Fate of linoleic, arachidonic, and docosa-7,10,13,16-tetraenoic acids in rat testicles

Effect of a pescetarian and vegan diet on fatty acid composition in blood and spermatozoa in young healthy men

INTRODUCTION

There is a growing interest in vegetarian and vegan diets, but both can potentially affect tissue fatty acids (FA) composition. We aimed to evaluate the effect of vegetarian diets on plasma, erythrocytes, and sperm n-3 polyunsaturated fatty acids (n-3 PUFA) status in healthy young men.

METHODS

Four groups were studied: i) men consuming a regular omnivore diet (OMV-1, n = 35); ii) men consuming an omnivore diet but excluding fish and seafood (OMV-2, n = 34); iii) men consuming a pescetarian diet (including dairy, eggs, fish, and seafood) (PESC, n = 36); and iv) men following a strict vegan diet (VEG, n = 35). Participants in each group should follow their diet for at least the previous 12 months. Diet evaluation used a structured validated food frequency questionnaire. FA composition was measured in plasma, erythrocyte phospho-lipids, and spermatozoa by gas-liquid chromatography, expressed as a mole percentage of the total FA content.

RESULTS

Main findings showed higher alpha-linolenic fatty acid (ALA) and total n-3 PUFA dietary intake in the VEG group. In plasma, arachidonic and eicosapentaenoic acids were higher in OMV and PESC groups, whereas docosahexaenoic acid (DHA) level was lower in VEG. Higher ALA, but reduced DHA and total n-3 PUFA levels were found in erythrocytes and spermatozoa in the VEG group.

CONCLUSION

Higher dietary ALA intake was found in pescetarians and vegan men. However, the higher ALA intake was not reflected in higher DHA content in the evaluated tissues. PUFA assessment, with particular emphasis in DHA, are necessary to improve PUFA status in vegan men.

Liver desaturase activities and FA composition in monkeys. Effect of a low-protein diet

The aim of the present study was to measure delta9-, delta6-, and delta5-desaturase activities in liver microsomes, as well as phospholipid FA composition of liver and erythrocytes in monkeys fed a control or low-protein diet during the postweaning period. Ten Saimiri sciureus boliviensis (Cebidae) of both sexes were employed; at 12 mon of age they were separated into two groups fed ad libitum on a control or a low-protein diet for 24 mon. Saimiri sciureus had active delta9, delta6, and delta5 liver desaturase enzymes, and these activities were influenced by the diet. A low-protein diet produced a significant reduction in delta5-desaturation capacity, an increase in delta9-desaturase activity, and no change in delta6-desaturase activity (P < 0.05). These changes, evoked by protein deprivation, were reflected in the liver phospholipid FA composition. Increases in the proportion of saturated FA and in monounsaturated oleic acid (18:1n-9) and a decrease in the proportion of PUFA of the n-6 and n-3 series were produced in the animals fed a low-protein diet (P < 0.0001).

Hormonal modulation of delta6 and delta5 desaturases: case of diabetes

Animal biosynthesis of high polyunsaturated fatty acids from linoleic, alpha-linolenic and oleic acids is mainly modulated by the delta6 and delta5 desaturases through dietary and hormonal stimulated mechanisms. From hormones, only insulin activates both enzymes. In experimental diabetes mellitus type-1, the depressed delta6 desaturase is restored by insulin stimulation of the gene expression of its mRNA. However, cAMP or cycloheximide injection prevents this effect. The depression of delta6 and delta5 desaturases in diabetes is rapidly correlated by lower contents of arachidonic acid and higher contents of linoleic in almost all the tissues except brain. However, docosahexaenoic n-3 acid enhancement, mainly in liver phospholipids, is not explained yet. In experimental non-insulin dependent diabetes, the effect upon the delta6 and delta5 desaturases is not clear. From all other hormones glucagon, adrenaline, glucocorticoids, mineralocorticoids, oestriol, oestradiol, testosterone and ACTH depress both desaturases, and a few hormones: progesterone, cortexolone and pregnanediol are inactive.

Regulation of the biosynthesis of 22:5n-6 and 22:6n-3: a complex intracellular process

Both 22:4n-6 and 22:5n-3 are synthesized from n-6 and n-3 fatty acid precursors in the endoplasmic reticulum. The synthesis of both 22:5n-6 and 22:6n-3 requires that 22:4n-6 and 22:5n-3 are metabolized, respectively, to 24:5n-6 and 24:6n-3 in the endoplasmic reticulum. These two 24-carbon acids must then move to peroxisomes for partial degradation followed by the movement of 22:5n-6 and 22:6n-3 back to the endoplasmic reticulum for use as substrates in membrane lipid biosynthesis. Clearly an understanding of the control of intracellular fatty acid movement as well as of the reactions carried out by microsomes, peroxisomes, and mitochondria are all required in order to understand not only what regulates the biosynthesis of 22:5n-6 and 22:6n-3 but also why most tissue lipids selectively accumulate 22:6n-3.

A comparison of the metabolism of [3-14C]-labeled 22- and 24-carbon (n-3) and (n-6) unsaturated fatty acids by rat testes and liver

The unsaturated fatty acid composition of phospholipids from different tissues frequently varies. Rat liver phospholipids contain esterified 22:6(n-3) while 22:5(n-6) is the major esterified 22-carbon acid in testes phospholipids. Both testes and liver synthesize polyunsaturated fatty acids. Microsomes, particularly from liver, have been used extensively to measure reaction rates as they relate to polyunsaturated fatty acid and phospholipid biosynthesis. None of these rate studies explain why specific acids are synthesized and subsequently esterified. In this study we compared the metabolism of [3-14C]-labeled (n-3) and (n-6) acids when injected via the tail vein, as a measure of hepatic metabolism, versus when they were injected directly into the testes. Liver preferentially metabolizes [3-14C]-labeled 24:5(n-3) and 24:6(n-3) to yield esterified 22:6(n-3), when compared with the conversion of [3-14C]-labeled 24:4(n-6) and 24:5(n-6) to yield 22:5(n-6). Both 24-carbon (n-3) acids were also converted to 22:5(n-3) but no labeled 22:4(n-6) was detected after injecting the two 24-carbon (n-6) acids. Differences in the hepatic metabolism of 24-carbon (n-3) and (n-6) acids to 22:6(n-3) and 22:5(n-6), versus their partial beta-oxidation to 22:5(n-3) and 22:4(n-6), are important in vivo controls. Surprisingly, in testes a higher percentage of radioactivity was found in esterified 22:6(n-3) versus 22:5(n-6) following injections, respectively, of [3-14C]-labeled 22:5(n-3) versus 22:4(n-6), which is the corresponding metabolic analog. Corresponding pairs of 24-carbon (n-3) and (n-6) acids, as they relate to metabolism, were processed in similar ways by testes. The relative absence of esterified 22-carbon (n-3) fatty acids, versus the abundance of 22- and 24-carbon (n-6) acids in testes phospholipids, does not appear per se to be due to differences in the ability of testes to metabolize (n-3) and (n-6) fatty acids. It remains to be determined if there is selective uptake of specific fatty acids by testes for use as precursors to synthesize polyunsaturated fatty acids.

A function for the vitamin E metabolite alpha-tocopherol quinone as an essential enzyme cofactor for the mitochondrial fatty acid desaturases

A critical analysis of the changes in fatty acid patterns and their metabolism elicited by vitamin E deficiency leads to the proposal that a major role of dietary RRR-alpha-tocopherol (alpha-TOC) is as an enzymatic precursor of alpha-tocopherolquinone (alpha-TQ) whose semiquinone radical functions as an essential enzyme cofactor for the fatty acid desaturases of the recently elucidated carnitine-dependent, channeled, mitochondrial desaturation-elongation pathway; a detailed mechanism for its function is proposed. Pathophysiological states produced by vitamin E deficiency and alpha-TOC transfer protein defects, such as ataxia, myopathy, retinopathy, and sterility are proposed to develop from the effects of impaired alpha-TQ-dependent desaturases and the resulting deficiency of their polyenoic fatty acid products.

An update on the pathways of polyunsaturated fatty acid metabolism

The biosynthesis of 4, 7, 10, 13, 16-22:5 and 4, 7, 10, 13, 16, 19-22:6 from dietary linoleate and linolenate, respectively, does not totally take place in the endoplasmic reticulum but does require the participation of enzymes in the endoplasmic reticulum and peroxisomes. The absence of an endoplasmic reticulum-associated acyl-CoA-dependent delta 4 desaturase also requires the controlled movement of 22- and 24-carbon polyunsaturated fatty acids between the endoplasmic reticulum and peroxisomes.

Analysis of the putative role of 24-carbon polyunsaturated fatty acids in the biosynthesis of docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids

The recent literature on the putative involvement of a single cycle of peroxisomal beta-oxidation of 24:5n-6 and 24:6n-3 polyunsaturated fatty acids in the biosynthesis of the respective docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) fatty acids is critically reviewed. Present evidence suggests that in vitro data in support of the above proposition is an artifact of a low 2,4-dienoyl-CoA reductase activity due to depletion of NADPH resulting from incubation conditions. Kinetic studies with radiolabeled precursors in cell cultures have shown lower initial rates of labeling of 24:6n-3 than that of 22:6n-3, indicating that 24:6n-3 is an elongation product of 22:6n-3 rather than its precursor. Analysis of other literature data supports the proposal that 22:5n-6 and 22:6n-3 are synthesized in mitochondria via channeled carnitine-dependent pathways involving separate n-6- and n-3-specific desaturases. It is proposed that impaired peroxisomal function in some peroxisomal disorders is a secondary consequence of defective mitochondrial synthesis of 22:6n-3; moreover, some disorders of peroxisomal beta-oxidation show normal or increased 22:5n-6 concentrations, indicating that 22:5n-6 is synthesized by independent desaturases without peroxisomal involvement.

Studies to determine if rat liver contains chain-length-specific acyl-CoA 6-desaturases

According to the revised pathway for 22:6(n - 3) biosynthesis in liver (Voss et al. (1991) J. Biol. Chem. 266, 19995-20000) both 18:3(n - 3) and 24:5(n - 3) serve as substrates for desaturation at position-6. The present study was undertaken to determine whether microsomes contain chain-length-specific 6-desaturases. Addition of [1-14C]20:3(n - 6), a substrate for desaturation at position-5, did not depress desaturation of either [1-14C]18:3(n - 3) or [3-14C]24:5(n - 3). An unexplained observation was that both 18:3(n - 3) and 24:5(n - 3) inhibited the metabolism of 20:3(n - 6) to 20:4(n - 6). When an enzyme-saturating level of [3-14C]24:5(n - 3) was now incubated alone and with 40, 80 and 120 nmol of [1-14C]18:3(n - 3), the production of 24:6(n - 3) was inhibited by 43, 67 and 81%. Conversely, when [1-14C]18:3(n - 3) was incubated with 40, 80 or 120 nmol of [3-14C]24:5(n - 3), the synthesis of 18:4(n - 3) was inhibited by only 15, 20 and 27%. These and other competitive studies showed that there was always preferential desaturation of 18:3(n - 3) rather than 24:5(n - 3). In addition, competitive studies between 18:2(n - 6) and 18:3(n - 3), as well as with 24:4(n - 6) and 24:5(n - 3) showed that there was always preferential desaturation of the (n - 3) acid. Although our results are consistent with a single 6-desaturase, further studies, including the isolation of the 6-desaturases(s), is obviously required to determine whether multiple forms of the 6-desaturase exist.

The biosynthesis of polyunsaturated fatty acids by rat sertoli cells

1. The biosynthesis of polyunsaturated fatty acids (PUFA) of the n-6 and n-3 series was investigated in cultured Sertoli cells. 18:2n-6, 18:3n-6, 20:2n-6, 18:3n-3 and 20:3n-3 were added individually at a concentration of 20 mumol to culture media. 2. Maximum incorporation of 20- and 22-carbon PUFA into membrane lipids was observed after 72 hr of incubation with all the exogenous substrates used. 3. As reported in other cell systems, the delta 6 desaturation was the first rate-limiting step; the major factor regulating this activity was the concentration of linoleic acid or alpha-linolenic acid in the medium. 4. Our data show that the delta 5-desaturation represents a second regulatory step in PUFA biosynthesis. 5. The sum of n-6 and n-3 PUFA of the 22 carbon chain length constantly represented between 11 and 12% of total fatty acids, regardless of the exogenous substrate used. 6. Our kinetic studies of the incorporation of PUFA of the n-6 and n-3 series did not permit detection of a delta 8 desaturase activity.

Arachidonic acid biosynthesis in non-stimulated and adrenocorticotropin-stimulated Sertoli and Leydig cells

The biosynthesis of arachidonic acid in Sertoli and Leydig cells isolated from the testes of mature rats has been investigated. Both types of cells incorporated [2-14C]eicosatrienoic acid from the incubation medium and transformed it into arachidonic acid. The administration of adrenocorticotropin (ACTH) to the rats decreased the delta 5 desaturating activity in the isolated testicular cells, while ACTH produced no changes in the uptake of the substrate. Similar results were obtained when ACTH was added to the incubation medium of cells isolated from non-hormone treated rats. The total fatty acid composition of the Sertoli cells isolated from ACTH-treated rats showed a significant increase in the relative percentage of 18:2n-6 and a decrease in the C20 and C22 polyenes. This may indicate that ACTH exerts an inhibitory effect on delta 6, delta 5 and delta 4 desaturase activities. Addition of corticosterone to the incubation medium also produced a significant decrease in arachidonic acid biosynthesis. Because ACTH is known to stimulate the release of corticosterone in vivo, both hormones may act cumulatively in the regulation of arachidonic acid metabolism in Sertoli and Leydig cells.

Differences in the interconversion between 20- and 22-carbon (n - 3) and (n - 6) polyunsaturated fatty acids in rat liver

When male weanling rats were fed diets containing either 5% corn oil or a diet in which half of the corn oil was replaced by fish oil, the 20:5(n - 3) in liver choline and ethanolamine phosphoglycerides, not only partially replaced arachidonate but also paired with palmitic and stearic acids in the same molar ratio as did arachidonate. The 22:5(n - 3)/22:6(n - 3) ratio in the liver phospholipids of corn oil fed rats was similar to that found when the esterified levels of these two acids were increased 5-fold by feeding fish oil. Moreover, the pairing of both 22:5(n - 3) and 22:6(n - 3) with palmitic and stearic acids, on a molar ratio basis, was relatively independent of the total amount of esterified 22:5(n - 3) and 22:6(n - 3). When (3-14C)-labeled 22:4(n - 6) was injected into rats raised on a chow diet or incubated with hepatocytes from these animals, its primary metabolic fate was retroconversion to arachidonate followed by esterification. Conversely, [3-14C]22:5(n - 3) was a poorer substrate for retroconversion with a larger amount being esterified directly into phospholipids and, in addition, this acid served as a precursor for 22:6(n - 3). The enhanced metabolism of both [3-14C]22:4(n - 6) to 22:5(n - 6) and of [3-14C]22:5(n - 3) to 22:6(n - 3) in animals raised on a diet devoid of fat or in their hepatocytes may possibly be due to elevated 6-desaturase activity and/or the level of this enzyme or enzymes. This hypothesis is based on studies showing that the synthesis of 22:6(n - 3) proceeds via a pathway independent of a 4-desaturase but requires the use of a 6-desaturase at two steps (Voss, A., Reinhart, M., Sankarappa, S. and Sprecher, H. (1991) J. Biol. Chem. 266, 19995-20000).

Developmental changes in the fatty acids of rat uterus and the influence of dietary essential fatty acids

The effect of age on uterine fatty acid composition was studied in rats fed diets of differing fatty acid composition. Uteri of newly weaned 23-day rats had a higher fatty acid content and a higher proportion of short-chain (less than or equal to C18) fatty acids. Higher incorporation of C less than or equal to 18 fatty acids into neutral lipid (NL) and phospholipid (PL) of young 42-day rats compared with adult 240-day rats was detected. Uterine NL incorporated predominantly C less than or equal to 18 fatty acids which may be an important metabolic energy store in developing uterine tissue. Incorporation of C less than or equal to 18 fatty acids by uterine PL and NL was relatively unselective. In contrast, there was selective retention of arachidonic acid (AA) and docosahexanoic acid (DHA) throughout uterine development. An effect of dietary EFA on uterine n-3 and n-6 EFA was detected in each age group. There was marked retention of uterine AA when dietary supplies of n-6 EFA were low, but the total AA, eicosapentaenoic acid (EPA) and DHA in uterine PL remained constant in the three diet groups, and a constant content of AA, EPA and DHA was maintained throughout uterine development, regardless of diet. The degree of n-3 substitution achieved in this study inhibited uterine release of PG and parturition in adult rats.

The influence of dietary essential fatty acids on uterine C20 and C22 fatty acid composition

The effect of dietary fatty acids on uterine fatty acid composition was studied in rats fed control diet or semi-synthetic diet supplemented with 1.5 microliter/g/day evening primrose oil (EPO) or fish oil (FO). Diet-related changes in uterine lipid were detected within 21 days. Changes of 2- to 20-fold were detected in the uterine n-6 and n-3 essential fatty acids (EFA) and in certain saturated and monounsaturated fatty acids. The FO diet was associated with higher uterine C20 and C22 n-3, and the EPO diet, with higher uterine n-6 fatty acid. High uterine C18:2 n-6 was detected in neutral lipid (NL) of rats fed high concentrations of this fatty acid, but there was little evidence of selective incorporation or retention of C18:2 n-6 by uterine NL. The incorporation of EFA into uterine phospholipids (PL) was greater than NL EFA incorporation, and uterine PL n-3/n-6 ratios showed greater diet dependence. Tissue/diet fatty acid ratios in NL and PL also indicated preferential incorporation/synthesis of C16:1 n-9, and C16:0, and there was greater incorporation of C12:0 and C14:0 into uteri of rats fed EPO and FO. Replacement of 50-60% of arachidonate with n-3 EFA in uterine PL may inhibit n-6 EFA metabolism necessary for uterine function at parturition.

Comparison of the conversion rates of alpha-linolenic acid (18:3(n - 3)) and stearidonic acid (18:4(n - 3)) to longer polyunsaturated fatty acids in rats

The delta 6-desaturase reaction is regarded to be the rate-limiting step in the conversion of linoleic acid (18:2(n - 6)) to arachidonic acid (20:4(n - 6)). The same is probably also the case with the conversion of alpha-linolenic acid (18:3(n - 3)) to eicosapentaenoic acid (20:5(n - 3)). However, there are very few in vivo studies that directly compared the conversion rate between 18:3(n - 3) and stearidonic acid (18:4(n - 3)), which is the delta 6-desaturated product of 18:3(n - 3). We compared this rate by feeding rats on a lipid-free diet supplemented with lard (9%, w/w) and 18:3(n - 3) ethyl ester (1%) diet or on a diet containing lard (9%) and 18:4(n - 3) ethyl ester (1%). A lard (10%)-supplemented diet was used as the control diet. The fatty acid compositions of total phospholipids, triglycerides and free fatty acids of both liver and plasma were measured after 1 or 3 weeks on different diets. The molar ratio of 20:5(n - 3) of most lipid fractions was about 2-fold higher in rats fed the 18:4(n - 3)-supplemented diet than in rats fed the 18:3(n - 3)-supplemented diet. 18:4(n - 3) was found in the liver lipid fraction in only a very small amount, even in the 18:4(n - 3)-supplemented groups. Thus, desaturation at C-6 is suggested to be the rate-limiting step in the conversion of 18:3(n - 3) to 20:5(n - 3).

Elongation and desaturation of arachidonic and eicosapentaenoic acids in rat liver. Effect of clofibrate feeding

The fatty acid elongation-desaturation ability of 5,8,11,14-eicosatetraenoic (20:4(n-6)) and 5,8,11,14,17-eicosapentaenoic (20:5(n-3)) acids was determined in both liver microsomal and light mitochondrial (rich in peroxisomes) fractions of untreated and clofibrate treated rats. The elongation and the subsequent desaturation steps were performed in the corresponding favorable media. 20:5(n-3) elongation was about 2-times more extensive than that of 20:4(n-6). Clofibrate feeding for 10 days resulted in a marked decrease in the elongation rate with the two substrates, while the delta 4 desaturation rate was increased. There were small differences in the elongation rate between the microsomal and light mitochondrial fractions, however, the relative delta 4 desaturation rate was higher in the light mitochondrial fraction than microsomes.

Kinetic analysis of delta-6-desaturation in liver microsomes: influence of gamma-linoleic acid dietary supplementation to young and old rats

Previous experiments demonstrated the ability-of a gamma-linoleic acid (GLA) dietary supplementation (as evening primrose oil--EPO) to counteract the fall off in delta-6-desaturase (D6D) activity of linoleic acid and alpha-linoleic acid in aged rats. Kinetic parameters of the D6D were determined in order to test the possibility that there may be a significant influence of GLA administration to young and aged rats on the Vm and Km values for 6-desaturation of both the substrates. In young rats GLA supplementation did not affect the kinetic parameters, while in old rats it produced an increase of Vm values of 6-desaturation for both the substrates. Thus the administration of small doses of GLA to old rats might offer substantial protection against the loss of D6D affinity observed in aging, enhancing the capacity of the enzyme itself.

Physiological importance of omega-3/omega-6 polyunsaturated fatty acids in man. An overview of still unresolved and controversial questions

The 'essentiality' of (omega-6) and (omega-3) fatty acids in mammals is well known. Nevertheless, some important points remain unclear concerning their implication in physiology. After a short discussion about the definition of essential fatty acids deficiency, this brief overview deals with some of these points, pointing out some of the unresolved questions. Different subjects are approached concerning the (omega-6) and (omega-3) fatty acids metabolism: desaturases, eicosanoids, production, as well as some of their metabolic effects on cell membranes, intestinal function, glucose and lipid metabolism, haemorheology.

Black currant seed oil feeding and fatty acids in liver lipid classes of guinea pigs

Guinea pigs were fed one of three diets containing 10% black currant seed oil (a source of gamma-linolenic (18:3 n-6) and stearidonic (18:4 n-3) acids), walnut oil or lard for 40 days. The fatty acid composition of liver triglycerides, free fatty acids, cholesteryl esters, phosphatidylinositol, phosphatidylserine, cardiolipin, phosphatidylcholine and phosphatidylethanolamine were determined. Dietary n-3 fatty acids found esterified in liver lipids had been desaturated and elongated to longer chain analogues, notably docosapentaenoic acid (22:5 n-3) and docosahexaenoic acid (22:6 n-3). When the diet contained low amounts of n-6 fatty acids, proportionately more of the n-3 fatty acids were transformed. Significantly more eicosapentaenoic acid (EPA) (20:5 n-3) was incorporated into triglycerides, cholesteryl esters, phosphatidylcholine and phosphatidylethanolamine of the black currant seed oil group compared with the walnut oil group. Feeding black currant seed oil resulted in significant increases of dihomogamma-linolenic acid (20:3 n-6) in all liver lipid classes examined, whereas the levels of arachidonic acid (20:4 n-6) remained relatively stable. The ratio dihomo-gamma-linolenic acid/arachidonic acid was significantly (2.5-fold in PI to 17-fold in cholesteryl esters) higher in all lipid classes from the black currant seed oil fed group.

Linoleic acid-induced fatty acid changes in platelet and aorta of the rat: effect of age and cholesterol

The influence of age and cholesterol on polyunsaturated fatty acids (PUFA) levels was studied in young and old male Sprague-Dawley rats. Animals were fed a fat-free diet supplemented with 10% (by wt) safflower oil with or without 1% cholesterol for 8 wk. As a result of cholesterol feeding, proportions of linoleic acid (18:2n-6) and dihomo-gamma-linolenic acid (20:3n-6) were increased and that of arachidonic acid (20:4n-6) was decreased in the liver and platelet phospholipids in 64-wk-old rats, suggesting inhibitory effects of cholesterol on 20:4n-6 synthesis from 18:2n-6. The prominent age-dependent effect on the levels of PUFA was a retention of C-22 n-3 PUFA, accompanied by decreased C-22 n-6 PUFA and increased 20:3n-6 in the liver and platelet phospholipids. Ratio of 20:3n-6/20:4n-6 increased in 64-wk-old rats regardless of dietary cholesterol, suggesting depressed delta 5-desaturase with age. In aorta phospholipids, 20:3n-6 content and 20:3n-6/20:4n-6 ratio increased with cholesterol supplementation, but not with age. These results suggest that changes of PUFA composition of platelet phospholipids with age are closely linked with changes in liver phospholipids. The 20:4n-6 content in both platelet and aorta phospholipids is kept constant, despite other n-6 and n-3 PUFA being affected by age.

The role of linoleic acid and its metabolites in the lowering of plasma cholesterol and the prevention of cardiovascular disease

An increase in linoleic acid intake lowers plasma cholesterol and is one of the safest methods for achieving this end. However, the amounts that must be consumed are large. Linoleic acid is metabolized via several routes and it is probable that a metabolite, rather than linoleic acid itself, is responsible for the cholesterol-lowering effect. If that metabolite could be identified, safe, drug-free, cholesterol-lowering might be achieved with much lower doses. Evidence is reviewed which suggests that a long-chain polyunsaturated fatty acid and/or a prostaglandin metabolite may be responsible for the cholesterol-controlling action of linoleic acid. Such metabolites may be effective also in controlling other risk factors for cardiovascular disease, such as elevated blood pressure and enhanced platelet aggregation. Epidemiological studies suggest that low levels of those metabolites, especially dihomogammalinolenic acid and arachidonic acid, are powerful independent risk factors for development of ischaemic heart disease. Further research in this area is urgently needed now that it is broadly accepted that cholesterol-lowering does indeed reduce the risk of cardiovascular disease.

Inability of skin enzyme preparations to biosynthesize arachidonic acid from linoleic acid

The lack of any information as to the origin of epidermal arachidonic acid, an important precursor of eicosanoids in the epidermis, prompted us to determine in vitro whether or not microsomal preparations from rat and guinea pig epidermis possess the delta 6 and delta 5 desaturase activities. The incubations were performed in parallel with microsomal preparations from liver of these animals where activities for these enzymes have previously been reported. The conversions of radioactive fatty acids were determined after methylation and separation of the 14C-fatty acid methyl esters by argentation thin layer chromatography. Data from these studies demonstrated that delta 5 desaturase activity is markedly lower in guinea pig liver than in rat liver. Interestingly, preparations from rat and guinea pig epidermis at all concentrations tested lacked the capacity to transform either linoleic acid into gammalinolenic acid or dihomogammalinolenic acid into arachidonic acid. This observation implies that arachidonic acid that is present in the epidermal phospholipids is biosynthesized elsewhere endogenously and transported to the epidermis for esterification into the phospholipids. The site of this biosynthesis is presumably the liver and the mode of transport to the epidermis remains to be determined. These studies indicate arachidonic acid per se as an essential fatty acid for the epidermis.

Effect of dietary fats on arachidonic acid and eicosapentaenoic acid biosynthesis and conversion to C22 fatty acids in isolated rat liver cells

The desaturation, chain elongation and esterification of [1-14C]eicosapentaenoic acid, [1-14C]arachidonic acid, [1-14C]eicosatrienoic acid, [1-14C]linolenic acid and [1-14C]linoleic acid were studied in isolated liver cells. Rats fed diets with either 15% hydrogenated coconut oil or 15% partially hydrogenated marine oil, both deficient in essential fatty acids, 15% soybean oil or standard pellet diet with 6% fat, were used. The delta 4-desaturation of 22:5(n - 3) and 22:4(n - 6) as well as the delta 6-desaturase activity was distinctly higher in cells from animals fed coconut or marine oil than with soybean oil or standard pellet. The rate of delta 5-desaturation of 20:3(n - 6) and 20:4(n - 3) was nearly the same in cells from rats fed coconut, marine and soybean oils and higher than with standard pellet. The chain elongation of 20:5(n - 3) to 22:5(n - 3) was distinctly more pronounced than the elongation of 20:4(n - 6) with all four diets. 20:5(n - 3) was mainly esterified in the phospholipids with marine and coconut oils, and mainly in triacylglycerol with standard pellet and soybean oils. The proportion of [1-14C]20:4(n - 6) in the phospholipids to that in triacylglycerol decreased in the order marine oil greater than coconut oil greater than standard pellet greater than soybean oil. The different endogenous arachidonic acid content in the phospholipids induced by the different diets increased in the same order. 20:5(n - 3) was rapidly esterified in triacylglycerol and phospholipids, then liberated especially from the triacylglycerol fraction, chain elongated to 22:5(n - 3) and reesterified.

Essential fatty acid metabolism in diseases of connective tissue with special reference to scleroderma and to Sjogren's syndrome

Drugs which modify the conversion of essential fatty acids to prostaglandins and leukotrienes are the mainstays of treatment in rheumatology. Yet these drugs have little or no action in scleroderma or Sjogren's syndrome and under some circumstances may have adverse effects. Patients with scleroderma have been shown to have very high levels of circulating prostaglandins, coupled with depletion of the prostaglandin precursors, dihomogammalinolenic acid and arachidonic acid. Levels of the metabolites of arachidonic acid, 22:4n-6 and 22:5n-6, which have major roles in maintaining normal cell membrane characteristics were exceptionally low in both plasma and red cell membranes. Others have observed that various functions are highly resistant to normal actions of PGs in scleroderma. This raises the possibility that the high rate of PG formation in scleroderma may be beneficial, in compensation, and that clinical symptoms develop when PG precursors begin to be depleted. Red cell membrane fatty acids patterns in Sjogren's syndrome are almost identical to those in scleroderma. Placebo-controlled trials of supplementation with essential fatty acids have been found to be beneficial in both scleroderma and Sjogren's syndrome.

Dietary lead: effects on hepatic fatty acid composition in chicks

Arbor Acre broiler chicks were fed diets containing 0, 500, 750, 1000, 2000, or 4000 ppm lead (as Pb acetate X 3 H2O) from day-old through 21 days of age. There were 8 groups of 10 male chicks per lead level. Eight chicks from each dietary lead level were killed at 21 days, and hepatic fatty acid composition was determined for each chick by gas-liquid chromatography. Increasing dietary lead levels decreased the concentrations of 16:1 and 18:1 fatty acids (first No. = No. carbons; second No. = No. double bonds) and increased the concentrations of 18:0 and 20:4. The concentration of 18:2 fatty acids did not differ significantly from control values for any level of lead. However, the ratio 18:2/20:4 declined from a control value of 3.3 to approximately 2 for all lead treatments. The ratio of saturated/monoenoic fatty acids increased with dietary lead levels above 1000 ppm. In a second experiment 10 male broiler chicks per treatment were fed either a control diet or the control diet plus 2000 ppm lead, 60 ppm cadmium, 500 ppm mercury, or 10 ppm selenium (as Pb acetate X 3 H2O, CdSO4, HgCl2, or Na2SeO3, respectively) for 21 days. Six chicks from each group were killed at 21 days, and hepatic fatty acid composition was determined for each chick. In comparison to control, the ratio 18:2/20:4 was lowered by lead but unaffected by cadmium, mercury, and selenium. The data suggest that lead may increase tissue peroxidation (as noted by other workers) via a relative increase of 20:4 fatty acid and that a decrease of hepatic ratio 18:2/20:4 may be a specific sign of lead toxicity.

Loss of delta-6-desaturase activity as a key factor in aging

Aging is characterized by a wide variety of defects, particularly in the cardiovascular and immune systems. Cyclic AMP levels fall, especially in lymphocytes. Delta-6-desaturase (D6D) levels have been found to fall rapidly in the testes and more slowly in the liver in aging rats. D6D is an enzyme which converts cis-linoleic acid to gamma-linolenic acid (GLA). Other factors which inhibit D6D activity are diabetes, alcohol and radiation, all of which may be associated with accelerated aging. In meat eaters or omnivores which can acquire arachidonic acid from food, the main consequences of D6D loss will be deficiencies of GLA, dihomogamma-linolenic acid (DGLA) and prostaglandin (PG) E1. PGE1 activates T lymphocytes, inhibits smooth muscle proliferation and thrombosis, is important in gonadal function and raises cyclic AMP levels in many tissues. It is a good candidate for a key factor lost in aging. Moderate food restriction, the only manoeuvre which consistently slows aging in homoiotherms, raises D6D activity by 300%. Other factors important in regulating D6D and the conversion of GLA to PGE1 are zinc, pyridoxine, ascorbic acid, the pineal hormone, melatonin, and possibly vitamin B3. GLA administration to humans has been found to lower blood pressure and cholesterol, and to cause clinical improvement in patients with Sjogren's syndrome, scleroderma and alcoholism. These diseases are associated with some features of accelerated aging. The proposition that D6D loss is not only a marker of aging but a cause of some of its major manifestations is amenable to experimental test even in humans. The blocked enzyme can be by-passed by giving GLA directly.

The metabolism of alpha-linolenic acid by the foetal rat

The metabolism of [I-C14]linolenic acid (18:3 omega 3) by the foetal rat was studied in vivo and in vitro It is suggested that foetal brain has the capacity to convert linolenic to docosahexaenoic (22: 6 omega 3) acid to meet its requirements for docosalhexaenoic acid.

Lipid and arachidonic acid accumulation in naturally regressing porcine corpora lutea

Patterns of luteal lipid and arachidonic acid accumulation were examined in relation to luteal progesterone and prostaglandin F synthesis in 30 sows and gilts between days 8 and 18 of the estrous cycle. Net in vitro release of progesterone from luteal tissue declined from 722 ng/100 mg tissue at day 8 to 81 ng/100 mg tissue at day 18. Although statistical significance was not present, net prostaglandin F release increased slightly from 8.6 to 13.9 ng/100 mg tissue. Luteal free cholesterol, esterified cholesterol, and free fatty acid contents did not change between days 8 and 18 whereas triglycerides accumulated rapidly between days 14 and 18 of the estrous cycle. Phospholipids increased between days 8 and 12, plateaued at 20.2 mg/g between days 14 and 16, and decreased to 15.4 mg/g on day 18. Between days 12 and 18, arachidonic acid increased from 19.4 to 34.8% in cholesterol esters, from 10.1 to 22.5% in triglycerides, and from 12.3 to 27.2% in luteal free fatty acids. Arachidonic acid in luteal phosphol lipids increased from 21.3 to 25.1% between days 14 and 16 of the estrous cycle. Luteal regression was associated with conservation of arachidonic acid. Based on blood plasma lipid fatty acid compositions, the corpus luteum elongated and desaturated essential fatty acids. Within porcine corpora lutea, calculated free arachidonic acid content was adequate for maintenance of prostaglandin synthesis.

Lipid metabolism and in vitro production of progesterone and prostaglandin F during induced regression of porcine corpora lutea

The effects of Cloprostenol administration on porcine luteal lipid and arachidonic acid accumulation were examined in relation to luteal in vitro progesterone and prostaglandin F synthesis in 18 mature gilts at day 12 of the estrous cycle. Basal and net in vitro release of progesterone from luteal tissue was depressed at 8 hr after treatment whereas net in vitro release of prostaglandin F was elevated at 8 hr. Inclusion of copper dithiothreitol or reduced glutathione in the incubation media resulted in minor alterations of in vitro release of progesterone and prostaglandin F and no changes in composition of luteal lipids or fatty acids. Luteal contents of triglyceride had increased by 8 hr after treatment whereas contents of free and esterified cholesterols had increased by 32 hr after Cloprostenol administration. Luteal contents of phospholipid and free fatty acids were not affected by Cloprostenol administration. At 32 hr after treatment, percentages and content of arachidonic acid had increased in luteal cholesterol esters and triglycerides. Although arachidonic acid percentages increased in luteal free fatty acids and phospholipids, calculated arachidonic acid contents did not change following Cloprostenol administration. Induced luteal regression was associated with decreased in vitro progesterone release, increased in vitro prostaglandin F release, and accelerated lipid and arachidonic acid accumulation within the corpus luteum. The effects of altered lipid metabolism on release of prostaglandin F could not be defined. However, availability of arachidonic acid did not appear to be rate-limiting in relation to luteal in vitro prostaglandin F synthesis.

Desaturation of eicosa-11,14-dienoic acid in human testes

The metabolism of [1-14C]eicosa-11,14-dienoic acid was investigated in human testes using whole tissue minces and microsomal preparations. Both types of preparations catalyzed the desaturation of the labeled diene to eicosa-8,11,14-trienoic as well as eicosa-5,11,14-trienoic acid. The reported results, therefore, indicate that human testicular tissue, as well as rat testicular tissue (reported previously), is capable of utilizing eicosa-11,14-dienoic acid as a precursor of arachidonic acid. Since it is known that there is no delta 8 desaturase activity in rat liver and brain, these studies support the concept that there is a tissue variation in this enzymatic pathway.

Metabolism of eicosa-11,14-dienoic acid in rat testes. Evidence for delta8-desaturase activity

The metabolism of [14C]eicosa-11,14-dienoic acid was investigated in rat testes in vivo and in vitro. Intratesticular injection of [1-14C]eicosa-11,14-dienoic acid resulted in the appearance of radioactivity (4-30% of 14C in total fatty acids) in 20-carbon trienoic fatty acids and a small amount (2-3.5%) in arachidonic acid. Analysis of the 20-carbon trienoic acid fraction by ozonolysis indicated that 15 to 34% of the 14C in this fraction was in an 8-carbon fragment originating from eicosa-8,11,14-trienoic acid. The rest (66 to 84%) was in a 5-carbon fragment, presumably originating from eicosa-5,11,14-trienoic acid. Incubation of testicular tissue minces or microsomes with [1-14C]eicosa-11,14-dienoic acid yielded labeled eicosa-8,11,14- and eicosa-5,11,14-trienoic acids in proportions similar to those obtained in vivo. Added unlabeled acetate had no effect on the formation of [14C]eicose-8,11,14-trienoic acid in vitro. Therefore, it is unlikely that the labeled eicosa-8,11,14-trienoic acid arose from elongation of octadeca-6,9,12-trienoic acid with labeled acetate derived from bio-oxidation of the labeled substrate. These results are compatible with a limited desaturation of eicosa-11,14-dienoic acid to eicosa-8,11,14-trienoic acid and provide evidence for delta8 desaturate activity in rat testis.

Effect of polyunsaturated fatty acids of the alpha-linolenic series on the development of rat testicles

The effect of 22:6 omega 3 acid provided by dietary fish oil on the development of germinal tissue of rat testes, fatty acid composition of lipids, and linoleic or alpha-linolenic acid delta 6 desaturation capacity was investigated. Results were compared to those obtained in animals fed methyl palmitate and sunflower seed oil (linoleate). At 7 and 9 weeks of age, development of germinal tissue of animals fed fish oil was normal. The fatty acid composition showed a decrease in 22:5 omega 6 acid content and an increase in 22:6 omega 3 acid in triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine. The fatty acid delta 6 desaturation capacity of testicules microsomes was increased. It is suggested that 22:6 omega 3 acid may functionally replace 22:5 omega 6 acid in germinal tissue.

Positional isomers of unsaturated fatty acids in rat liver lipids

The fatty acids of liver lipids from rats raised on a fat free diet from the 30th to the 90th day after birth were analyzed with special regard to the detection of positional isomers of mono-, di-, tri-, and tetraenoic fatty acids. The methyl esters obtained after transesterification of total lipids were separated by argentation chromatography into five fractions: I saturated, II monoenoic, III dienoic, IV dienoic nonmethylene interrupted, V tri- and tetraenoic fatty acid esters. After hydroxylation of the double bonds with osmium tetroxide, the analysis of the poly-O-trimethylsilyl derivatives by gas liquid chromatography on S.C.O.T. columns combined with mass spectrometry revealed the presence of 19 monoenoic, 15 dienoic, and 9 trienoic as well as 3 tetraenoic fatty acid isomers including the normally occurring representatives of the (n-3), (n-6), (n-7), and (n-9) fatty acid families. The majority of the identified isomers can be coordinated to one of these families like 7-16:1; 11-20:1; 6,9-18:2; 8,11-20:2; 5,11-20:2; 5,8,11-20:3; 7,10,13-22:3 to the (n-9) family, 11-18:1; 13-20:1; 5,11-18:2; 7,13-20:2; 6,11-18:2; 6,9-16:2; 8,11-18:2; 10,13-20:2; 5,8,11-18:3; 7,10,13-20:3; 4,7,10,13-20:4 to the (n-7) family and 11,14-20:2; 5,11,14-20:3; 6,9,12-18:3; 8,11,14-20:3; 5,8,11,14-20:4; 7,10,13,16-22:4 to the (n-6) family. All these naturally occuring isomers can be placed into a network of desaturation and chain elongation steps which allows certain conclusions about the substrate specificity of the delta6-, delta5- and delta4-desaturase systems. The great number of isomers found in the (n-7) family indicates that the members of this family are actively metabolized in partial essential fatty acid deficiency.

Regulatory function of delta6 desaturate -- key enzyme of polyunsaturated fatty acid synthesis

The delta6 desaturation of unsaturated acyl-CoA is the first reaction involved in the normal biosynthesis of all polyunsaturated fatty acids families in animal microsomes. Due to this key position it can regulate the biosynthesis of the fatty acids of the series. The reaction is modified by competition with substrates and products, ATP, and acyl-CoA acceptors. Dietary glucose and fructose inhibit the enzyme whereas protein diets and essential fatty acid deficient diets enhance the reaction independently of hormonal effects. The enzyme is sensitive to hormones concentration. Insulin enhance the reaction but the effect is eliminated by protein synthesis inhibition. Hyperglucemic hormones as glucagon, and epinephrine depress the activity of the delta6 desaturase by reactions triggers by an increase of cAMP concentration. The lateral relation of linoleic or alpha-linolenic microsomal elongation is insensitive to insulin, glucagon, epinephrine and protein. All these effects have been proved by either in vivo experiments or cell culture using linoleic or alpha-linolenic acids as substrates.

Comparative effect of glucagon, dibutyryl cyclic AMP, and epinephrine on the desaturation and elongation of linoleic acid by rat liver microsomes

The effect of glucagon, dibutyryl cyclic adenosine 3',5'-monophosphate, and epinephrine on the biosynthesis of polyunsaturated fatty acids of the linoleic acid family was studied. The incubations were performed with rat liver microsomes and labeled linoleic acid under desaturating and elongating conditions. Under desaturating conditions linoleic acid was converted to gamma-linolenic acid, whereas under elongating conditions it was converted to 20:2omega6. Glucagon, dibutyryl cyclic AMP, and epinephrine decreased the oxidative desaturation of linoleic acid to gamma-linolenic acid while the elongating reaction was not modified in the experimental conditions tested. Consequently, the results support the hypothesis that the oxidative desaturation of linoleic acid to gamma-linolenic acid is the main controllable step in the biosynthesis of polyunsaturated fatty acids of the linoleic acid family in the microsomes.