatography, high performance thin-layer chromatography, high performance liquid chromatography, both normal and reverse-phase, UV spectroscopy, enzymatic assays, and acid hydrolysis. Due to the mild conditions employed, and to the relative ease of these reactions, these studies offer a simple attractive system for the nonenzymatic synthesis of phosphorylated high-energy metabolic intermediates under conditions considered to have been prevalent on the ancient Earth.     

Nonenzymatic recombination of RNA: possible mechanism for the formation of novel sequences

We report on the formation of novel RNA molecules in a recombination-like, nonenzymatic reaction proceeding in the complex of partially complementary RNA-oligonucleotides under very simple conditions. Analysis of the isolated products demonstrated that at least 5% of the formed linkages are of the (natural) 3',5'-phosphodiester type. We suggest that similar reactions could contribute to the development of the 'RNA world', but could also proceed in vivo within variously structured RNA or RNA complexes containing loops, bulges, or dangling ends, providing an emergence of novel RNA sequences.     

The so-called “interconversion” of stereoisomeric drugs: An attempt at clarification

A variety of reactions can be categorized under the global concept of the “interconversion of stereoisomers.” Thus, racemization or epimerization can result from inversion of labile chiral centers. From the examples available, some predictive rules are suggested for a chiral center of the type R″R′RC? H undergoing base-catalyzed inversion and a provisional table of affecting groups is presented. Unimolecular inversion of nonsymmetrical, nonplanar ring systems can also result in racemization or epimerization, but no generalization can yet be offered. Beside these cases of nonenzymatic reactions, a limited variety of enzymatic reactions can operate to interconvert stereoisomers, the outcome rarely being a racemic mixture. An important aspect of stereoisomer interconversion is the time scale in which the phenomenon is observed. Thus, several reactions to nonezymatic racemization or epimerization are fast compared to the duration of action of the drug and therefore have pharmacological significance, while other are slower and are of pharmaceutical relevance only. © 1993 Wiley-Liss, Inc.     

A biomimetic domino reaction for the concise synthesis of capreomycidine and epicapreomycidine

The non-proteinogenic amino acids capreomycidine and epicapreomycidine are constituents of antibiotically active natural products, but the synthesis of these unusual cyclic guanidine derivatives is challenging. The biosynthesis of capreomycidine has therefore been employed as a guideline to develop a concise biomimetic synthesis of both epimeric amino acids. The resulting domino-guanidinylation-aza-Michael-addition reaction provides the most convenient access to these amino acids in racemic form. Attempts to dissect the domino reaction into two separate transformations for a stereocontrolled version of this synthetic approach have also been made. The synthesized didehydro-arginine derivatives with urethane-protected guanidine moieties did not undergo the aza-Michael-addition anymore. These results may have wider implications for the 1,4-addition of guanidines to α,β-unsaturated carbonyl compounds, particularly to didehydro amino acids.     

A diversity-oriented synthesis approach to macrocycles via oxidative ring expansion

Macrocycles are key structural elements in numerous bioactive small molecules and are attractive targets in the diversity-oriented synthesis of natural product-based libraries. However, efficient and systematic access to diverse collections of macrocycles has proven difficult using classical macrocyclization reactions. To address this problem, we have developed a concise, modular approach to the diversity-oriented synthesis of macrolactones and macrolactams involving oxidative cleavage of a bridging double bond in polycyclic enol ethers and enamines. These substrates are assembled in only four or five synthetic steps and undergo ring expansion to afford highly functionalized macrocycles bearing handles for further diversification. In contrast to macrocyclization reactions of corresponding seco acids, the ring expansion reactions are efficient and insensitive to ring size and stereochemistry, overcoming key limitations of conventional approaches to systematic macrocycle synthesis. Cheminformatic analysis indicates that these macrocycles access regions of chemical space that overlap with natural products, distinct from currently targeted synthetic drugs.     

A nomenclature system for isofurans

Recently, the isolation of a new class of human arachidonic acid oxidation products, the isofurans, was reported. These are produced in vivo by a free radical mechanism independent of the cyclooxygenase enzymes. Because these compounds are tetrahydrofuran derivatives that are related biosynthetically to the isoprostanes, they were termed isofurans. There are eight different isofuran regioisomers, each of which can exist as 16 racemic diastereomers. Thus, 256 enantiomerically-pure isofurans can be formed. These molecules are of interest as measurement of isofurans provides a sensitive index of free-radical induced lipid peroxidation in vivo under conditions of elevated oxygen tension. They also, in analogy to isoprostanes, may have potent biological activity. To explore this, the chemical synthesis of the IsoFs has been initiated. As a result, there is a need for a systematic nomenclature for this class of natural products. A facile system that will allow the ready differentiation of each of the isomeric structures comprising the family of isofurans is presented.     

Complex molecules,clever solutions – Enzymatic approaches towards natural product and active agent syntheses

Natural compounds are often structurally complex and their synthesis is still highly challenging. The review intends to give an overview on developments in biotechnology and their role for the production of natural products and active agents. In vitro and in vivo methods are presented side by side beginning with rather simple but smart single step conversions, followed by cascade reactions, and finishing with complex bio-, semi- and mutasynthesis endeavours. All the enzymatic approaches do obviously complement traditional synthetic methods; with their particular strengths, the combined repertoire will lead to an increased efficiency in natural product synthesis as well as in providing analogues.     

Terpenoid bio-transformations and applications via cell/organ cultures: a systematic review

Abstract

Structurally diverse natural products are valued for their targeted biological activity. The challenge of working with such metabolites is their low natural abundance and complex structure, often with multiple stereocenters, precludes large-scale or unsophisticated chemical synthesis. Since select plants contain the enzymatic machinery necessary to produce specialized compounds, tissue cultures can be used to achieve key transformations for large-scale chemical and/or pharmaceutical applications. In this context, plant tissue-culture bio-transformations have demonstrated great promise in the preparation of pharmaceutical products. This review describes the capacity of cultured plant cells to transform terpenoid natural products and the specific application of such transformations over the past three decades (1988–2019).     

Post Iron Decoration of Mesoporous Nitrogen‐Doped Carbon Spheres for Efficient Electrochemical Oxygen Reduction

Iron–nitrogen–carbon (Fe–N–C) catalysts are considered as the most promising nonprecious metal catalysts for oxygen reduction reactions (ORRs). Their synthesis generally involves complex pyrolysis reactions at high temperature, making it difficult to optimize their composition, pore structure, and active sites. This study reports a simple synthesis strategy by reacting preformed nitrogen‐doped carbon scaffolds with iron pentacarbonyl, a liquid precursor that can effectively form active sites with the nitrogen sites, enabling more effective control of the catalyst. The resultant catalyst possesses a well‐defined mesoporous structure, a high surface area, and optimized active sites. The catalysts exhibit high ORR activity comparable to that of Pt/C catalyst (40% Pt loading) in alkaline media, with excellent stability and methanol tolerance. The synthetic strategy can be extended to synthesize other metal–N–C catalysts.     

Protein oxidation at the air-lung interface

Summary. Whilst performing its normal functions the lung is required to deal with a range of toxic insults. Whether these are infectious agents, allergens or air pollutants they subject the lung to a range of direct and indirect oxidative stresses. In many instances these challenges lead to oxidative alterations of peptides and proteins within the lung. Measurement of protein oxidation products permits the degree of oxidative stress to be assessed and indicates that endogenous antioxidant defences are overwhelmed. The range of protein oxidation products observed is diverse and the nature and extent of specific oxidation products may inform us about the nature of the damaging ROS and NOS. Recently, there has been a significant shift away from the measurement of these oxidation products simply to establish the presence of oxidative stress, to a focus on identifying specific proteins sensitive to oxidation and establishing the functional consequences of these modifications. In addition the identification of specific enzyme systems to repair these oxidative modifications has lead to the belief that protein function may be regulated through these oxidation reactions. In this review we focus primarily on the soluble protein components of within the surface liquid layer in the lung and the consequence of their undue oxidation.     

Does the chemical instability of aspartyl and asparaginyl residues in proteins contribute to erythrocyte aging? The role of protein carboxyl methylation reactions

As erythrocytes age in the circulation, their proteins are subjected to a wide variety of spontaneous reactions that lead to the formation of covalent derivatives. In this article, we concentrate on nonenzymatic reactions at aspartyl and asparaginyl residues, both of which are especially vulnerable targets on the protein. These residues can be altered by a combination of deamidation, isomerization, and racemization reactions that form D- and L-aspartyl and D- and L-isoaspartyl residues. We present evidence that two of these modified residues are targets for an enzymatic methyl esterification reaction, and that methylation may represent the means by which cells respond to this type of protein damage. The metabolic fate of the methyl ester is unclear, but in vitro model studies with peptides and proteins suggest that this methylation can lead to the partial repair of the altered protein and can mitigate the loss of protein function.     

2-ketoacid dehydrogenase complexes of Escherichia coli: stereospecificities of the three components for (R)-lipoate

Stereospecificities of component enzymes in the pyruvate dehydrogenase complex and 2-ketoglutarate dehydrogenase complex from Escherichia coli for lipoate and dihydrolipoate are determined. Assays of the component enzymes using R,S-, R-, or S-lipoate or the enantiomers of dihydrolipoate show that only the R-enantiomers are substrates for these enzymes. Nonenzymatic reactions involving acetyl group transfer and coupled electron and acetyl group transfer between enantiomeric molecules of lipoate or/and dihydrolipoate proceed at significant rates. Coupled acetyl group and electron transfer from enzyme-bound acetyldihydrolipoyl moieties to free lipoate is also observed. The S-enantiomers are neither substrates nor inhibitors; however, products of S-enantiomers are slowly generated in enzymatic reactions owing to nonenzymatic reactions between enzyme-bound acetyldihydrolipoyl-groups and free S-lipoate or S-dihydrolipoate.     

Tools and ingredients for the biocatalytic synthesis of metabolites

Metabolic networks have been an interesting starting point not only for the design of synthetic routes in a similar sequence of reactions, e.g., in biomimetic syntheses, but also for assembling a number of biocatalytic steps by preparing the required enzymes and auxiliary reagents. Retrosynthetic analysis involving multiple biocatalytic reactions steps therefore needs to consider the practically realized biocatalytic single steps. The opportunities for route selection are enlarged if novel synthetic reactions connecting easily available starting materials and products are found, and/or both biocatalytic and classical reactions of organic chemistry are utilized. Tools and ingredients for biocatalytic synthesis are of special interest for reactions difficult to achieve by classical organic synthesis. Densely and differentially functionalized small molecules do not allow much space for protecting or activating groups. Biocatalytic reactions have therefore performed well for a number of useful metabolites in enantiopure form to achieve full functionality. Although many well-known metabolites from classical biochemistry have only been prepared in racemic form, it is of fundamental interest to have these available in enantiomerically pure form. Biocatalytic reactions with nature's privileged chiral catalysts appear to be a promising synthetic strategy towards these metabolites, especially when sensitive or stable-isotope-labeled metabolites are to be prepared. The main applications for these metabolites are as references materials in metabolomics, as enzyme substrates for the characterization of metabolic enzyme activities and as potential pharmaceuticals in biomedical research. The use of stable-isotope-labeled metabolites can thereby simplify in vivo applications and metabolic flux analyses.     

Managing predators: The influence of kangaroo rat antipredator displays on sidewinder rattlesnake hunting behavior

Upon sensing predators in their vicinity, many prey species perform antipredator displays that are thought to provide information to the predator that deters it from attacking (predator‐deterrent signals). These displays can be complex, incorporating a variety of signaling elements as well as direct physical harassment of the predator. Although the display behaviors in these communication systems are often well characterized, evidence of the efficacy of these displays in deterring predators is limited due to the challenges associated with studying free‐ranging predators. Here, we examine how the anti‐snake signals of the desert kangaroo rat (Dipodomys deserti) influence the ambush hunting behaviors of sidewinder rattlesnakes (Crotalus cerastes). We found that, although desert kangaroo rats incorporate a number of signal elements into their antipredator display, only sand kicking behavior was a significant factor in motivating sidewinder rattlesnakes to cease hunting: high rates of sand kicking led to early abandonment of ambush coils. These results indicate that anti‐snake displays of small mammals may be especially effective at mitigating the threat posed by rattlesnakes when those displays incorporate physical harassment as well as signaling.     

Metabolic network sensitivity analysis

A linear sensitivity analysis of metabolic regulation in nonsteady states is described. This treatment considers the effects of enzymatic and nonenzymatic reactions and spontaneous rapid equilibria. Sensitivity coefficients summarizing the influence of metabolite concentrations on reaction rates and pathway net flux are defined, as are sensitivity coefficients summarizing the effects of enzymes on metabolite concentrations and net flux. The sensitivity analysis is implemented in an easily used set of computer programs. A four-enzyme test model was shown to be resistant to intuitive interpretation. Sensitivity analysis showed a shift of control from the end of the enzymic sequence to the beginning of the sequence with changing metabolic state. The homeostatic behavior of the test system was shown to depend on the nonenzymatic reactions as well as on the enzymes. Under certain conditions metabolic regulation is shared so intimately among enzymes and spontaneous reactions that separation of their effects is impossible.     

Quantification of N-(glucitol)ethanolamine and N-(carboxymethyl)serine: two products of nonenzymatic modification of aminophospholipids formed in vivo.

Chemical, nonenzymatic modification of protein and lipids by reducing sugars, such as glucose, is thought to contribute to age-related deterioration in tissue protein and cellular membranes and to the pathogenesis of diabetic complications. This report describes the synthesis and quantification of N-(glucitol)ethanolamine (GE) and N-(carboxymethyl)serine (CMS), two products of nonenzymatic modification of aminophospholipids. GE is the product of reduction and hydrolysis of glycated phosphatidylethanolamine (PE), while CMS is formed through reaction of phosphatidylserine (PS) with products of oxidation of either carbohydrate (glycoxidation) or lipids (lipoxidation). Gas chromatography/mass spectrometry procedures for quantification of the N,O-acetyl methyl ester derivatives of the modified head groups were developed. GE and CMS were quantified in samples of PE and PS, respectively, following incubation with glucose in vitro; CMS formation was dependent on the presence of oxygen during the incubation. Both GE and CMS were detected and quantified in lipid extracts of human red blood cell membranes. The content of GE, but not CMS, was increased in the lipids from diabetic compared to nondiabetic subjects. Measurement of these modified lipids should prove useful for assessing the role of carbonyl-amine reactions of aminophospholipids in aging and age-related diseases.