Rapid screening of urinary proline-hydroxyproline dipeptide in bone turnover studies

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).

Monitoring urinary collagen metabolite changes following collagen peptide ingestion and physical activity using ELISA with anti active collagen oligopeptide antibody

Active collagen oligopeptides (ACOP) are bioactive collagen-derived peptides detected by a recently-established ELISA. To facilitate studies of the function and metabolism of these products, this study aims to determine which of these peptides is recognized by a novel anti-ACOP antibody used in this ELISA. We then investigate the effect of collagen peptide (CP) ingestion and exercise on urinary ACOP concentrations in a cohort of university student athletes using colorimetric, LC–MS/MS, and ELISA. We observed that the antibody showed strong cross-reactivity to Pro-Hyp and Gly-Pro-Hyp and weak cross-reactivity to commercial CP. CP ingestion increased the urinary level of ACOP over time, which correlated highly with urinary levels of peptide forms of Hyp and Pro-Hyp. Physical activity significantly decreased the urinary ACOP level. This study demonstrates changes in urinary ACOP following oral CP intake and physical activity using ELISA with the novel anti-ACOP antibody. Thus, ACOP may be useful as a new biomarker for collagen metabolism.

1H-NMR analysis of the human urinary metabolome in response to an 18-month multi-component exercise program and calcium–vitamin-D3supplementation in older men

The musculoskeletal benefits of calcium and vitamin-D3supplementation and exercise have been extensively studied, but the effect on metabolism remains contentious. Urine samples were analyzed by1H-NMR spectroscopy from participants recruited for an 18-month, randomized controlled trial of a multi-component exercise program and calcium and vitamin-D3fortified milk consumption. It was shown previously that no increase in musculoskeletal composition was observed for participants assigned to the calcium and vitamin-D3intervention, but exercise resulted in increased bone mineral density, total lean body mass, and muscle strength. Retrospective metabolomics analysis of urine samples from patients involved in this study revealed no distinct changes in the urinary metabolome in response to the calcium and vitamin-D3intervention, but significant changes followed the exercise intervention, notably a reduction in creatinine and an increase in choline, guanidinoacetate, and hypoxanthine (p < 0.001, fold change > 1.5). These metabolites are intrinsically involved in anaerobic ATP synthesis, intracellular buffering, and methyl-balance regulation. The exercise intervention had a marked effect on the urine metabolome and markers of muscle turnover but none of these metabolites were obvious markers of bone turnover. Measurement of specific urinary exercise biomarkers may provide a basis for monitoring performance and metabolic response to exercise regimes.

Determination of proline-hydroxyproline dipeptide in rat urine by high-performance anion-exchange chromatography coupled with pulsed amperometric detection

A method for determination of proline-hydroxyproline dipeptide (PHP) was developed using high-performance anion-exchange chromatography-pulsed amperometric detection (HPAEC-PAD). This method resulted in good separation of proline (Pro), hydroxyproline (Hyp), and PHP within 20min using a mobile phase of 1.2mM Ba(OH)2+1.5mM Ba(OAc)2. The linear dynamic ranges and their detection limits (S/N=3) were 1-100 (r(2)=0.9990-0.9999) and 0.05-0.3μM, respectively. Mean recoveries were 91.6-121.3% and 92.2-110.3% for intra- and inter-day assays, respectively. Our HPAEC-PAD method showed clear differences in the corrected PHP levels measured in urine samples from two groups of rats, sham-operated and ovariectomized, without the need for prior acid hydrolysis or sample derivatization.

Paracetamol prevents hyperglycinemia in vervet monkeys treated with valproate

Valproate administration increases the level of the inhibitory transmitter, glycine, in the urine and plasma of patients and experimental animals. Nonketotic hyperglycinemia (NKH), an autosomal recessive disorder of glycine metabolism, causes increased glycine concentrations in blood, urine, and cerebrospinal fluid (CSF), most likely due to a defect in the glycine cleavage enzyme or possibly deficits in glycine transport across cell membranes. We investigated the relationship between the hyperglycinemic effect of valproate and induced pyroglutamic aciduria via paracetamol in the vervet monkey. Firstly it was determined if valproate could induce hyperglycinemia in the monkey. The second aim was to increase glutamic acid (oxoproline) urine excretion using paracetamol as a pre-treatment and to assess whether valproate has an influence on the γ-glutamyl cycle. Hyperglycinemia was induced in healthy vervet monkeys when treated with a single oral dose of 50 mg/kg valproate. An acute dose of 50 mg/kg paracetamol increased oxoproline in the urine. Pre-treatment with paracetamol opposed the hyperglycinemic effect of valproate. However, the CSF:serum glycine ratio in a nonketotic monkey increased markedly after paracetamol treatment and remained high following valproate treatment. These results indicate that the γ-glutamyl cycle does indeed play a role in the hyperglycinemic effect of valproate treatment, and that paracetamol may have value in preventing and/or treating valproate-induced NKH.

In situ derivatization-liquid liquid extraction as a sample preparation strategy for the determination of urinary biomarker prolyl-4-hydroxyproline by liquid chromatography-tandem mass spectrometry

Polar analytes that possess protic functional groups have often been treated with alkyl chloroformates to decrease their polarity and increase their volatility prior to gas chromatography-mass spectrometry analysis. This derivatization reaction has two distinct advantages. It proceeds smoothly in aqueous media, and the desired reaction products are efficiently separated from interfering ionic components by their extraction into a water-immiscible organic phase. In the present work, the derivatization-liquid liquid sample preparation was examined in detail for analysis of a potential urinary dipeptide biomarker L-prolyl-4-L-hydroxyproline (PHP) by downstream liquid chromatography coupled to electrospray mass spectrometry. PHP was treated with a series of alkyl and fluoroalkyl chloroformates in aqueous media, and the detected reaction products were investigated. Smooth conversion of PHP into the N-isobutyloxycarbonyl isobutyl ester was accomplished by the coupled action of isobutanol, isobutyl chloroformate and the pyridine catalyst. This derivative afforded a highest detector response from all the derivatized forms examined, including the nonderivatized PHP. A simple isocratic elution on a common RP-C18 HPLC column coupled with tandem mass spectrometry, and use of the synthesized heptadeuterated analog (D7-PHP) as an internal standard, enabled validation of the method and determination of PHP in human urine in less than 5 min. The in situ derivatization-liquid liquid extraction has thus been demonstrated to be a useful sample preparation strategy for the analysis of polar metabolites by liquid chromatography-tandem mass spectrometry in the complex urine matrix.

Prolyl-hydroxyproline dipeptide in non-hydrolyzed morning urine and its value in postmenopausal osteoporosis

BACKGROUND

Owing to the high correlation between the level of prolyl-4-hydroxyproline dipeptide in non-hydrolyzed urine and that of 4-hydroxyproline in hydrolyzed urine, we examined whether the dipeptide might function as a valuable marker of bone turnover.

METHODS

Based on densitometric measurements, 68 postmenopausal women were divided into groups of non-osteopathic, osteopenic and osteoporotic subjects. The dipeptide and current urinary resorption markers were assayed in morning urine, the former using liquid chromatography, the others plus serum formation markers by means of immunoassay procedures. Together with the assay of basal levels, diet-related changes and healing effect of yearly alendronate therapy were assessed.

RESULTS

Concentration levels in controls and osteoporotic subjects differed significantly; receiver operating characteristics yielded sensitivity of 0.743, specificity of 0.908, area under curve of 0.903, and cut-off of 10.2 micromol/mmol of creatinine. Spearman rank correlation showed the highest pair coefficient between the dipeptide and osteocalcin. Diet-related changes were not found. Following therapy, a significant decline occurred already within a trimester, whilst with the other resorption markers not until 6 months.

CONCLUSIONS

The ease of the dipeptide assay in non-hydrolyzed urine surpasses that of hydroxyproline, and the results present the compound as a real competition to other commonly assessed markers in osteopathies.

Optimum conditions of autoclaving for hydrolysis of proteins and urinary peptides of prolyl and hydroxyprolyl residues and HPLC analysis

A method for urinary peptide(s) and protein hydrolysis, involving autoclaving at 15psi (121 degrees C) for 60min, is described. Using three candidate proteins (bovine serum albumin, casein and gelatin) and urine specimens, the effect of autoclaving with respect to the optimum time required for hydrolysis under both acidic (6N HCl) and alkaline (6N KOH) conditions was studied. Recoveries of total amino acids from proteins and urine hydrolysate(s) suggest that complete hydrolysis of proteins and urinary peptides could be achieved by autoclaving for 30-60min instead of 16h of incubation at 110 degrees C. Further, stability of some of the individual amino acids was also studied. The observed differential stability of amino acids under acidic and alkaline conditions, as demonstrated in this study by HPLC analysis, makes it imperative to choose the appropriate hydrolytic condition while studying the composition of any given amino acids in urinary peptide(s)/protein hydrolysates. Further, the finding that both Pro and Hyp were stable under alkaline conditions of hydrolysis by autoclaving renders this method suitable for assaying these two amino acids from urine hydrolysates, hence its utility in the study of urinary peptide derived Hyp and Pro in bone/cartilage disorders.

Quantitation of hydroxyproline in bone by gas chromatography-mass spectrometry

A validated gas chromatography (GC)-mass spectrometric (MS) method for the analysis of hydroxyproline in rat femur is reported. Hydroxyproline in bone hydrolysates was extracted with an anion exchange resin and the N(O)-tert-butyldimethylsilyl derivatives analyzed by GC-MS. The hydroxyproline concentration was estimated relative to pipecolic acid, 3,4-dehydroproline and n-tetracosane as internal standards. The mass-to-charge ratios (m/z) for the ions used for quantitation by single ion monitoring were 314 m/z for hydroxyproline, 198 m/z for pipecolic acid, 256 m/z for dehydroproline and 57 m/z for n-tetracosane. A coefficient of variation of 5.8% was achieved and the limit of detection was calculated to be 0.233 micromol/l bone hydrolysate.

Comparison of different chloroformates for the derivatisation of seleno amino acids for gas chromatographic analysis

Three chloroformate reagents, ethyl chloroformate (ECF), methyl chloroformate (MCF) and menthyl chloroformate (MenCF), have been used for the derivatisation of seleno amino acids and their performance was compared. Chromatographic parameters and the inertness of the different instrumental configurations used (gas chromatography-atomic emission detection (GC-AED), and GC-MS) were shown to have a significant influence on the detection of various seleno amino acids (selenomethione, selenoethione and selenocysteine) and some sulphur-containing amino acids (methionine, cysteine, cystine and methylcysteine) which were included in the experiments for comparison. Methyl chloroformate was the preferred derivatisation reagent, since it generally performed best in terms of derivatisation yield and reproducibility and also showed less significant conditioning effects than ethyl chloroformate. Methyl and ethyl chloroformate derivatives of selenomethionine, selenoethionine, cysteine and methionine were detectable, while the detection of the menthyl chloroformate derivatives of selenocystine and cystine was not reproducible. Overall efficiencies for the determination of selenomethionine and selenoethionine from aqueous extracts ranged from 40 to 100% for methyl chloroformate, over 30-75% for ethyl chloroformate to 15-70% for menthyl chloroformate for different series measured over a period of months. The relative standard deviation of the method for the methyl and menthyl chloroformate derivatisation ranged from 7 to 13% without internal standard and was improved to 2% for the determination of selenomethionine using selenoethionine as internal standard. This indicates that, despite the limited reproducibility of the method, its repeatability is good enough to allow accurate determination of seleno amino acids, which was also demonstrated by the analysis of selenium supplementation tablets for human diet that contained selenomethionine.

Simple plasma work-up for a fast chromatographic analysis of homocysteine, cysteine, methionine and aromatic amino acids

Simplified sample workup obviating protein precipitation and eluent evaporation commonly employed in earlier reports using chloroformate-mediated derivatization of aminothiols prior to mass spectrometric (MS) detection is presented. The reduction of disulfides in plasma is accomplished with dithiothreitol within minutes. A simultaneous derivatization with ethyl chloroformate (ECF) and extraction of derivatives into organic phase takes place within seconds. Along with S-amino acids, also aromatic amino acids can be determined during a 5-min run. Gas chromatography with flame ionization detection (GC-FID) proved to be sensitive enough to reach plasma homocysteine levels. A prerequisite for a reliable quantitation was fulfilled under the given conditions. Intra-assay precision was <5%, recoveries from spiked plasma complete (101.2%), detection and quantitation limits for homocysteine came to <1 and 3 micro mol/l. Our results were in full agreement with those obtained by liquid chromatography (r=0.999 for homocysteine and 0.987 for cysteine), and were close to two homocysteine immunoassays (r=0.991 and 0.939, respectively).

Novel and traditional biomarkers of bone turnover in postmenopausal women

Urinary markers of bone collagen degradation such as N-terminal telopeptide, deoxypyridinoline, hydroxyproline and proline-hydroxyproline dipeptide as a novel marker were measured in both morning and daily urine samples collected from 40 postmenopausal women on 10 consecutive days. In addition, osteocalcin, bone alkaline phosphatase and procollagen C-terminal propeptide were analyzed in one serum sample from each of the women. DXA densitometry was employed to differentiate between osteopenic and osteoporotic women. Spearman rank correlation, factor analysis and receiver-operating characteristics were computed to evaluate the results. The highest correlation with a single common factor was found between telopeptide assayed in daily urine, followed by the dipeptide and hydroxyproline assayed in morning urine. These markers also showed high sensitivity for the diagnosis of osteoporosis, while osteocalcin had a high sensitivity for the diagnosis of bone disease. The concentrations of telopeptide, dipeptide and osteocalcin differed significantly between the osteopenic and osteoporotic subjects. Because of the ease of non-hydrolyzed urine processing, the speed of chromatographic analysis and the negligible costs, the dipeptide assay opens new possibilities in the field.