Phosphatic metabolites in keratoconus

The 31P Spectral Modulus (PSM) as an Assay of Metabolic Status

The phosphorus-31 (31P) spectral modulus (PSM) is a measure of the metabolic status of cells, tissues, and organs. The PSM can be calculated from 31P nuclear magnetic resonance (31P NMR) spectra obtained from cell, tissue, or organ preparations. These 31P NMR spectra can be a measure of intact living cells, tissues, or organs, or appropriate biochemical extracts of such preparations. The 31P NMR spectrum is comprised of signals derived from organophosphate metabolites that resonate from 10 δ to -25 δ on the phosphorus chemical shift δ scale. The PSM is the ratio of the high-energy phosphate to that of the low-energy phosphate spectral integrals. These integrals may be conveniently grouped into high-energy and low-energy spectral regions, respectively, into 31P chemical shifts located between -0.13 δ to -25 δ and between 10 δ to -0.13 δ. High-energy phosphates are typically described as providing the energy necessary for the activity of cellular metabolism; chemically, they contain one or more phosphate anhydride bonds. This study demonstrates that, (1) in general, the higher the metabolic activity, the higher the PSM, and (2) the modulus calculation does not require a highly resolved 31P spectrum and can be calculated solely from the integral. The PSM was calculated among cells, tissues, and organs considered normal, diseased, and stressed. In diseased (mean 1.29 ± 0.73) and stressed (mean 1.23 ± 0.75) cells, tissues, and organs, PSM values are typically low or low relative to normal cells, tissues, or organs (mean 1.65 ± 0.90), following time-course measurements, in dynamic decline. The PSM is useful in determining the metabolic status of cells, tissues, or organs and can be employed as a calculable numeric assay for determining health status statically or over time. Calculation of the PSM can be carried out with spectra of low signal-to-noise; it relies on the minimal resolution required to detect an integral curve having a clear spectral integral inflection point at ca. -0.13 δ. Detection of an integral curve alone enables the calculation of a PSM even at levels of phosphorus concentration so low as to prevent detection of the individual or groups of metabolites, such as with in vivo or ex vivo cell, tissue, or organ determinations. This study (1) presents the foundations and fundamentals of the PSM, a living index of tissue metabolic health, and (2) demonstrates the use of spectral scan analysis in opening new vistas of biology and medicine for measuring the metabolic status of stressed and diseased tissues at a range of detectable levels for monitoring therapeutic interventions.

ATP, the 31P Spectral Modulus, and Metabolism

Adenosine triphosphate (ATP) has a high intracellular millimolar concentration (ca. 2.4 mM) throughout the phylogenetic spectrum of eukaryotes, archaea, and prokaryotes. In addition, the function of ATP as a hydrotrope in the prevention of protein aggregation and maintenance of protein solubilization is essential to cellular, tissue, and organ homeostasis. The 31P spectral modulus (PSM) is a measure of the health status of cell, tissue, and organ systems, as well as of ATP, and it is based on in vivo 31P nuclear magnetic resonance (31P NMR) spectra. The PSM is calculated by dividing the area of the 31P NMR integral curve representing the high-energy phosphates by that of the low-energy phosphates. Unlike the difficulties encountered in measuring organophosphates such as ATP or any other phosphorylated metabolites in a conventional 31P NMR spectrum or in processed tissue samples, in vivo PSM measurements are possible with NMR surface-coil technology. The PSM does not rely on the resolution of individual metabolite signals but uses the total area derived from each of the NMR integral curves of the above-described spectral regions. Calculation is based on a simple ratio of the high- and low-energy phosphate bands, which are conveniently arranged in the high- and low-field portions of the 31P NMR spectrum. In practice, there is essentially no signal overlap between these two regions, with the dividing point being ca. -3 δ. ATP is the principal contributor to the maintenance of an elevated PSM that is typically observed in healthy systems. The purpose of this study is to demonstrate that (1) in general, the higher the metabolic activity, the higher the 31P spectral modulus, and (2) the modulus calculation does not require highly resolved 31P spectral signals and thus can even be used with reduced signal-to-noise spectra such as those detected as a result of in vivo analyses or those that may be obtained during a clinical MRI examination. With increasing metabolic stress or maturation of metabolic disease in cells, tissues, or organ systems, the PSM index declines; alternatively, with decreasing stress or resolution of disease states, the PSM increases. The PSM can serve to monitor normal homeostasis as a diagnostic tool and may be used to monitor disease processes with and without interventional treatment.

Phospholipid analyses of rabbit ocular surface tissues

The tissues of the integument covering the ocular surface comprise a mucus membrane functioning as a protective physical barrier and has the ability to mount a defensive inflammatory response. Since lipid metabolism has a role in both of these functions, we studied normal membrane phospholipids (PL) of the cornea and bulbar conjunctiva to (1) determine baseline PL profiles of these tissues, (2) compare and contrast these individual PL metabolite profiles as well as groups of metabolites, and (3) describe pathway-specific metabolic interrelations among these tissues. Corneal and conjunctival tissue samples were isolated from rabbit eyes (n = 30) and extracted with chloroform-methanol using a modified Folch procedure. 31P nuclear magnetic resonance spectroscopy was used to qualitatively and quantitatively measure tissue PL profiles. The cornea and conjunctiva, respectively, have the following PL composition (mole % of total detected phospholipid): phosphatidylglycerol (PG) -, 0.4; lysophosphatidylethanolamine 1.2, -; phosphatidic acid -, 0.4; diPG (cardiolipin) 2.1, 3.5; unknown PL at the chemical shift of 0.13 δ 1.5, 0.9; ethanolamine plasmalogen 11.2, 13.0; phosphatidylethanolamine 11.5, 12.8; phosphatidylserine 8.9, 10.1; sphingomyelin 10.2, 10.7; lysophosphatidylcholine 0.9, 1.4; phosphatidylinositol 5.3, 5.3; phosphatidylcholine (PC) plasmalogen or alkylacylPC 2.2, 1.9; PC 45.1, 40.0. In addition, 28 PL metabolic indices were calculated from these data, which permitted pathway-specific lipid analyses. This study (1) establishes PL profiles of the two ocular tissues of the integument that cover the surface of the eye, (2) compares and contrasts indices comprised of ratios and combinations of PL, and (3) describes pathway-specific metabolic interrelations among these tissues to serve as baselines for studies involving the distribution of tissue phospholipids.

Metabolomic Signature Discriminates Normal Human Cornea from Keratoconus-A Pilot GC/MS Study

The molecular etiology of keratoconus (KC), a pathological condition of the human cornea, remains unclear. The aim of this work was to perform profiling of metabolites and identification of features discriminating this pathology from the normal cornea. The combination of gas chromatography and mass spectrometry (GC/MS) techniques has been applied for profiling and identification of metabolites in corneal buttons from 6 healthy controls and 7 KC patients. An untargeted GC/MS-based approach allowed the detection of 377 compounds, including 46 identified unique metabolites, whose levels enabled the separation of compared groups of samples in unsupervised hierarchical cluster analysis. There were 13 identified metabolites whose levels differentiated between groups of samples. Downregulation of several carboxylic acids, fatty acids, and steroids was observed in KC when compared to the normal cornea. Metabolic pathways associated with compounds that discriminated both groups were involved in energy production, lipid metabolism, and amino acid metabolism. An observed signature may reflect cellular processes involved in the development of KC pathology, including oxidative stress and inflammation.

Corneal Cryopreservation Using Glycerylphosphorylcholine-Enriched Medium

PURPOSE

To determine the effects of prolonged cryopreservation at subzero-degree temperatures on corneal transparency and histology after treatment with preservation medium containing the phosphodiester glycerylphosphorylcholine (GPC).

METHODS

Rabbit corneas (n = 30) were immersed for 3 hours in K-Sol preservation medium containing 30 mM GPC. Three groups with 6 corneas each were refrigerated at -8°C for 2 weeks and liquid nitrogen temperature for 2 and 6 weeks, respectively. Two groups with 6 corneas each immersed in K-Sol preservation medium only were refrigerated at -8°C for 2 weeks and liquid nitrogen temperature for 6 weeks, respectively. Postthawing corneal transparency was measured on a grading scale after which corneas were prepared for and analyzed by light and transmission electron microscopy.

RESULTS

All 3 groups of corneas preserved with GPC maintained a greater degree of corneal transparency compared with corneas preserved without GPC. The number of corneas retaining epithelial and endothelial layers increased in all groups where corneas were preserved in medium containing GPC, in contrast to corneas preserved in medium without GPC. Cytoplasmic vacuolization or nuclear damage was greater in corneas preserved without GPC. Similar findings were found in corneas stored at -8°C and liquid nitrogen temperatures.

CONCLUSIONS

This study demonstrates a cryoprotective effect of corneas preserved in K-Sol containing the phosphodiester GPC at subzero-degree temperatures. In corneas immersed in preservation medium containing GPC, a higher degree of transparency is maintained and a lesser degree of histopathologic changes is observed with storage at both -8°C and in liquid nitrogen.

Metabolic profile of keratoconic cornea

PURPOSE

To investigate the difference in metabolic profile of keratoconic and normal corneas using two different analysis methods.

METHODS

Keratoconic corneas were obtained from patients (aged 19-27) during transplantation surgery. Control samples were obtained from human donors (aged 61-75) 1-8 h post-mortem. The metabolic profile of tissues was investigated with high-resolution magic angle spinning (1)H nuclear magnetic resonance (NMR) spectroscopy and high performance liquid chromatography (HPLC).

RESULTS

Nine amino acids and 20 metabolites were assigned with HPLC and NMR spectroscopy, respectively. No significant biochemical difference was revealed between keratoconic and control samples, which represent distant age groups.

CONCLUSIONS

It suggests that development of keratoconus might be related to the accelerated ageing of the cornea. This issue warrants further studies.

Metabonomics--a new approach in ophthalmology

Metabonomics is a new technology providing broad information about dynamic metabolic responses in living systems to pathophysiological stimuli or genetic modification. Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful methods in metabonomics; it is utilized to establish the metabolic profiles of biofluids, and is practically the only method capable of examining intact tissue samples. Experience with the application of metabonomics in eye research is still limited, yet this method provides the possibility of exploring metabolic processes in the eye in vivo. This article presents a brief background to the usefulness of metabonomics, and the possible applications of an NMR-based technique in eye research and clinical practice.

Phosphatic intermediate metabolites of the porcine ocular tunica fibrosa

This study compares porcine scleral and corneal phosphorylated intermediate metabolites determined using 31P NMR. These tissues comprise the tunica fibrosa (outer coat) of the eye. Since the sclera does not possess epithelia as does the cornea, comparative analysis of these tissues included examination of the cornea with and without its epithelia. The phosphorylated intermediates detected include: dihydroxyacetone phosphate, hexose 6-phosphates, alpha-glycerol phosphate, beta-glycerol phosphate, ethanolamine and choline phosphates, nucleoside mono-, di-, and tri-phosphates, inorganic orthophosphate, glycerol 3-phosphoryl-ethanolamine and -choline, phosphoglycans, phosphocreatine, nucleoside diphosphosugars, dinucleotides, and four uncharacterized (unknown) signals. Metabolic indices, comprised of individual or grouped metabolites, were calculated to further compare and contrast metabolites and to provide more pathway specific metabolic interrelations for phosphorylated metabolites. Significant differences exist between the corneal stroma and the sclera in 16 of the 22 phosphorylated metabolites determined, whereas differences exist between the whole cornea and the sclera in six of the 22 phosphorylated metabolites. Considering all metabolite levels and metabolic indices in aggregate, the sclera is most similar to the whole cornea and least similar to the corneal stroma.

P-31 NMR analysis of phospholipids from cultured human corneal epithelial, fibroblast and endothelial cells

Corneal epithelial, fibroblast and endothelial cells, cultured from human donors, were analyzed to determine their characteristic phospholipid profiles by 31P NMR. Tissue phospholipid profiles from epithelial, fibroblast and endothelial cell cultures were evaluated to differentiate the individual cell types and to identify resonances that typically appear in high-resolution phospholipid profiles of whole corneas. Phosphatidylcholine, phosphatidylethanolamine plasmalogen, an uncharacterized phospholipid at 0.13 delta, phosphatidylinositol, phosphatidylserine and sphingomyelin were determined to be, in decreasing order of concentration, the major phospholipids detected in these three cultured corneal cell types. Indices of phospholipid metabolism representing total plasmalogen content, total choline-containing lipids and the total choline-containing lipids less those synthesized through the plasmalogen pathway were found to differentiate the three cell types. Minor phospholipids cardiolipin, lysophosphatidylcholine, phosphatidylethanolamine, lysophosphatidylcholine (LPC) and LPC plasmalogen not usually reported in studies of corneal phospholipids using other techniques, were useful in discriminating between cell types. Phospholipid profiles of the whole cornea provide important information concerning the biochemistry and pathology of the tissue, however, phospholipid analysis of individual components of the cornea, such as the epithelial, fibroblast and endothelial cells, makes it possible to understand the contribution of specific cellular constituents to the spectral information obtained from the whole cornea.