High-performance liquid chromatographic determination of free fatty acids with 1-naphthylamine

Quantification of carboxyl groups in carbodiimide cross-linked collagen sponges

Glutaraldehyde (GA) fixation of bioprosthetic tissue is a well adapted technique, with commercial products on the market for almost 40 years. Amine groups present in tissue react with GA to form different types of cross-links. An estimation of the degree of cross-linking of the tissue can be obtained by measuring the concentration of residual amine groups, which is frequently carried out with the 2,4,6 trinitrobenzene sulphonic acid (TNBS) assay. Cross-linked tissue and collagen matrices are usually further characterized by determining their physical properties (such as the shrinkage temperature), biological properties (such as resistance to enzymatic degradation), and mechanical properties before in vivo evaluation takes place. In an effort to improve the properties of cross-linked tissue and collagen, alternative cross-linking methods have been developed. One of these methods is based on the use of water soluble carbodiimides (CDI). It is generally accepted that this cross-linking method leads only to the formation of amide linkages between tissue carboxyl and amine groups. Therefore, until recently the TNBS assay was also used to determine the degree of cross-linking of CDI cross-linked tissue and collagen. However, it cannot be excluded that after activation of carboxyl groups of tissue and collagen by CDI, these groups can react with other nucleophiles (like hydroxyl groups) present in the matrix. To obtain a better insight in the degree of cross-linking of CDI cross-linked matrices a reliable assay for quantification of residual carboxyl groups is required. Up to now such an assay was not available. In this study a new assay to determine residual carboxyl groups in CDI cross-linked collagen matrices is presented. Reconstituted dermal bovine collagen matrices (RDBC) were cross-linked with a water soluble CDI and N-hydroxysuccinimide (NHS) and residual carboxyl groups were labeled using 5-bromomethyl fluorescein. Subsequently, the fluorescent label was released by mild hydrolysis and quantified with capillary zone electrophoresis. A calibration curve relating the concentration of carboxyl groups with peak intensities was obtained using Sephadex standards with known concentrations of carboxyl groups. The concentration of carboxyl groups in unprocessed RDBC as determined with this new technique was equal to the concentration of carboxyl groups measured by amino acid analysis. On the basis of the concentration of residual carboxyl groups determined for CDI/NHS cross-linked RDBC and RDBC, in which the amine groups were blocked with propionaldehyde before CDI/NHS cross-linking, it was concluded that activated carboxyl groups can also react with other groups (such as hydroxyl groups) present in the matrix. This implies that the crosslink density of RDBC matrices after treatment with CDI/NHS is higher than expected on the basis of amide bond formation only, as determined by the TNBS assay.

High-performance liquid chromatographic determination of polysorbate 80 in pharmaceutical suspensions

A quick HPLC method is reported for the analysis of polysorbate 80 in pharmaceutical suspensions. A typical pharmaceutical suspension was mixed with dilute potassium hydroxide, and heated at 40 degrees C for 6 h. This procedure resulted in quantitative hydrolysis of polysorbate 80 to release oleic acid. A quick HPLC procedure was used to analyze the hydrolyzed samples without further sample treatment. Polysorbate 80 USP, treated in the same way as the pharmaceutical suspensions, was used as standard. Full validation tests were carried out and the validation studies demonstrated that this method is suitable for accurate and reproducible analysis of polysorbate 80 in pharmaceutical suspensions.

Rapid quantitation of free fatty acids in human plasma by high-performance liquid chromatography

We report a rapid and sensitive method for separation and quantitation of free fatty acids (FFAs) in human plasma using high-performance liquid chromatography (HPLC). Two established techniques of lipid extraction were investigated and modified to achieve maximal FFA recovery in a reasonably short time period. A modified Dole extraction method exhibited greater recovery (approximately 90%) and short processing times (30 min) compared to the method of Miles et al. Reversed-phase HPLC using UV detection was used for plasma FFA separation and quantitation. Two phenacyl ester derivatives, phenacyl bromide and p-bromophenacyl bromide, were investigated in order to achieve optimal separation of individual plasma FFAs (saturated and unsaturated) with desirable detection limits. Different chromatographic parameters including column temperature, column type and elution profiles (isocratic and gradient) were tested to achieve optimal separation and recovery of fatty acids. Phenacyl bromide esters of plasma fatty acids were best resolved using an octadecylsilyl column with endcapped silanol groups. An isocratic elution method using acetonitrile-water (83:17) at 2 ml/min with UV detection at 242 nm and a column temperature of 45 degrees C was found to optimally resolve the six major free fatty acids present in human plasma (myristic [14:0], palmitic [16:0], palmitoleic [16:1], stearic [18:0], oleic [18:1] and linoleic [18:2]), with a run time of less than 35 min and detection limits in the nmol range. The entire process including plasma extraction, pre-column derivatization, and HPLC quantitation can be completed in approximately 90 min with plasma samples as small as 50 microl. Over a wide physiological range, plasma FFA concentrations determined using our HPLC method agree closely with measurements using established TLC-GC methods (r2 < or = 0.95). In addition, by measuring [14C] or [3H] radioactivity in eluent fractions following HPLC separation of plasma FFA, this method can also quantitate rates of FFA turnover in vivo in human metabolic studies employing isotopic tracers of one or more fatty acids.

An oxazine reagent for derivatization of carboxylic acid analytes suitable for liquid chromatographic detection using visible diode laser-induced fluorescence

This study reports the use of Nile Blue, an oxazine dye, as a derivatization reagent that fluoresces in the far-red spectral region and is suitable for derivatization with carboxylic-acid-containing analytes. Model carboxylic acid analytes such as benzoic acid, acetic acid, phenylacetic acid and hexanoic acid have been reacted as acid chlorides to form Nile Blue derivatives. The synthesis product of the Nile Blue benzoic acid derivative was confirmed using electrospray-mass spectrometry, infrared spectrometry, 1H and 13C nuclear magnetic resonance, reversed phase liquid chromatography (RP-HPLC), normal phase-thin layer chromatography, and spectral characterization. The synthesized Nile Blue derivatives, separated from reaction by-products with RP-HPLC, all demonstrated an approximately 10-fold drop in molar absorptivity and relative quantum yield. In addition, a 40 nm increase in Stokes shift was observed. A portion of the fluorescence was regained through post-column ionization of the Nile Blue benzoic acid derivative at pH 12. A RP-HPLC limit of detection of 88.25 fmol on column has been reported with conventional fluorescence detection-post-column ionization of the Nile Blue benzoic acid derivative. A limit of detection of 1.99 fmol on column (3.98 x 10(-11) M) has been demonstrated for the Nile Blue benzoic acid derivative with the use of a laboratory-constructed visible diode laser fluorescence detector.

Ultraviolet and fluorescence derivatization reagents for carboxylic acids suitable for high performance liquid chromatography: a review

Pre- and postcolumn derivatization with a suitable chromophore or fluorophore have often been utilized to enhance the sensitivity and selectivity of detection for analytes possessing a carboxyl group. The major classes of UV and fluorescent derivatization reagents include the coumarin analogues, alkyl halides, diazoalkanes, and amines. The derivatization reaction conditions, HPLC systems for separation of the conjugates and detection limits of various analytes are presented in this review. High mass sensitivity for various carboxylic acids have been achieved with the hydrazides, anthracenes, and diazoalkanes.

Determination of polyoxyethyleneglycerol triricinoleate 35 (Cremophor EL) in plasma by pre-column derivatization and reversed-phase high-performance liquid chromatography

A sensitive and selective reversed-phase high-performance liquid chromatographic (HPLC) method for the determination of polyoxyethyleneglycerol triricinoleate 35 (Cremophor EL; CrEL), which requires only microvolumes (20 microliters) of plasma, has been developed and validated. The procedure is based on saponification of CrEL in alcoholic KOH, followed by extraction of the released fatty acid ricinoleic acid with chloroform and derivatization with 1-naphthylamine. Margaric acid was used as the internal standard. The products are separated using an HPLC system consisting of an analytical column packed with Spherisorb ODS-1 material and a mobile phase of methanol-acetonitrile-10 mM potassium phosphate buffer pH 7.0 (72:13:15, v/v). Detection was executed by UV absorption at 280 nm. The lower limit of quantitation and the lower limit of detection in plasma are 0.01 and 0.005% (v/v) of CrEL, respectively. The percentage deviation and precision of the procedure, over the validated concentration range of 0.01 to 1.0% (v/v) of CrEL in plasma, are < or = 8.0% and < or = 6.6%, respectively. Compared to the previously described bioassay, the presented HPLC method possesses superior sensitivity and reliability. Preliminary pharmacokinetic studies of CrEL in mice and patients receiving paclitaxel formulated in CrEL have demonstrated the applicability of the presented assay.

Effects of obesity and ancillary variables (dialysis time, drug, albumin, and fatty acid concentrations) on theophylline serum protein binding

The effect of obesity on the serum protein binding of theophylline was investigated in man and rat along with other ancillary variables such as dialysis time, theophylline concentration, albumin concentration, and fatty acid type and concentration. The percent binding of theophylline first increased with dialysis time, reached equilibrium over 2 to 6 h, then diminished. This decrease was not due to instability of theophylline. Theophylline binding was linear over a concentration range of 15 to 150 micrograms ml-1. A similar degree of binding was found in normal humans (44.4 +/- 1.0%) and rats (41.5 +/- 0.5%). The binding ratio (bound/free) of theophylline was proportional to the albumin concentration (1 to 5%) and yielded a binding parameter (NK) of 1.47 x 10(-3) M-1. Over a normal physiological range, individual and mixed fatty acids had minimal effects on theophylline binding to albumin. However, binding significantly decreased as fatty acid (FFA) concentrations increased. The magnitude of the effect appeared to parallel the carbon chain number of the fatty acid. Theophylline binding in obese subjects decreased to a mean (SD) of 35.8 +/- 8.0 per cent compared to 43.0 +/- 6.1 per cent in normal subjects (p less than 0.05). Similar decreases were found in normal versus obese rats and in the saliva: serum ratio following theophylline administration to normal and obese human subjects. Obesity causes a moderate decrease in serum binding of theophylline which may be attributed to increased FFA rather than in vitro artifacts.

Determination of fatty acids as phenacyl esters in rat adipose tissue and blood vessel walls by high-performance liquid chromatography

Twenty-two biologically relevant (6:0-22:6) saturated, monounsaturated and polyunsaturated fatty acids were separated by reversed-phase high-performance liquid chromatography after derivatization with phenacyl bromide. An optimal resolution of the critical combinations linolenic-myristic, docosahexaenoic-palmitoleic-arachidonic and palmitic-oleic acids and cis and trans isomers of octadecenoic (n9) and octadecadienoic (n9, 12) acids was achieved by continuous gradient elution with methanol-acetonitrile-water. Elution of mixtures of 6:0-22:1 fatty acids was completed within 80 min at a flow-rate of 1 ml/min. By the use of UV detection at 242 nm the detection limits for short- and long-chain fatty acids were found to be about 0.8 and 12 ng per injection, respectively. Linearity was tested up to 100 ng. The method was applied to the determination of fatty acids in rat adipose tissue and blood vessel walls of animals fed hydrogenated fat diets. The results are comparable to those obtained by gas chromatography and surpass the latter for the resolution of oleic and elaidic acids.

Uptake of oleate by isolated rat adipocytes is mediated by a 40-kDa plasma membrane fatty acid binding protein closely related to that in liver and gut

A portion of the hepatocellular uptake of nonesterified long-chain fatty acids is mediated by a specific 40-kDa plasma membrane fatty acid binding protein, which has also been isolated from the gut. To investigate whether a similar transport process exists in other tissues with high transmembrane fatty acid fluxes, initial rates (Vo) of [3H]oleate uptake into isolated rat adipocytes were studied as a function of the concentration of unbound [3H]oleate in the medium. Vo reached a maximum as the concentration of unbound oleate was increased (Km = 0.30 +/- 0.03 microM; Vmax = 2470 +/- 90 pmol/min per 5 X 10(4) adipocytes) and was significantly inhibited both by phloretin and by prior incubation of the cells with Pronase. A rabbit antibody to the rat liver plasma membrane fatty acid binding protein inhibited adipocyte fatty acid uptake by up to 63% in dose-dependent fashion. Inhibition was noncompetitive; at an immunoglobulin concentration of 250 micrograms/ml Vmax was reduced from 2480 +/- 160 to 1870 +/- 80 pmol/min per 5 X 10(4) adipocytes, with no change in Km. A basic (pI approximately equal to 9.1) 40-kDa adipocyte plasma membrane fatty acid binding protein, isolated from crude adipocyte plasma membrane fractions, reacted strongly in both agar gel diffusion and electrophoretic blots with the antibody raised against the corresponding hepatic plasma membrane protein. These data indicate that the uptake of oleate by rat adipocytes is mediated by a 40-kDa plasma membrane fatty acid binding protein closely related to that in liver and gut.

Analysis of fatty acids as their anthrylmethyl esters by high-performance liquid chromatography with fluorescence detection

A reversed-phase high-performance liquid chromatographic method for the analysis of free fatty acids in plasma has been developed which allows the resolution of the major plasma fatty acids in man. A mixture of thirteen fatty acid anthrylmethyl esters is resolved on a Spherisorb 3-micron C8 column at a flow-rate of 1 ml/min by gradient elution. The solvent system consists of acetonitrile-water (93:7) for 12 min, followed by 5 min at acetonitrile-water (86:14) and 23 min at 100% acetonitrile. The eluent is monitored with a fluorescence detector (excitation 360 nm, emission 420 nm). Three different C18 columns were evaluated, and none were able to resolve all the esters. To optimize the separation on the C8 columns it was necessary to construct plots of log capacity factor vs. percent water in the mobile phase. The slope of the line for arachidonic acid (C20:4) was markedly different from that for the saturated acid derivatives. The method was applied to the analysis of plasma free fatty acids in normal and diabetic subjects. As expected, elevated levels of free fatty acids were found in diabetic subjects. The results were shown to correlate well with an established gas chromatographic assay.

Microanalysis of free fatty acids in plasma of experimental animals and humans by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method was developed for microanalysis of thirteen free fatty acids using 200 microliter of plasma. Fatty acids were derivatized with 9-anthryldiazomethane for HPLC analysis. Use of an ODS minicolumn for pretreatment of plasma gave a more accurate determination of free fatty acids in plasma than by chloroform extraction. Using this method, thirteen free fatty acids in the plasma of normal human, dog, rabbit, guinea pig and rat were determined.

Hepatocellular uptake of oleate is energy dependent, sodium linked, and inhibited by an antibody to a hepatocyte plasma membrane fatty acid binding protein

Several studies suggest that a portion of hepatocellular nonesterified fatty acid uptake may be carrier mediated. To further investigate this process, initial rates (Vo) of [14C]oleate uptake into rat hepatocytes, isolated by collagenase perfusion and incubated at 37 degrees C with oleate in the presence of bovine serum albumin, were studied as a function of the concentration of unbound [14C]oleate in the medium. Vo was saturable with increasing unbound oleate concentration (Km = 8.3 X 10(-8) M; Vmax = 197 pmol per min per 5 X 10(4) hepatocytes) and was not inhibited by up to 40 microM sulfobromophthalein, taurocholate, or cholic acid. Oleate uptake was sodium dependent. Vo was significantly diminished when Li+, K+, choline, or sucrose were substituted for Na+ in the incubation medium and was reduced 46% by 1 mM ouabain. Uptake was also markedly reduced after exposure of cells to metabolic inhibitors (e.g., 2,4-dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone, antimycin, KCN). To evaluate the physiologic significance of the previously isolated rat liver plasma membrane fatty acid-binding protein, the effect of an antibody directed against this protein on hepatocellular [14C]oleate uptake was examined. Preincubation of hepatocytes with the IgG fraction of this antiserum inhibited Vo of [14C]oleate by up to 65% in dose-related fashion, without altering Vo for [35S]sulfobromophthalein, [14C]taurocholate, or [3H]cholate. These data indicate that at least a portion of hepatocellular oleate uptake is energy dependent, sodium linked, and mediated by a specific liver plasma membrane-fatty acid-binding protein.

A comparison of methods for the high-performance liquid chromatographic and capillary gas-liquid chromatographic analysis of fatty acid esters

A reversed-phase high-performance liquid chromatographic (RP-HPLC) method for the analysis of free fatty acids in plasma was compared with a method using capillary gas-liquid chromatography (GLC). The same extraction procedure was used for both assays. In the RP-HPLC method, the acids were separated as their anthrylmethyl esters on a C18 reversed-phase column, and detected by fluorescence. The coupling agent 2-bromo-1-methylpyridinium iodide was used with 9-(hydroxymethyl)anthracene. A mobile phase of acetonitrile-water (98:2) was used with flow programming. The derivatives of the C14:0, C16:1 and C18:2 acids could not be fully resolved. For capillary GLC, the acids were separated as their methyl esters following on-column injection into a 25-m OV-101 glass capillary column and detected using flame ionization detection. The esterifying agent used was diazomethane. The C18:2 and C18:3 esters were not fully resolved. The precision and sensitivity of both methods were similar. In an application of the methods, the free fatty acid concentrations in the plasma of a group of diabetic patients and their age-matched controls were estimated. Fatty acid concentrations tended to be higher in the diabetic group but, in the small number of patients studied, wide inter-individual variations prevented a significant difference from being detected. Estimates of individual fatty acids were higher by the RP-HPLC method. The identity of the acids in the extract was confirmed by gas chromatography-mass spectrometry of their methyl esters.

Highly sensitive determination of free fatty acids in human serum by high-performance liquid chromatography with fluorescence detection

A highly sensitive and rapid high-performance liquid chromatographic method for the determination of free fatty acids in human serum is described. The fatty acids are converted into the corresponding fluorescent derivatives by the reaction with 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone in the presence of potassium carbonate and 18-crown-6 in acetonitrile. The derivatives are separated simultaneously within 44 min on a reversed-phase column (YMC-Pack C8) with a gradient elution of aqueous methanol and detected fluorimetrically. The detection limits are 0.5-2 fmol in a 10-microliters injection volume. This sensitivity permits precise determination of free fatty acids including lauric, myristoleic and linolenic acids, which occur in serum at very low concentrations, in 5 microliters of sera from healthy subjects and patients with diabetes.

Liquid chromatography/mass spectrometry of fatty acids as their anthrylmethyl esters

The mass spectra of a series of saturated and unsaturated fatty acids have been recorded as their anthrylmethyl esters using a liquid chromatographic mass spectrometric interface. The spectra show an intense peak for the aromatic nucleus, and a molecular ion. The liquid chromatographic/mass spectrometric separation was performed on a reverse phase column using a solvent system of acetone + acetonitrile. While a complete separation of the fatty acids known to occur in man was not achieved, the recognition of all of these acids is possible using a scanning mode or by ion monitoring.

Simultaneous separation and sensitive determination of free fatty acids in blood plasma by high-performance liquid chromatography

Fatty acids are separated by reversed-phase high-performance liquid chromatography after derivatization with a fluorescence reagent, 4-bromomethyl-7-acetoxycoumarin. Each derivative eluted from a column is successively hydrolysed by mixing it with an alkaline solution, and the produced fluorescence is detected. The derivatives of series of both saturated and unsaturated fatty acids (C6:0--C20:4) are simultaneously separated by a continuous gradient elution method using a methanol-based solvent containing acetonitrile. The quantitative detection of fatty acids is over a range of 5-1000 pmol per derivatization mixture. This method is applicable to the quantitative analysis of free fatty acids in normal human blood samples and blood samples from diabetic patients. Ten microliters of blood plasma are sufficient to carry out the determination. The analytical results show good recovery and good reproducibility. This sensitive method is very useful for the analysis of fatty acids in very low concentrations.

Fluorometric high-performance liquid chromatography of 9-aminophenanthrene-derivatized free fatty acids

The application of 9-aminophenanthrene (9-AP), a fluorescence-labeling reagent for free fatty acids (FFA), was examined. 9-AP dissolved in benzene was added to a benzene solution of FFA chlorides derived from FFA and oxalyl chloride. The mixture was allowed to react for 45 min at 70 degrees C. By the method, 9-AP-tagged FFA with a strong fluorescence was formed. The materials thus obtained have a lambda max at around 303 nm for excitation and 376 nm for emission. By using this derivatization method, recoveries were measured for seven kinds of FFA added to 0.5 ml of healthy human serum. Significant recoveries ranging from 96 to 107% (coefficient of variation 1.4-5.0%) were obtained for each FFA. The proposed method was clinically applied to the determination of FFA in 0.5 ml of healthy human serum, and almost satisfactory results were obtained. Detection limits of FFA by this derivatization method were 10 pmol for C14:0, C16:0, C16:1, C18:1 and C18:2, and 15 pmol for C18:0 and C20:4. As a quantitative measurement of FFA, gas chromatography and high-performance liquid chromatography with fluorescence detection, which have been routinely used, were chosen for comparison with the present method.