Metabolite quantification: A fluorescence-based method for urine sample normalization prior to 1H-NMR analysis

INTRODUCTION

Metabolomics is a multi-discipline approach to systems biology that provides a snapshot of the metabolic status of a cell, tissue, or organism. Metabolomics uses mass spectroscopy (MS) and nuclear magnetic resonance (NMR) to analyze biological samples for low molecular weight metabolites.

OBJECTIVE

Normalize urine sample pre-acquisition to perform a targeted quantitative analysis of selected metabolites in rat urine.

METHODS

Urine samples were provided from rats on a control diet (n = 10) and moderate sucrose diet (n = 8) collected in a metabolic cage during an eight hour fast. Urine from each sample was prepared by two different methods. One sample was a non-normalized sample of 1200 µL and the second sample was a variable volume-normalized to the concentration of urobilin in a standard sample of urine. The urobilin concentration in all samples was determined by fluorescence. Ten metabolites for each non-normalized and normalized urine sample were quantified by integration to an internal standard of DSS.

RESULTS

Both groups showed an improvement in pH range going from non-normalized to normalized samples. In the group on the control diet, eight metabolites had significant improvement in range, while the remaining two metabolites had insignificant improvement in range comparing the non-normalized sample to the normalized sample. In the group on the moderate sucrose diet all ten metabolites showed significant improvement in range going from non-normalized to normalized samples.

CONCLUSIONS

These findings describe a pre-acquisition method of urine normalization to adjust for differences in hydration state of each organism. This results in a narrower concentration range in a targeted analysis.

Rapid and sensitive naked eye detection of faecal pigments using their enhanced solid-state green fluorescence on a zinc acetate substrate

The identification of trace faecal pigments in real-time and on-site detection remains a challenge for water quality monitoring. Herein, a simple, low-cost and rapid fluorescence-based analytical method has been developed in a solid matrix for faecal pigments like stercobilin and urobilin detection. This was made possible due to significant enhancement of green solid-state fluorescence (520 nm) by zinc(II) complexation with faecal pigments embedded in the surface of zinc acetate crystals. It enables naked-eye detection of these pigments even at a 10 μM level when excited with 365 nm blue-UV. It was demonstrated that easily available white cellulose paper strips or TLC silica plates coated with zinc acetate can be used as substrates. A photophysical study of solid-state faecal pigments-zinc(II) complexes suggests that green fluorescence enhancement results from the complexation, which can be attributed to the substantial decrease of the non-radiative decay rate (k_nr) as well as more efficient use of excitation light. The observation of reduced interference of humic acid fluorescence makes faecal pigment detection more efficient by this proposed method.

Photophysics of faecal pigments stercobilin and urobilin in aliphatic alcohols: introduction of a sensitive method for their detection using solvent phase extraction and fluorometry

Faecal pigments (FPs) are ubiquitous in the environment and are a primary contaminant in groundwater and surface water. This article presents a new analytical paradigm by a fluorescence coupled extraction-based method involving FP fluorescence enhancement and minimization of background fluorescence for high sensitivity detection. FPs show higher fluorescence intensity in aliphatic alcohols due to the breaking down of higher-order H-aggregates into lower-order H-aggregates (dimers). DFT studies using the B3LYP functional and LANL2DZ basis set show π-π stacking and hydrogen-bonding contributions towards forming H-aggregated dimers of FPs in the implicit and explicit solvent environments of 1-hexanol. This study is the first report on the extractability of FPs using 1-hexanol as an efficient extraction medium in comparison to higher-order aliphatic alcohols (1-butanol, 1-hexanol and 1-octanol). Furthermore, FP-Zn(II) complexes in 1-hexanol medium significantly enhance the fluorescence emission intensity (∼14-17 times), and the emission intensity remains stable over time. This further helps to increase the detection limit of FPs in the picomolar to sub-picomolar concentration range. This study proposes a protocol involving extraction of FPs by 1-hexanol followed by the complexation of FPs with Zn(II) in the alcohol media and subsequent fluorimetric detection of the FP-Zn(II) complex with a high level of sensitivity, enabled by reduced interference from the background fluorescence of humic acid. The complexation behaviour of FPs with various metal salts was also examined, which provided an understanding of the fluorescence behaviour of FPs with various other metal ions commonly present in natural environmental water. The proposed analytical method has been further validated using real water samples.

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis

A detailed photophysical study of two faecal pigments (FPs), Urobilin (UB) and Stercobilin (SB), and their zinc complexes [FP-Zn(II)] was carried out. The enhancement of UB and SB fluorescence resulting from the formation of their Zn(II) complexes was attributed to the complexation-induced rigidity of the chromophoric units, and the corresponding decrease of nonradiative decay rate constants of the excited singlet states (k_nr). The effect of various physicochemical environments was also studied in detail in order to understand the fluorescence behaviour of the Zn(II) complexes. FP-Zn(II) complexes have a lower solubility in water that results in the formation of molecular aggregates. The aggregation-induced loss of fluorescence of FP-Zn(II) complexes could be overcome by using the appropriate mixture of ethanol and water (70:30). Molecular orbital calculations on the FP-Zn(II) complexes provided a good idea of the geometry of the complexes and helped rationalise the enhancement of fluorescence after complexation. This study could pave the way towards developing a convenient non-extraction aqueous phase analytical procedure for detection of FPs using Zn(II) complexation method.

On the existence of a mono-vinyl d-urobilin

Chromic acid degradation of a d-urobilin, obtained after incubation of bilirubin in fecal bacterial cultures, gave methylvinylmaleimide and methylethylmaleimide. The d-urobilin, molecular weight 588, C(33)H(40)-N(4)O(6), clearly showed the presence of both vinyl and ethyl resonances in the nuclear magnetic resonance spectrum. These results point unambiguously to a urobilin structure with one vinyl and one ethyl beta-substituent.

The treatment of gout and disorders of uric acid metabolism with allopurinol

Allopurinol (4-hydroxypyrazolo (3,4-d)-pyrimidine) is a potent xanthine oxidase inhibitor which inhibits the oxidation of naturally occurring oxypurines, thus decreasing uric acid formation. The clinical and metabolic effects of this agent were studied in 80 subjects with primary and secondary gout and other disorders of uric acid metabolism. Allopurinol has been universally successful in lowering the serum uric acid concentration and uric acid excretion to normal levels, while not significantly affecting the clearance of urate or other aspects of renal function. Oxypurine excretion increased concomitantly with the fall in urine uric acid. The agent is particularly valuable in the management of problems of gout with azotemia, acute uric acid nephropathy and uric acid urolithiasis. The minor side effects, clinical indications and theoretical complications are discussed.