Biochemistry of unsaturated fatty acid isomers

Quantitation of alpha-linolenic acid elongation to eicosapentaenoic and docosahexaenoic acid as affected by the ratio of n6/n3 fatty acids

BACKGROUND

Conversion of linoleic acid (LA) and alpha-linolenic acid (ALA) to their higher chain homologues in humans depends on the ratio of ingested n6 and n3 fatty acids.

DESIGN AND METHODS

In order to determine the most effective ratio with regard to the conversion of ALA to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), human hepatoma cells were incubated with varying ratios of [¹³C] labeled linoleic acid ([¹³C]LA)- and alpha-linolenic acid ([¹³C]ALA)-methylesters. Regulative cellular signal transduction pathways involved were studied by determinations of transcript levels of the genes encoding delta-5 desaturase (D5D) and delta-6 desaturase (D6D), peroxisome proliferator-activated receptor alpha (PPARα) and sterol regulatory element binding protein 1c (SREBP-1c). Mitogen-activated protein kinase kinase 1 (MEK1) and mitogen-activated protein kinase kinase kinase 1 (MEKK1) were also examined.

RESULTS

Maximum conversion was observed in cells incubated with the mixture of [¹³C]LA/[¹³C]ALA at a ratio of 1:1, where 0.7% and 17% of the recovered [¹³C]ALA was converted to DHA and EPA, respectively. Furthermore, differential regulation of enzymes involved in the conversion at the transcript level, dependent on the ratio of administered n6 to n3 fatty acids in human hepatocytes was demonstrated.

CONCLUSION

Formation of EPA and DHA was highest at an administered LA/ALA ratio of 1:1, although gene expression of PPARα, SREBP-1c and D5D involved in ALA elongation were higher in the presence of ALA solely. Also, our findings suggest that a diet-induced enhancement of the cell membrane content of highly unsaturated fatty acids is only possible up to a certain level.

Effect of a low-fat diet on fatty acid composition in red cells, plasma phospholipids, and cholesterol esters: investigation of a biomarker of total fat intake

BACKGROUND

The utility of fatty acids (FAs) as biomarkers of total fat intake is unknown.

OBJECTIVE

We compared FA changes in red cells (RCs), plasma phospholipids (PLs), and cholesterol esters (CEs) in response to a low-fat diet (LFD) and a moderate-fat diet (MFD) and assessed whether individual or combination of FAs predict LFD.

DESIGN

Postmenopausal women (n = 66) were randomly assigned to receive an LFD (17% of energy from fat) or an MFD (34% of energy from fat) for 6 wk. All foods were provided. FAs in diets and blood were determined by gas-liquid chromatography. FA changes between baseline and end of study were compared across diets by using t tests. FA predictors of an LFD were selected by logistic regression.

RESULTS

Many FAs in RCs, PLs, and CEs responded differently to the 2 diets. Changes from baseline with an LFD for palmitic acid (16:0) (3-11% increase), behenic (22:0) and lignoceric (24:0) acids (3-20% decrease, in RCs and PLs only), cis-monounsaturated FA (MUFA) (25-35% increase), linoleic acid (18:2n-6) (11-13% decrease), trans octadecanoic acids (trans 18:1) (7-20% decrease), and n-6 highly unsaturated FA (HUFA) (2-8% increase) were significantly different from changes with an MFD. Individually, 18:2n-6 and trans 18:1 were strong predictors of an LFD [receiver operating characteristic (ROC) curves: 0.92-0.80). A logistic regression model with trans 18:1, 18:2n-6, and vaccenic acid (18:1n-7) predicted an LFD with high specificity and sensitivity (ROC curves: 0.99).

CONCLUSIONS

Saturated FA, cisMUFA, n-6 HUFA, and exogenous FAs greatly differed in their response to the LFD and MFD. Parallel responses were observed in RCs, PLs, and CEs. A model with a combination of FAs almost perfectly differentiated the consumption of 34% fat from that of 17% fat.

Effect of oleic and linoleic acids on the production of deep-fried odor in heated triolein and trilinolein

To determine sources of desirable deep-fried flavor in frying oils, degradation products from heated triolein and trilinolein with 5-31% polar compounds representing low to high deterioration were evaluated by purge-trap gas chromatography-mass spectrometry-olfactometry. (E,E)-2,4-Decadienal, 2-heptenal, 2-octenal, 2,4-nonadienal, and 2,4-octadienal produced deep-fried odor at moderate-strong intensities in heated trilinolein. However, unexpected aldehydes-2,4-decadienal, 2,4-undecadienal, 2,4-nonadienal, and 2-octenal (all <15 ppm)-were produced in triolein heated for 6 h. These dienals possibly were produced by hydroperoxidation and/or hydroxylation followed by dehydration of 2-alkenals. The 2-alkenals were produced from thermal decomposition of hydroperoxides, epoxides, and keto and dimeric compounds produced during the heating of triolein. These aldehydes produced low intensities of deep-fried odor in triolein. This information helps to explain sources of the deep-fried flavor that is characteristic of high linoleic frying oils but which is only at low intensity levels in high oleic frying oils.

Trans fatty acid isomers in Canadian human milk

The fatty acid composition, total trans content (i.e., sum of all the fatty acids which may have one or more trans double bonds) and geometric and positional isomer distribution of unsaturated fatty acids of 198 human milk samples collected in 1992 from nine provinces of Canada were determined using a combination of capillary gas-liquid chromatography and silver nitrate thin-layer chromatography. The mean total trans fatty acid content was 7.19 +/- 3.03% of the total milk fatty acids and ranged from 0.10 to 17.15%. Twenty-five of the 198 samples contained more than 10% total trans fatty acids, and thirteen samples contained less than 4%. Total trans isomers of linoleic acid were 0.89% of the total milk fatty acids with 18:2 delta 9c,13t being the most prevalent isomer, followed by 18:2 delta 9c,12t and 18:2 delta 9t,12c. Using the total trans values in human milk determined in the present study, the intake of total trans fatty acids from various dietary sources by Canadian lactating women was estimated to be 10.6 +/- 3.7 g/person/d, and in some individuals, the intake could be as high as 20.3 g/d. The 18:1 trans isomer distribution differed from that of cow's milk fat but was remarkably similar to that in partially hydrogenated soybean and canola oils, suggesting that partially hydrogenated vegetable oils are the major source of these trans fatty acids.

Trans-fatty acid patterns in patients with angiographically documented coronary artery disease

The plasma trans-fatty acids of 47 patients with angiographically documented coronary artery disease were compared with those of 56 reference subjects using high-resolution capillary column gas-liquid chromatography to test the hypothesis that trans-fatty acid intake is a risk factor for cardiovascular disease. Individual and total trans-fatty acids were higher in patients than in reference subjects (1.38 vs 1.11% for total trans-fatty acids, p < 0.003; 0.40 vs 0.31% for palmitoleic acid trans, p < 0.001; and 0.28 vs 0.22% for linoleic acid trans, p < 0.007). High-density lipoprotein (HDL) cholesterol and HDL cholesterol/total cholesterol were negatively correlated (r = -0.29, p < 0.004; and r = -0.35, p < 0.001, respectively), whereas triglycerides, total cholesterol and low-density lipoprotein cholesterol were positively correlated (r = 0.47, p < 0.001; r = 0.22, p < 0.03; r = 0.20, p < 0.05, respectively) with palmitoleic acid trans. The correlations were similar and significant for linoleic acid trans, but less strong for total trans-fatty acids (which is more difficult to measure and has greater variability). Saturated and trans-fatty acids and total cholesterol are positively associated, whereas HDL/total cholesterol and polyunsaturated fatty acids are negatively associated with coronary artery disease. These results are consistent with the hypothesis that dietary trans-fatty acids are a cardiovascular risk factor.

Lack of effects of trans fatty acids on eicosanoid biosynthesis with adequate intakes of linoleic acid

The minimum requirement of linoleic acid to prevent effects of dietary C18 trans fatty acids on eicosanoid biosynthesis in rats was assessed. In a first experiment, six groups of animals were fed diets with a high content of trans fatty acids [20% of energy (en%)], and increasing amounts of linoleic acid (0.4 to 7.1 en%). In a second experiment, four groups of rats were fed diets designed to compare trans fatty acids with saturated and cis-monounsaturated fatty acids of the same chain length at the 2 en% linoleic acid level. After 9-14 weeks the biosynthesis of prostacyclin by pieces of aorta and the biosynthesis of hydroxy-heptadecatrienoic acid and 12-hydroxy-eicosatetraenoic acid by platelets were measured. The fatty acid compositions of aorta phospholipid and platelet lipid were also determined. Both the prostacyclin-production by aorta pieces and the production of hydroxy-heptadecatrienoic acid and 12-hydroxy-eicosatetraenoic acid by platelets appeared to be a linear function of the arachidonic acid level in aorta phospholipid and platelet lipid, irrespective of the trans fatty acid content in the diet. This indicates that trans fatty acids do not directly influence enzymes involved in eicosanoid biosynthesis. In a direct comparison with cis-monounsaturated or saturated fatty acids with 2 en% linoleic acid in the diet, only a moderate reduction in arachidonic acid level in aorta phospholipids in the group fed trans fatty acids was observed. The geometry of the double bond did not influence the arachidonic acid level in platelet lipid, although the diet rich in saturated fatty acids increased arachidonic acid levels significantly compared with all other diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary trans-fatty acids on reproductive performance of Wistar rats

1. Wistar rats were fed for three successive generations on a semi-purified diet, in which the fat was provided by butter, sunflower oil, rapeseed oil or hydrogenated vegetable fat, differing in the content of cis,cis-18:2 and trans-18:1 fatty acids. Effects of these fats on the composition of adipose tissue and reproductive performance were studied. Fatty acids were analysed using high-performance liquid chromatography. 2. The fatty acid pattern of adipose tissue was closely related to dietary fat composition and, established in the first generation, did not change significantly in successive generations of rats. 3. Hydrogenated fat adversely affected litter size, sperm morphology and regularity of oestrous cycle, and prolonged the period of gestation in experimental animals. Differences observed between the generations were not significant. 4. Hydrogenated fat decreased the level of serum testosterone in males, but the differences observed in levels of serum progesterone in females were not apparently related to the dietary trans-fatty acids.

Linoleic acid requirement of rats fed trans fatty acids

The amount of linoleic acid required to prevent undesirable effects of C18 trans fatty acids was investigated. In a first experiment, six groups of rats were fed diets with a high content of trans fatty acids (20% of energy [en%]), and increasing amounts of linoleic acid (0.4 to 7.1 en%). In a second experiment, four groups of rats were fed diets designed to compare trans fatty acids with saturated and cis-monounsaturated fatty acids of the same chain length at the 2 en% linoleic acid level. After 9-14 weeks, the oxygen uptake, lipid composition and ATP synthesis of heart and liver mitochondria were determined. The phospholipid composition of the mitochondria did not change, but the fatty acid compositions of the two main mitochondrial phospholipids were influenced by the dietary fats. Trans fatty acids were incorporated in all phospholipids investigated. The linoleic acid level in the phospholipids, irrespective of the dietary content of linoleic acid, increased on incorporation of trans fatty acids. The arachidonic acid level had decreased in most phospholipids in animals fed diets containing 2 en% linoleic acid. At higher linoleic acid intakes, the effect of trans fatty acids on the phospholipid arachidonic acid level diminished. However, in heart mitochondrial phosphatidylethanolamine, trans fatty acids significantly increased the arachidonic acid level. Despite these changes in composition, neither the amount of dietary linoleic acid nor the addition of trans fatty acids influenced the mitochondrial function. For rats, a level of 2 en% of linoleic acid is sufficient to prevent undesirable effects of high amounts of dietary C18 trans fatty acids on the mitochondrial function.

Modification of alkenyl chain profile in plasmalogens of rat heart mitochondria by dietary trielaidin

Effects of dietary trielaidin upon the alkenyl chain profile of plasmalogens were studied using heart mitochondria of rats fed a semi-purified diet containing 10% of fat supplement in which elaidic acid accounted for 69% of total fatty acids. Alkyl substituted dioxane (ASD) derivatives of the alkenyl groups of plasmalogens were prepared and analyzed by silver nitrate TLC and by GLC on different phases (BDS and OV-275). After two months of feeding the experimental diet, 40% of the ASD contained a trans-octadecenyl chain, suggesting that dietary elaidic acid was reduced in vivo to the corresponding alcohol and incorporated into plasmalogens. There was a simultaneous decrease in the percentage of ASD containing saturated chains, but the percentage of ASD substituted with cis-octadecenyl chains was not significantly affected. These observations suggested that elaidic acid may compete with saturated fatty acids, but not with cis-octadecenoic acids during the plasmalogen biosynthesis. Feeding trielaidin did not seem to have any significant influence on the relative proportions of plasmalogens, which accounted for 11-12%, on a phosphorus basis, of total heart mitochondria phospholipids.

Dietary fats containing concentrates of cis or trans octadecenoates and the patterns of polyunsaturated fatty acids of liver phosphatidylcholine and phosphatidylethanolamine

The effects of the mixed cis- 18:1 isomers and mixed trans- 18:1 isomers present in partially hydrogenated soybean oil (PHSO) upon the patterns of polyunsaturated fatty acids (PUFA) in liver phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were studied in rats fed concentrates of cis- 18:1 or trans- 18:1 isomers isolated as triacylglycerides from PHSO. The cis- 18:1 and trans- 18:1 concentrates were fed at levels equal to those present in PHSO fed at 17.9% of the diet. All diets contained the required amounts of both linoleic and linolenic acids. The trans- 18:1 concentrate was found to suppress the levels of 20:4 omega 6 and 20:3 omega 9, and to increase the levels of 18:2 omega 6 and 20:5 omega 3 in PC and PE. The cis- 18:1 concentrate suppressed 20:4 omega 6 in PC, 20:5 omega 3 in PC and PE, and 18:2 omega 6 in PC, but increased the levels of 20:4 omega 6 in PE, and 20:3 omega 9 in PC and PE. The cis- 18:1 concentrate was more effective than the trans concentrate in suppressing 22:6 omega 3. The trans- 18:1 concentrate was more effective in suppressing 20:4 omega 6. The trans- 18:1 isomers appear to modify PUFA metabolism by inhibition of PUFA synthesis, whereas the cis- 18:1 isomers appear to complete with 2-position fatty acyl transfer and to inhibit omega 3 PUFA acylation.