Long-chain fatty acids and alcohols from gerbil meibomian lipids

Toward an animal model of the human tear film: biochemical comparison of the mouse, canine, rabbit, and human meibomian lipidomes

PURPOSE

Secretions that are produced by meibomian glands (also known as meibum) are a major source of lipids for the ocular surface of humans and animals alike. Many animal species have been evaluated for their meibomian lipidomes. However, there have been a very small number of studies in which the animals were compared with humans side by side. Therefore, the purpose of this study was to compare meibum collected from humans and three typical laboratory animals, canines, mice, and rabbits, for their meibomian lipid composition in order to determine which animal species most resembles humans.

METHODS

High pressure liquid chromatography (HPLC) and gas-liquid chromatography (GLC) in combination with mass spectrometry were used to evaluate lipidomes of all tested species.

RESULTS

Among three tested animal species, mice were found to be the closest match to humans in terms of their meibomian lipidomes, while canines were the second closest species. The lipids of these three species were close to each other structurally and, for most lipid classes, quantitatively. The rabbit meibomian lipidome, on the other hand, was vastly different from lipidomes of all other tested species. Interestingly, a previously described class of lipids, acylated omega-hydroxy fatty acids (OAHFA), was found to be present in every tested species as the major amphiphilic component of meibum.

CONCLUSIONS

Our side by side comparison of the rabbit and the human meibum demonstrated their vast differences. Thus, the rabbit seems to be a poor animal model of the human tear film, at least when studying its biochemistry and biophysics.

Derivatization in mass spectrometry--1. Silylation

This is the first of a series of reviews on the application of derivatization in mass spectrometry. A description is given of advances in silylation as a powerful tool used for increasing the volatility, thermal and thermo-catalytic stability, and chromatographic mobility of polar and unstable organic compounds. In addition to chemical aspects of silylation, mass spectral properties of silyl derivatives useful for structure determination and quantitation of various organic and biologically-active compounds, mainly by GC/MS, are described. Practically all tested and widely used silylating agents are described. The role of comprehensive libraries containing reference mass spectra for various silyl derivatives and search systems in structure determination is emphasized. Applications of silylation for particular analyses are summarised.

Trace determination of 1-octacosanol in rat plasma by solid-phase extraction with Tenax GC and capillary gas chromatography

1-Octacosanol is the major component of policosanol, a new natural lowering-cholesterol agent. A sensitive solid-phase extraction with a Tenax GC capillary gas chromatography method for determining the proportion of this fatty alcohol in plasma after its denaturation with trichloroacetic acid was developed. The trimethylsilyl ether derivative of the target analyte obtained from the organic extract showed excellent chromatographic properties and was detectable in the low nanogram range (1 ng/ml). Adequate separation from plasma's extract was achieved with a fused-silica capillary column (30m x 0.25 mm I.D.) with SPB-5 (0.5 microm film thickness) and operated with temperature programming from 100 to 200 degrees C at 40 degrees C/min and from 200 degrees C increased at 10 degrees C/min to 320 degrees C, then held for 30 min, the carrier gas flow-rate (argon) was 1 ml/min. Quantification was performed by the internal standard method using 1-hexacosanol. The reliable relative retention parameters and the mass response factors values, and their confidence levels, ensure a proper GC sensitivity, necessary for the determination of the alcohol being analyzed. The method was evaluated to a concentration range from 6 to 47.6 ng/ml of plasma obtaining recoveries from 95 to 98%. The correlation between the theoretical concentration values and the corresponding experimental values was appropriate (gamma=0.9718 x -0.0915; r2=0.9998). The method showed a good within-day (RSD=4.3%) and between-day (RSD=6.0%) precision according to the acceptance criteria (<10%). This procedure was successfully applied to the study of 1-octacosanol in rat plasma samples after a single oral administration (40 mg/kg) of policosanol.

Analysis of alcohols, as dimethylglycine esters, by electrospray ionization tandem mass spectrometry

Dimethylglycine (DMG) esters are new derivatives for the rapid, sensitive and selective analysis of primary and secondary alcohols, in complex mixtures, by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Their development was inspired by the use of the complementary dimethylaminoethyl esters for the trace, rapid analysis of fatty acids. DMG esters are simply prepared by heating a dichloromethane solution of the imidazolide of dimethylglycine, containing triethylamine, and an alcohol. DMG esters of long-chain fatty alcohols, isoprenoidal alcohols and hydroxy-acids are analysed by electrospray ionization tandem mass spectrometry with a precursor ion of m/z 104 scan. Diols, glyceryl esters, glyceryl ethers and some sterols are analysed by a neutral loss of 103 Da scan. Trimethylglycine (TMG) ester iodides, prepared by alkylation of DMG esters with methyl iodide, are more sensitive derivatives for molecules containing secondary alcohol groups, such as cholesterol and gibberellic acid. They are analysed by a precursor ion of m/z 118 scan. DMG or TMG derivatives were shown to be at least comparable and sometimes an order of magnitude more sensitive than N-methylpyridyl ether derivatives for ESI-MS/MS analysis of the different classes of alcohols. Applications of these derivatives for the diagnosis of inherited disorders and the analysis of natural products are presented.

2-(4-Carboxyphenyl)-6-N,N-diethylaminobenzofuran: a useful reagent for the sensitive determination of alcohols by high-performance liquid chromatography with fluorimetric detection

A simple and highly sensitive method for the determination of short, medium and long-chain alcohols using high-performance liquid chromatography with fluorimetric detection is described. The alcohols were derivatized to their corresponding esters with (4-carboxyphenyl)-6-N,N-diethylaminobenzofuran. The esterification reaction proceeded rapidly and smoothly in acetonitrile at 60 degrees C with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (a coupling agent) in the presence of 4-dimethylaminopyridine (a base catalyst). The resulting esters of alcohols from methanol to eicosanol (C1-C20-ol) were separated on a reversed-phase column (Ultrasphere C8) with gradient elution (acetonitrile-water) and detected fluorometrically (excitation 387, emission 537 nm). The lower limits of detection (signal-to-noise ratio of 3) for the derivatized alcohols were in the range of 0.2-0.5 pg.

Application of triple quadrupole tandem mass spectrometry to the analysis of pyridine-containing derivatives of long-chain acids and alcohols

An improved derivatization procedure for the preparation of nicotinate and 3-picolinyl esters from mixtures of fatty alcohols and acids has been developed. The derivatives can be analysed by capillary gas chromatography on an SE-54 type column, which affords separation of the acid and alcohol derivatives with the same carbon chain. Detection with tandem mass spectrometric techniques on a triple quadrupole instrument is feasible, and yields informative spectra devoid of coeluting interferences.