Dual-labeled technique for human lipid metabolism studies using deuterated fatty acid isomers

Stable isotope methods for the in vivo measurement of lipogenesis and triglyceride metabolism1,2

Synthesis of fatty acids (via de novo lipogenesis) and triglycerides are important factors in fat accumulation and the efficiency of animal production. Recently, new stable isotope methods using heavy water (2H2O) have made possible the safe, and relatively easy, measurement of both of these processes in vivo in animals and humans over prolonged periods. These methods also provide information on the relative contribution of glycolysis and glyceroneogenesis to triglyceride synthesis under different physiological settings. The data suggest that numerous dietary factors, including nutrient composition and caloric content, may affect de novo lipogenesis. Significant differences in de novo lipogenesis have also been seen across species and in different tissues. The rates of triglyceride synthesis have been shown to be affected by diet and to differ significantly between different adipose depots, with metabolically active depots (e.g., visceral fat) having much more rapid triglyceride turnover than subcutaneous depots. Dietary fat and the peroxisome proliferator-activated-gamma agonist rosiglitazone have both been shown to influence triglyceride synthesis rates and to increase glyceroneogenesis. A significant portion of triglyceride synthesis is not related to triglyceride accumulation but rather is secondary to active lipolysis and reesterification. The application of these new techniques to animals other than rodents will undoubtedly enhance our understanding of adipose tissue biology and could lead to new methods for improving animal production.

Simultaneous quantitative determination of deuterium- and carbon-13-labeled essential fatty acids in rat plasma

This study reports methods for the quantitative determination of stable isotope-labeled essential fatty acids (EFAs) as well as an experiment in which deuterium-labeled linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3) were compared with those labeled with carbon-13 in rat plasma in vivo. Standard curves were constructed to compensate for concentration and plasma matrix effects. It was observed that endogenous pools of fatty acids had a greater suppressing effect on the measurements of 13C-U-labeled EFAs relative to those labeled with 2H5. Using these methods, the in vivo metabolism of orally administered deuterated-linolenate, 13C-U-labeled linolenate, deuterated-linoleate, and 13C-U-labeled linoleate was compared in adult rats (n = 11). There were no significant differences in the concentrations of the 2H versus 13C isotopomers of 18:2n-6, 18:3n-3, arachidonic acid (20:4n-6), and docosahexaenoic acid (22:6n-3) in rat plasma samples at 24 h after dosing. Thus, there appears to be little isotope effect for 2H5- versus 13C-U-labeled EFAs when the data are calculated using the conventional standard curves and corrected for endogenous fatty acid pool size and matrix effects.

In vivo conversion of 18- and 20-C essential fatty acids in rats using the multiple simultaneous stable isotope method

An important question for mammalian nutrition is the relative efficiency of C18 versus C20 essential fatty acids (EFAs) for supporting the tissue composition of n-3 and n-6 pathway end products. One specific question is whether C22 EFAs are made available to tissues more effectively by dietary alpha-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) or by dietary eicosapentaenoic acid (20:5n-3) and dihomo-gamma-linolenic acid (20:3n-6). To address this question in a direct manner, four stable isotope compounds were given simultaneously in a novel paradigm. A single oral dose of a mixture of 2H5-18:3n-3, 13C-U-20:5n-3, 13C-U-18:2n-6, and 2H5-20:3n-6 was administered to rats given a defined diet. There was a preferential in vivo conversion of arachidonic acid (20:4n-6) to docosatetraenoic acid (22:4n-6) and of 22:4n-6 to n-6 docosapentaenoic acid (22:5n-6) when the substrates originated from the C18 precursors. However, when the end products docosahexaenoic acid (22:6n-3) or 22:5n-6 were expressed as the total amount in the plasma compartment divided by the dosage, this parameter was 11-fold greater for 20:5n-3 than for 18:3n-3 and 14-fold greater for 20:3n-6 than for 18:2n-6. Thus, on a per dosage basis, the total amounts of n-3 and n-6 end products accreted in plasma were considerably greater for C20 EFA precursors relative to C18.

Influence of dietary arachidonic acid on metabolism in vivo of 8cis,11cis,14-eicosatrienoic acid in humans

This study investigated the influence of dietary arachidonic acid (20:4n-6) on delta 5 desaturation and incorporation of deuterium-labeled 8cis,11 cis, 14-eicosatrienoic acid (20:3n-6) into human plasma lipids. Adult male subjects (n = 4) were fed diets containing either 1.7 g/d (HI20:4 diet) or 0.21 g/d (LO20:4 diet) of arachidonic acid for 50 d and then dosed with a mixture containing ethyl esters of 20:3n-6[d4] and 18:1n-9[d2]. A series of blood samples was sequentially drawn over a 72-h period, and methyl esters of plasma total lipid, triacylglycerol, phospholipids, and cholesteryl ester were analyzed by gas chromatography-mass spectrometry. Based on the concentration of 20:3n6[d4] in total plasma lipid, the estimated conversion of 20:3n-6[d4] to 20:4n-6[d4] was 17.7 +/- 0.79% (HI20:4 diet) and 2.13 +/- 1.44% (LO20:4 diet). The concentrations of 20:4n-6[d4] in total plasma lipids from subjects fed the HI20:4 and LO20:4 diets were 2.10 +/- 0.6 and 0.29 +/- 0.2 mumole/mL plasma/mmole of 20:3n-6[d4] fed/kg of body weight. These data indicate that conversion of 20:3n-6[d4] to 20:4n-6[d4] was stimulated 7-8-fold by the HI20:4 diet. Phospholipid acyltransferase was 2.5-fold more selective for 20:3n-6[d4] than 18:1n-9[d2], and lecithin:cholesterol acyltransferase was 2-fold more selective for 18:1n-9[ds] than 20:3n-6[d4]. These differences in selectivity were not significantly influenced by diet. Absorption of ethyl 20:3n-6[d4] was about 33% less than ethyl 18:1n-9[d2]. The sum of the n-6 retroconversion products from 20:3n-6[d4] in total plasma lipids was about 2% of the total deuterated fatty acids. Neither absorption nor retroconversion appears to be influenced by diet.

Measurement of the metabolic interconversion of deuterium-labeled fatty acids by gas chromatography/mass spectrometry

An analytical method that was developed to analyze deuterium-labeled fatty acids in human blood has been extended to identify labeled fatty acids from C14 to C24 chain length which are formed by metabolic processes such as desaturation, elongation, or shortening of the labeled fatty acids fed. A new computer and a hardware adder have been utilized to assure reliable data acquisition. Relative standard deviations for the analysis of labeled fatty acids were measured at 0.02, 0.03, and 0.04 at the 5%, 1%, and 0.2% levels of the labeled fatty acid methyl esters, respectively. The method makes extensive use of standards and computer processing for accuracy and high productivity. Data from a chylomicron triacylglycerol fraction are included to demonstrate the sensitivity of detection of metabolites formed by desaturation and elongation.

Simultaneous analysis of low plasma levels of deuterium-labeled saturated and unsaturated fatty acids as t-butyldimethylsilyl esters

A sensitive and accurate method for detection and quantitation of deuterated fatty acids in the presence of large amounts of unlabeled fatty acids is described using mass fragmentography in combination with the preparation of tertiarybutyldimethylsilyl esters (t-BDMS). The method has been applied to determination of deuterated stearic, oleic, elaidic and linoleic acids in human plasma lipoproteins following duodenal perfusion with a micellar mixture of acids. Over a concentration range of 10-1000 ng/ml, the average coefficient of variation for the linoleate was 3% and for the oleate (elaidate) ester was 2%.

Analysis of deuterium labeled blood lipids by chemical ionization mass spectrometry

A quantitative analytical method has been developed to analyze methyl esters of blood fatty acids derived from human subjects fed deuterium-labeled fats. The GCMS computer method provides for the analysis of the fed deuterium-labeled fatty acids, the naturally occurring blood fatty acids and new fatty acids formed by chain elongation or shortening of the fed labeled fats. Approximately 20 fatty acids including 16, 17, 18 and 20 carbon chain acids were analyzed with a relative standard deviation of 0.02 at the microgram level and a sensitivity of less than one nanogram. The method uses capillary GC to separate the fatty acid esters and isobutane chemical ionization mass spectrometry with multiple ion detection to determine the isotopic constituents of the GC peaks. The technique provides for the determination of overlapping GC peaks labeled with 2, 4 and 6 deuterium atoms and makes extensive use of computers both for data acquisition and processing.

Selective deposition of trans-8- and cis-9-octadecenoates in egg and tissue lipids of the laying hen

The disposition of trans-8-octadecenoate-8(9)-3H (8t-18:1-3H) was compared to cis-9-octadecenoate-10-14C (9c-18:.1-14C) in the major egg yolk neutral lipids and in organ lipids from the laying hen. trans-8-Octadecenoate was preferentially incorporated into only the phosphatidylethanolamines (PE), whereas discrimination against 8t-18:1-3H occurred in the phosphatidylcholines (PC), triglycerides (TG) and cholesteryl esters (CE). The a-acyl position of both PE and PC from the 2-acyl position of these phospholipids was found. Preferential incorporation of 9c-18:1-14C occurred at the combined 1- and 3-acyl positions and at the 2-acyl position of yolk TG. Tissue lipid analyses indicated that there was preferential deposition of 9c-18:1-14C into all organs. Individual liver lipid classes displayed the same relative order of discrimination against 8t-18:1-3H as did egg yolk lipids (CE greater than TG greater than PC greater than PE).

Competitive deposition of trans-12 and cis-9-octadecenoates into egg yolk lipids

The deposition of trans-12-octadencenoate-12(13)-3H (12t-18:1-3H) was compared to cis-9-octadecenoate-10-14C (9c-18:1-14C) in the major egg yolk neutral lipids and phospholipids trans-12-Octadecenoate was preferentially incorporated into cholesteryl esters (CE), phosphatidylcholines (PC), and phosphatidylethanolamines (PE) but was discriminated against in triglycerides (TG). Isotopic ratios indicate that 5.9 and 5.6 times more 12t-18:1-3H than 9c-18:1-14C was esterified at the 1-acyl position of PE and PC, respectively. The combined 1- and 3-acyl positions of TG and the 2-acyl position of TG, PE and PC were each preferentially esterified with 9c-18:1-14C.

Distribution of deuterium-labeled cis- and trans-12-octadecenoic acids in human plasma and lipoprotein lipids

Triglycerides containing cis- and trans-12-octadecenoic acid (12c-18:1 and 12t-18:1) and cis-9-octadecenoic acid (9c-18:1) labeled with deuterium were fed to 2 young adult male subjects. These fatty isomers each contained a different number of deuterium lables, which allowed mass spectrometric analysis to distinguish among them after they were fed as a mixture. This approach results in a direct comparison of the absorption and distribution of these 3 monenoic acids into blood plasma and lipoprotein lipids. Plasma lipid data indicated that all phospholipid fractions selectively incorporate 12c-18:1 and 12t-18:1 in preference to 9c-18:1. Discrimination against 12c-18:1 and 12t-18:1 compared to 9c-18:1 was found in the plasma neutral lipids, with a strong discrimination against 12t-18:1 incorporation into the cholesteryl ester fraction. Considerable reduction in the percentage of linoleic and arachidonic acid was observed when 12-18:1 isomers were incorporated in plasma triglyceride, phosphatidylcholine and spingomyelin samples. Chylomicron lipid analyses indicated that all isomers were well absorbed. Variation was observed in the relative distribution of 12c-18:1, 12t-18:1 and 9c-18:1 between the very low density, low density and high density lipoprotein lipid classes. No desaturation of 12c-18:1 to linoleic acid was detected.

Selective ion monitoring in clinical chemistry

The principles of selective ion monitoring are described. Choice of instrumentations, derivatives, and internal standards is discussed. The most important factors influencing sensitivity, specificity, and precision are summarized. Applications of selective ion monitoring for quantitative assay of steroids, fat soluble vitamins, triglycerides, prostaglandins, biogenic amines, amino acids, carbohydrates, and several other organic compounds of clinical interest are critically reviewed. It is concluded that isotope dilution selective ion monitoring is one of the most sensitive and accurate techniques presently available for quantitation of a large number of endogenous compounds of clinical interest. In view of the high accuracy, the technique is useful not only for the clinical chemists dealing with determination of specific compounds which are difficult to analyze by other methods, but also for those dealing with quality control of routine analyses of simple organic compounds.

Incorporation of deuterium-labeled cis- and trans-9-octadecenoic acids in humans: plasma, erythrocyte, and platelet phospholipids

The objective of this study was to follow the uptake and distribution of oleic and elaidic acids into human erythrocytes, platelets, and plasma phospholipids. The use of dual and triple labeling methodology permitted a precise comparison of elaidic and oleic acid utilization. Elaidic acid (EI) was selectively concentrated in all the plasma phospholipids except for lysophosphatidylcholine. Three times more elaidic than oleic acid (OI) accumulated in the 1-acyl position of phosphatidylcholine, as determined by hydrolysis with phospholipase A2. Rapid incorporation and removal of elaidate were observed for all samples. These results support the concept that enzymes responsible for acylation of phospholipids are sensitive to double bond configuration and the physical properties of the fatty acid moieties. Labeled fatty acid levels in red cell and platelet phospholipids were much lower than for plasma phospholipids, indicating a relatively slow rate for the in vivo incorporation of fatty acids into blood cell membrane phospholipids. No isotope effect was found when oleic acid labeled with deuterium on the double bond was used.

Mass spectrometric analysis of deuterium dual labeled blood lipids

A gas chromatography selected ion monitoring mass spectrometer technique was developed to analyze deuterium dual labeled blood lipid samples from a human feeding experiment. In the metabolism experiment, described elsewhere. [2H2] labeled cis and [2H4] labeled trans fatty acids were fed to a human sujbect as a single pulse in order to determine whether the human body differentiates between cis and trans fatty acids in the diet. The analytical method described here was developed to accurately measure the ratio of [2H2]methyl oleate to [2H4]methyl elaidate in the presence of large amounts of [2H0] methyl oleate in samples derived from separated fractions of blood lipids. The technique is different from most selected ion monitoring methods in that the internal standard was fed to the subject along with the experimental material; the samples contained large amounts of unlabeled material chemically identical to the labeled material and the ratio of [2H2] to [2H4] fatty esters was of principal interest rather than the absolute value. Nine standards were analyzed eight or more times to form a basis for statistical evaluation of the method. Analysis of the plasma phosphatidyl ethanolamine fraction from the metabolic experiment is given as an example.

A selected bibliography of biomedical and environmental applications of stable isotopes. I--Deuterium 1971-1976

A compilation of selected references to the use of deuterium in biochemical, pharmacological, clinical and environmental applications for the period 1971-1976 is presented. As a further aid, author and subject indices have been compiled.

Metabolism of linoleate versus linoelaidate in the laying hen

The metabolic fate in the laying hen of linolaidic acid, the trans,trans-geometric isomer of linoleic acid, was compared to that of the naturally occurring cis,cis linoleate. In two experiments, mixtures of radioisotope-labeled linoleate and linoelaidate were orally administered to a set of three laying hens. A third mixture consisting of linoleate-3H and linoleate-14C was fed to three hens to measure biological isotope effects. Isotopic ratios (3H/14C) of the neutral lipid and phospholipid fractions isolated from egg yolks and of the octadecadienoic acids from these fractions were compared to those of the administered mixtures. The 3H/14C ratios indicate that linoelaidic acid and linoleic acid are equally incorporated into egg yolk neutral lipids and phospholipids. Arachidonic acid was found exclusively in the phospholipid fraction and was radiolabeled with the isotope from the cis,cis octadecadienoate isomer only. Further detailed analysis of individual neutral lipid components indicated: (a) discrimination against the trans,trans isomer in cholesteryl esters and (b) no discrimination against either isomer in triacylglycerols.