Analysis of hydroxyproline isomers and hydroxylysine by reversed-phase HPLC and mass spectrometry

Development of a two-dimensional LC-MS/MS system for the determination of proline and 4-hydroxyproline enantiomers in biological and food samples

A two-dimensional LC-MS/MS system has been developed for the enantioselective determination of proline (Pro), cis-4-hydroxyproline (cis-4-Hyp) and trans-4-hydroxyproline (trans-4-Hyp) in a variety of biological samples. The amino acids were pre-column derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), and the NBD-derivatives were separated by a reversed-phase column (Singularity RP18) as their D plus L mixtures in the first dimension. The collected target fractions were then introduced into the second dimension where the enantiomers were separated by a Pirkle-type enantioselective column (Singularity CSP-001S) and determined by a tandem mass spectrometer (Triple Quad™ 5500). The method was validated by the standard amino acids and also by human plasma, and sufficient results were obtained for the calibration, precision and accuracy. The method was applied to human plasma and urine, bivalve tissues and fermented food/beverages. D-Pro was widely found in the human physiological fluids, bivalves and several fermented products. Although trans-4-D-Hyp was not found in all the tested samples, cis-4-D-Hyp was present in human urine and tissues of the ark shell, and further studies focusing on the origin and physiological significance of these D-enantiomers are expected.

Chromatographic separation by RPLC-ESI-MS of all hydroxyproline isomers for the characterization of collagens from different sources

During collagen biosynthesis, proline is post-translationally converted to hydroxyproline by specific enzymes. This amino acid, unique to collagen, plays a crucial role in stabilizing the collagen triple helix structure and could serve as an important biomarker for collagen content and quality analysis. Hydroxyproline has four isomers, depending on whether proline is hydroxylated at position 4 or 3 and on whether the cis- or trans- conformation is formed. Moreover, as extensive hydrolysis of collagen is required for its amino acid analysis, epimerization may also occur, although to a lesser extent, giving a total of eight possible isomers. The aim of the present study was to develop a reversed-phase high-performance liquid chromatography-UV-mass spectrometry (RPLC-UV-MS) method for the separation and quantification of all eight hydroxyproline isomers. After the chiral derivatization of the hydroxyproline isomers with Nα-(2,4-dinitro-5-fluorophenyl)-L-valinamide (L-FDVA), to enable their UV detection, the derivatized diastereoisomers were separated by testing different C18 column technologies and morphologies and optimizing operative conditions such as the mobile phase composition (solvent, additives), elution mode, flow rate and temperature. Baseline resolution of all eight isomers was achieved on a HALO® ES-C18 reversed-phase column (150×1.5 mm, 2.7 μm, 160 Å) using isocratic elution and MS-compatible mobile phase. The optimized method was validated for the quantification of hydroxyproline isomers and then applied to different collagen hydrolysates to gain insight and a deeper understanding of hydroxyproline abundances in different species (human, chicken) and sources (native, recombinant).

Inferring Pathways of Oxidative Folding from Prefolding Free Energy Landscapes of Disulfide-Rich Toxins

Short, cysteine-rich peptides can exist in stable or metastable structural ensembles due to the number of possible patterns of formation of their disulfide bonds. One interesting subset of this peptide group is the conotoxins, which are produced by aquatic snails in the family Conidae. The μ conotoxins, which are antagonists and blockers of the voltage-gated sodium channel, exist in a folding spectrum: on one end of the spectrum are more hirudin-like folders, which form disulfide bonds and then reshuffle them, leading to an ensemble of kinetically trapped isomers, and on the other end are more BPTI-like folders, which form the native disulfide bonds one by one in a particular order, leading to a preponderance of conformations existing in a single stable state. In this Article, we employ the composite diffusion map approach to study the unified free energy surface of prefolding μ-conotoxin equilibrium. We identify the two most important nonlinear collective modes of the unified folding landscape and demonstrate that in the absence of their disulfides, the conotoxins can be thought of as largely disordered polymers. A small increase in the number of hydrophobic residues in the protein shifts the free energy landscape toward hydrophobically collapsed coil conformations responsible for cysteine proximity in hirudin-like folders, compared to semiextended coil conformations with more distal cysteines in BPTI-like folders. Overall, this work sheds important light on the folding processes and free energy landscapes of cysteine-rich peptides and demonstrates the extent to which sequence and length contribute to these landscapes.

Chiral secondary amino acids, their importance, and methods of analysis

Naturally occurring secondary amino acids, with proline as the main representative, contain an alpha-imino group in a cycle that is typically four-, five-, and six-membered. The unique ring structure exhibits exceptional properties-conformational rigidity, chemical stability, and specific roles in protein structure and folding. Many proline analogues have been used as valuable compounds for the study of metabolism of both prokaryotic and eukaryotic cells and for the synthesis of compounds with desired biological, pharmaceutical, or industrial properties. The D-forms of secondary amino acids play different roles in living organisms than the L-forms. They have different metabolic pathways, biological, physiological, and pharmacological effects, they can be indicators of changes and also serve as biomarkers of diseases. In the scientific literature, the number of articles examining D-amino acids in biological samples is increasing. The review summarises information on the occurrence and importance of D- and L-secondary amino acids-azetidic acid, proline, hydroxyprolines, pipecolic, nipecotic, hydroxypipecolic acids and related peptides containing these D-AAs, as well as the main analytical methods (mostly chromatographic) used for their enantiomeric determination in different matrices (biological samples, plants, food, water, and soil).

Recharacterization of the Mammalian Cytosolic Type 2 (R)-β-Hydroxybutyrate Dehydrogenase (BDH2) as 4-Oxo-L-Proline Reductase (EC 1.1.1.104)

Early studies revealed that chicken embryos incubated with a rare analog of l-proline, 4-oxo-l-proline, showed increased levels of the metabolite 4-hydroxy-l-proline. In 1962, 4-oxo-l-proline reductase, an enzyme responsible for the reduction of 4-oxo-l-proline, was partially purified from rabbit kidneys and characterized biochemically. However, only recently was the molecular identity of this enzyme solved. Here, we report the purification from rat kidneys, identification, and biochemical characterization of 4-oxo-l-proline reductase. Following mass spectrometry analysis of the purified protein preparation, the previously annotated mammalian cytosolic type 2 (R)-β-hydroxybutyrate dehydrogenase (BDH2) emerged as the only candidate for the reductase. We subsequently expressed rat and human BDH2 in Escherichia coli, then purified it, and showed that it catalyzed the reversible reduction of 4-oxo-l-proline to cis-4-hydroxy-l-proline via chromatographic and tandem mass spectrometry analysis. Specificity studies with an array of compounds carried out on both enzymes showed that 4-oxo-l-proline was the best substrate, and the human enzyme acted with 12,500-fold higher catalytic efficiency on 4-oxo-l-proline than on (R)-β-hydroxybutyrate. In addition, human embryonic kidney 293T (HEK293T) cells efficiently metabolized 4-oxo-l-proline to cis-4-hydroxy-l-proline, whereas HEK293T BDH2 KO cells were incapable of producing cis-4-hydroxy-l-proline. Both WT and KO HEK293T cells also produced trans-4-hydroxy-l-proline in the presence of 4-oxo-l-proline, suggesting that the latter compound might interfere with the trans-4-hydroxy-l-proline breakdown in human cells. We conclude that BDH2 is a mammalian 4-oxo-l-proline reductase that converts 4-oxo-l-proline to cis-4-hydroxy-l-proline and not to trans-4-hydroxy-l-proline, as originally thought. We also hypothesize that this enzyme may be a potential source of cis-4-hydroxy-l-proline in mammalian tissues.

A chiral GC-MS method for analysis of secondary amino acids after heptafluorobutyl chloroformate & methylamine derivatization

L-amino acids (L-AAs) play different important roles in the physiology of all living organisms. Their chiral counterparts, D-amino acids (D-AAs) are increasingly being recognized as essential molecules in many biological systems. Secondary amino acids with cyclic structures, such as prolines, exhibit conformational rigidity and thus unique properties in the structural and protein folding. Despite their widespread occurrence, much less attention was paid to their chiral analysis, particularly when the minor, typically D-enantiomer, is present in low amounts in a complex biological matrix. In this paper, a cost-effective, chiral GC-MS method is described for capillary Chirasil-L-Val separation of nine cyclic secondary amino acid enantiomers with four-, five-, and six-membered rings, involving azetidine-2-carboxylic acid, pipecolic acid, nipecotic acid, proline, isomeric cis/trans 3-hydroxy, 4-hydroxyproline, and cis/trans-5-hydroxy-L-pipecolic acid in the excess of its enantiomeric antipode. The sample preparation involves in-situ derivatization with heptafluorobutyl chloroformate, simultaneous liquid-liquid micro-extraction into isooctane followed by amidation of the arising low-polar derivatives with methylamine, an evaporation step, re-dissolution, and final GC-MS analysis. The developed method was used for analyses of human biofluids, biologically active peptides containing chiral proline constituents, and collagen.

Analysis of Hydroxyproline in Collagen Hydrolysates

Hydroxyproline (Hyp) is an imino acid posttranslationally formed by sequence-specific hydroxylases in the repeating collagen Gly-Xaa-Yaa triad present in all collagen types of all species. In both Xaa- and Yaa-positions, Pro is the most common residue, often oxidized to 4-Hyp in the Yaa- and rarely to 3-Hyp in the Xaa-positions. Here we describe the qualitative and quantitative analysis of 3- and 4-Hyp-isomers by separating the free imino acids either with hydrophilic interaction chromatography (HILIC) or after derivatization with reversed-phase chromatography (RPC). In both cases the compounds were detected by electrospray-ionization mass spectrometry.

A prominent glycyl radical enzyme in human gut microbiomes metabolizes trans-4-hydroxy-l-proline

The human microbiome encodes vast numbers of uncharacterized enzymes, limiting our functional understanding of this community and its effects on host health and disease. By incorporating information about enzymatic chemistry into quantitative metagenomics, we determined the abundance and distribution of individual members of the glycyl radical enzyme superfamily among the microbiomes of healthy humans. We identified many uncharacterized family members, including a universally distributed enzyme that enables commensal gut microbes and human pathogens to dehydrate trans-4-hydroxy-l-proline, the product of the most abundant human posttranslational modification. This "chemically guided functional profiling" workflow can therefore use ecological context to facilitate the discovery of enzymes in microbial communities.

The application of targeted mass spectrometry-based strategies to the detection and localization of post-translational modifications

This review describes some of the more interesting and imaginative ways in which mass spectrometry has been utilized to study a number of important post-translational modifications over the past two decades; from circa 1990 to 2013. A diverse range of modifications is covered, including citrullination, sulfation, hydroxylation and sumoylation. A summary of the biological role of each modification described, along with some brief mechanistic detail, is also included. Emphasis has been placed on strategies specifically aimed at detecting target modifications, as opposed to more serendipitous modification discovery approaches, which rely upon straightforward product ion scanning methods. The authors have intentionally excluded from this review both phosphorylation and glycosylation since these major modifications have been extensively reviewed elsewhere.

LC-MS/MS identification of the O-glycosylation and hydroxylation of amino acid residues of collagen α-1 (II) chain from bovine cartilage

O-Glycosylation of collagen is a unique type of posttranslational modifications (PTMs) involving the attachment of galactose (Gal) or glucose-galactose (Glc-Gal) moieties to hydroxylysine (HyK). Also, hydroxyproline (HyP) result from the posttranslational hydroxylation of some proline residues in collagen. Here, LC-MS/MS was effectively employed to identify 23 O-glycosylation sites and a large number of HyP residues associated with bovine type II collagen α-1 chain (CO2A1). The modifications of the 23 O-glycosylation sites varied qualitatively and quantitatively. Both Gal and Glc-Gal moieties occupied 22 of the identified glycosylation sites, while K773 was observed as unmodified. A large number of HyP residues at Yaa positions of Gly-Xaa-Yaa motif were detected. HyP residues at Xaa positions of Gly-HyP-HyP, Gly-HyP-Ala, and Gly-HyP-Val motifs were also observed. Notably, HyP residue of Gly-HyP-Gln motif was detected, which has not been previously reported. Moreover, the deamidation of 8 Asn residues was identified, of which 2 Asp residues were observed at different retention times because of isomerization (Asp vs isoAsp). Partial macroheterogeneities of some CO2A1 glycosylation sites were revealed by LC-MS/MS analysis. ETD experiments revealed partial macroheterogeneities associated with K299-K308, K452-K464, K464-K470, and K857-K884 glycosylation sites. Semiquantitative data suggest that the glycosylation of hydroxylysine residues is site-specific.

Chiral derivatizations applied for the separation of unusual amino acid enantiomers by liquid chromatography and related techniques

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the proteinogenic α-amino acids contain at least one stereogenic center (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past 25 years and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers of unusual amino acids and related compounds, with emphasis on the literature published from 1980s. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea and thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Bioanalytical LC separation techniques for quantitative analysis of free amino acids in human plasma

The quantitative analysis of free amino acids in human plasma has become an important and essential analysis parameter in different areas of life sciences. Free amino acid concentrations in human plasma samples are generally determined by means of GC or LC after chemical derivatization followed by UV, fluorescent or MS detection of the amino acid derivatives. Derivatization of free amino acids is done either pre- or post-column, and the amino acid derivatives obtained posess improved chromatographic behavior, increased detection sensitivity and selectivity compared with non-derivatized free amino acids. This work gives an overview of different chemical derivatization methods applied and their liquid separation techniques in bioanalytical assays for quantitative free amino acid analysis in human plasma samples. Important plasma preparation procedures, pre- and post-column derivatization, and different LC separation techniques are presented.

Oncocin derivative Onc72 is highly active against Escherichia coli in a systemic septicaemia infection mouse model

OBJECTIVES

The antimicrobial oncocin derivative Onc72 is highly active against a number of Gram-negative bacteria, including resistant strains. Here we study its toxicity and efficacy in a lethal mouse infection model.

METHODS

In an acute toxicity study, purified Onc72 was administered to NMRI mice in four consecutive injections within a period of 24 h as an intraperitoneal bolus. The animals' behaviour was monitored for 5 days, before several organs were examined by histopathology. A lethal Escherichia coli infection model was established and the efficacy of Onc72 was evaluated for different peptide doses considering the survival rates of each dose group and the bacterial counts in blood, lavage and organs.

RESULTS

Intraperitoneal bolus injections with single doses of 20 or 40 mg of Onc72 per kg of body weight did not result in any abnormal animal behaviour. No mouse became moribund or died within the studied period. Histopathological examinations revealed no toxic effects. When infected with E. coli at a lethal dose, none of the untreated animals survived the next 24 h, whereas all animals treated three times with Onc72 at doses of ≥5 mg/kg survived the observation period of 5 days. No bacteria were detected in the blood of treated animals after day 5 post-infection. The effective dose (ED(50)) was ∼2 mg/kg.

CONCLUSIONS

No toxic effects were observed for Onc72 within the studied dose range up to 40 mg/kg, indicating a safety margin of >20.

Analysis of hydroxyproline in collagen hydrolysates

Hydroxyproline (Hyp) is an imino acid post-translationally formed by sequence-specific hydroxylases in the repeating collagen Gly-Xaa-Yaa triad present in all collagen types of all species. In both Xaa- and Yaa-positions, Pro is the most common residue, often oxidized to 4-Hyp in the Yaa- and rarely to 3-Hyp in the Xaa-positions. Here, we describe the qualitative and quantitative analysis of 3- and 4-Hyp-isomers by separating the free imino acids either with hydrophilic interaction chromatography (HILIC) or after derivatization with reversed-phase chromatography (RPC). In both cases, the compounds were detected by electrospray ionization mass spectrometry (ESI-MS).

Carboxymethylation of the fibrillar collagen with respect to formation of hydroxyapatite

Control over crystal growth by acidic matrix macromolecules is an important process in the formation of many mineralized tissues. Highly acidic macromolecules are postulated intermediates in tissue mineralization, because they sequester many calcium ions and occur in high concentrations at mineralizing foci in distantly related organisms. A prerequisite for biomineralization is the ability of cations like calcium to bind to proteins and to result in concert with appropriate anions like phosphates or carbonates in composite materials with bone-like properties. For this mineralization process the proteins have to be modified with respect to acidification. In this study we modified the protein collagen by carboxymethylation using glucuronic acid. Our experiments showed unambigously, that N(epsilon)-carboxymethyllysine is the major product of the in vitro nonenzymatic glycation reaction between glucuronic acid and collagen. We hypothesized that the function of biomimetically carboxymethylated collagen is to increase the local concentration of corresponding ions so that a critical nucleus of ions can be formed, leading to the formation of the mineral. Thus, the self-organization of HAP nanocrystals on and within collagen fibrils was intensified by carboxymethylation.

Hydroxyproline quantification for the estimation of collagen in tissue using multiple reaction monitoring mass spectrometry

Collagens are highly abundant mammalian proteins that contain a high content of hydroxylated amino acids such as hydroxyproline. We have exploited the high hydroxyproline content of collagen and developed a method for hydroxyproline quantification as a measure of collagen content in muscle samples. The novel method utilizes a highly selective and sensitive method of multiple reaction monitoring (MRM) by mass spectrometry. The analytical method is simple, rapid (5min), convenient (no derivatization), precise (<17% RSD), accurate (90-108%), sensitive (4.88nmol/L) and linear (R(2)>0.999) over three orders of magnitude (5-5000nmol/L).