Interrelationship between the dietary regulation of fatty acid synthesis and the fatty acyl-CoA desaturases

Opportunities and challenges in developing stearoyl-coenzyme A desaturase-1 inhibitors as novel therapeutics for human disease

This review provides an overview of stearoyl-coenzyme A desaturase-1 (SCD1) as a novel therapeutic target for metabolic disorders and other indications. Target validation is reviewed, and limitations due to incomplete knowledge of the relevant biological systems are described. Assay development, particularly for high throughput screening, and characterization of SCD1 inhibition are summarized. The progress and evolution in medicinal chemistry are discussed, specifically focusing on key attributes of the most advanced SCD1 inhibitors described in the primary literature and in patent applications. This work culminated in numerous companies identifying potent selective inhibitors, some of which progressed to early clinical development. The status of current SCD1 drug discovery programs is reviewed. Challenges are discussed, and potential new directions are indicated.

Daily intake of cod or salmon for 2 weeks decreases the 18:1n-9/18:0 ratio and serum triacylglycerols in healthy subjects

Intake of fish and omega-3 (n-3) fatty acids is associated with a reduced concentration of plasma triacylglycerols (TAG) but the mechanisms are not fully clarified. Stearoyl-CoA desaturase-1 (SCD1) activity, governing TAG synthesis, is affected by n-3 fatty acids. Peripheral blood mononuclear cells (PBMC) display expression of genes involved in lipid metabolism. The aim of the present study was to estimate whether intake of lean and fatty fish would influence n-3 fatty acids composition in plasma phospholipids (PL), serum TAG, 18:1n-9/18:0 ratio in plasma PL, as well as PBMC gene expression of SCD1 and fatty acid synthase (FAS). Healthy males and females (n = 30), aged 20-40, consumed either 150 g of cod, salmon, or potato (control) daily for 15 days. During intervention docosahexaenoic acid (DHA, 22:6n-3) increased in the cod group (P < 0.05), while TAG concentration decreased (P < 0.05). In the salmon group both eicosapentaenoic acid (EPA, 20:5n-3) and DHA increased (P < 0.05) whereas TAG concentration and the 18:1n-9/18:0 ratio decreased (P < 0.05). Reduction of the 18:1n-9/18:0 ratio was associated with a corresponding lowering of TAG (P < 0.05) and an increase in EPA and DHA (P < 0.05). The mRNA levels of SCD1 and FAS in PBMC were not significantly altered after intake of cod or salmon when compared with the control group. In conclusion, both lean and fatty fish may lower TAG, possibly by reducing the 18:1n-9/18:0 ratio related to allosteric inhibition of SCD1 activity, rather than by influencing the synthesis of enzyme protein.

Obesity - a perspective based on the biochemical interrelationship of lipids and carbohydrates

Many factors affect the onset of obesity including satiety control, reduced levels of physical exercise as well as hormonal and genetic parameters which influence the metabolic pathways leading to the net accumulation of triacylglycerol (TAG). The predominant fatty acid of human adipose tissue TAGs is oleic acid, reflecting primarily the composition of the diet but also the product of de novo lipogenesis. Consequently, both carbohydrates and lipids are potential sources of these stored fats. Many studies have been carried out using a variety of differing experimental protocols on healthy, obese or diabetic humans and animals in positive or neutral energy balance to establish the underlying molecular basis for obesity particularly in humans. This short review discusses the interdependence and control of the metabolism of lipids and carbohydrates as it relates to lipogenesis and proposes a unified hypothesis for obesity which brings together a number of different approaches focusing on (i) the interaction of dietary fat and carbohydrate, which typically represent approximately 80% of the daily caloric intake, and their role in the synthesis of TAGs, (ii) the biochemical pathways which control the amount of TAG produced by controlling the composition of their fatty acids via the action of stearoyl-CoA desaturase (SCD), (iii) the control of lipogenesis and SCD by dietary polyunsaturated fatty acid (PUFA) and (iv) the interaction of PUFAs with the transcription factors, peroxisome proliferator activated receptors (PPAR) alpha and gamma, which maintain the balance between oxidation and storage of lipids. The hypothesis focuses on the central role of stearoyl-CoA desaturase (SCD) and its inhibition by polyunsaturated fatty acids (PUFAs) acting via transcription factors based upon data obtained from both animal and human studies. From these observations it should be possible to determine the relevance of the hypothesis to humans and to speculate how these aspects of metabolism may impact the risk of developing related diseases such as coronary heart disease, Type 2 diabetes and certain forms of cancer.

Cyp1a2 protects against reactive oxygen production in mouse liver microsomes

H(2)O(2) production was evaluated in liver microsomes prepared from Cyp1a1/1a2(+/+) wild-type and Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice pretreated with 5 microg dioxin (TCDD)/kg body wt or vehicle alone. NADPH-dependent H(2)O(2) production in TCDD-induced microsomes from wild-type mice was about one-third of that in noninduced microsomes. In Cyp1a2(-/-) mice, H(2)O(2) production was the same for induced and noninduced microsomes, with levels significantly higher than those in wild-type mice. Cyp1a1(-/-) microsomes displayed markedly lower levels of H(2)O(2) production in both induced and noninduced microsomes, compared with those in wild-type and Cyp1a2(-/-) microsomes. The CYP1A2 inhibitor furafylline in vitro exacerbated microsomal H(2)O(2) production proportional to the degree of CYP1A2 inhibition, and the CYP2E1 inhibitor diethyldithiocarbamate decreased H(2)O(2) production proportional to the degree of CYP2E1 inhibition. Microsomal H(2)O(2) production was strongly correlated to NADPH-stimulated production of thiobarbituric acid-reactive substances, as well as to decreases in microsomal membrane polarization anisotropy, indicative of peroxidation of unsaturated membrane lipids. Our results suggest that possibly acting as an "electron sink," CYP1A2 might decrease CYP2E1-and CYP1A1-mediated H(2)O(2) production and oxidative stress. In this regard, CYP1A2 may be considered an antioxidant enzyme.

Bioconversion of vaccenic acid to conjugated linoleic acid in humans

BACKGROUND

Vaccenic acid (11-trans octadecenoic acid; VA), a major trans fatty acid in the fat of ruminants, is produced in the rumen and converted in tissues to rumenic acid (9-cis, 11-trans octadecenoic acid; RA), an isomer of conjugated linoleic acid, by Delta(9)-desaturase. There are indications that this conversion also occurs in humans.

OBJECTIVE

The aim of this controlled intervention was to study the conversion of VA to RA in humans after consumption of diets with increasing amounts of VA.

DESIGN

Thirty healthy subjects consumed a baseline diet rich in oleic acid for 2 wk. The subjects were then divided into 3 groups (n = 10 per group) and provided a diet containing 1.5, 3.0, or 4.5 g VA/d for 9 d. All diets contained equal amounts of macronutrients and differed only in their fatty acid compositions. The fats were mixed into conventional foods, and nearly all food was provided during the study.

RESULTS

The proportion of VA in serum total fatty acids increased 94%, 307%, and 620% above baseline with the 1.5-, 3.0-, and 4.5-g diets, respectively. This was associated with a linear increase in the proportion of RA. The conversion rate was 19% on average, with significant interindividual differences with all 3 intakes of VA. The urinary excretion of 8-iso-prostaglandin F(2alpha) increased in all groups (P < 0.001).

CONCLUSIONS

The results quantify the desaturation of VA to RA in humans. Conversion is likely to contribute significantly to the amount of RA available to the body, and dietary intakes of VA should thus be taken into account when predicting RA status.

Effect of dietary mono- and polyunsaturated fatty acids on the fatty acid composition of pigs' adipose tissues

In two experiments with growing-finishing pigs six different dietary fats were added to a conventional diet (control--C) to study the effects of dietary monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA) on the fatty acid composition of backfat and kidney fat at similar amounts of double bonds in feed (Exp. 1:7% pork fat--PF, 4.95% olive oil--OO, 3.17% soybean oil--SO) or a constant amount of 5% of processed fats (Exp. 2: partially hydrogenated fat--SAT, fractionated pork fats: olein--OLE, stearin--STE). Compared with the control, PUFA were only slightly increased in backfat of pigs fed PF, OLE, STE or OO, although dietary PUFA intake was up to 70% higher. With SO PUFA were significantly increased in adipose tissues, predominantly at the expense of MUFA. Consequently, a non-linear relationship was found between PUFA intake and proportion in backfat. MUFA were incorporated at the expense of SFA, therefore, adipose tissues of OO fed animals were lowest in SFA. Despite comparable amounts of double bonds in feed (Exp. 1), the degree of unsaturation measured as fat score (sum of double bonds) was in the order SO > OO > PF > C. In contrast, the proportion of SFA was C > PF = SO > OO. Regarding the decisive role of SFA for fat consistency it may be concluded that MUFA should also be considered in feeding recommendations for pigs. Furthermore, in case of a high dietary supply of MUFA, a simple index of double bonds might not be sufficiently conclusive to judge pig fat quality.

Nitric oxide and peroxynitrite anion modulate liver plasma membrane fluidity and Na(+)/K(+)-ATPase activity

Free radicals attack membranes and frequently alter their fluidity and function. The aim of the present work was to study the effect of nitric oxide (NO) radical and peroxynitrite anion on basolateral liver plasma membrane fluidity and on the activity of Na(+)/K(+)-ATPase. Basolateral membranes (BM) were isolated by ultracentrifugation in sucrose gradients and characterized enzymatically. BM were incubated with SNAP (a NO donor) or SIN-1 (a peroxynitrite donor). The release of NO or peroxynitrite was monitored by measuring NO(-)(2) + NO(-)(3). Relative fluidity was measured by polarization of fluorescence. NO increased membrane fluidity while peroxynitrite decreased it in a concentration-dependent manner. Na(+)/K(+)-ATPase activity was reduced by NO or peroxynitrite. Peroxynitrite anion inhibits ATPase activity in part by decreasing fluidity. However, it is very likely that both compounds inhibit ATPase activity by oxidation of the thiol groups of the enzyme. Our results suggest that NO may exert part of its biological effects by modulating membrane fluidity and function.

Hormonal regulation of stearoyl coenzyme-A desaturase 1 activity and gene expression in primary cultures of chicken hepatocytes

Previous studies have provided evidence for the important role of liver stearoyl-CoA desaturase (SCD) in excessive adiposity in the chicken and suggest that the difference in SCD activity between fat and lean chickens could be explained by a difference in SCD1 gene expression. In the present study, the regulation of SCD1 gene expression was analyzed as the result of insulin and glucagon action, using primary cultures of 6-week-old chicken hepatocytes. Insulin increased SCD1 activity and mRNA levels, whereas glucagon decreased dramatically both the enzyme activity and the mRNA levels. Nuclear run-on transcription assays and mRNA stability investigations demonstrated that insulin and glucagon effects on SCD1 gene expression was primarily transcriptional. Furthermore, the results indicated that the glucagon-mediated inhibition of SCD1 gene transcription was more potent than just counteracting the insulin-mediated effect. These data represent the first demonstration that the glucagon effect on the SCD1 gene expression is primarily transcriptional. Moreover, among hepatic genes involved in lipid metabolism in chicken, SCD1 is the first gene shown to be regulated at the transcriptional level by insulin, in the absence of triiodothyronine. These data point out the potency of the growing chicken hepatocyte culture model in contrast with the embryonic cell culture model as regards the investigations of the insulin effect on gene expression.

Effects of a mixture of organisms, Lactobacillus acidophilus or Streptococcus faecalis on delta6-desaturase activity in the livers of rats fed a fat- and cholesterol-enriched diet

The effect of a mixture of organisms (a probiotic mixture) comprising Bacillus, Lactobacillus, Streptococcus, Clostridium, Saccharomyces, and Candida (10(7-8) colony-forming units/g rice bran of each component) on delta6-desaturase activity in liver microsomes was compared with those of Lactobacillus acidophilus and Streptococcus faecalis. There were four treatment groups. Each group of these rats received rice bran (control), the mixture of organisms, L. acidophilus, or S. faecalis (30 g/kg) along with a fat- and cholesterol-enriched diet for 4 wk. The serum total cholesterol concentration of the group fed the mixture of organisms was reduced by 15-33% compared with the other groups at the end of the 4-wk feeding period (P<0.05). The proportion of palmitic acid in the serum phosphatidylcholine (PC) for the control group was significantly higher than those of the other groups. The proportion of arachidonic acid in the serum PC for the mixed-organism group was also significantly higher than those of the other groups. The proportion of arachidonic acid in the liver PC for the mixed-organism group was significantly higher than those of the control and S. faecalis groups. The ratio of arachidonic acid/linoleic acid was significantly higher in the liver PC of rats fed the mixed organisms compared with the control group (P<0.05). The delta6-desaturase activity in the liver microsomal fraction of the mixed-organism group was significantly higher than those of the other groups. The delta6-desaturase activity correlated positively with the ratio of arachidonic acid/linoleic acid of liver PC, the correlation coefficient (r) being 0.819 (P<0.001). The results indicate that the effect of the mixture of organisms was to increase delta6-desaturase activity and serum arachidonic acid and decrease cholesterol compared to the other organisms and control, but the mechanism whereby the enzyme activity was related to serum cholesterol does not appear to have been explored.

Fatty acid composition in fractions of structural and storage lipids in liver and skeletal muscle of hereditary hypertriglyceridemic rats

The fatty acid (FA) compositions of liver and skeletal muscle structural lipids, overall phospholipids and phosphatidylcholine, and triglycerides (TG) were determined in the hereditary hypertriglyceridemic (HTG) rat, a nonobese animal model of the insulin resistance syndrome. Four groups of HTG rats and four groups of control animals were fed equal-energy diets for two weeks: basal (B), high-sucrose (HS), or fish oil-supplemented basal (BFO) or high-sucrose (HSFO) diets. In the liver of HTG rats, a decrease of n-6 long-chain polyunsaturated FA (PUFA), especially in 20:4n-6, in comparison with controls was found. Moreover, a concomitant accumulation of 18:2n-6 in structural lipids was observed. These differences were more pronounced in liver than in skeletal muscle. HS feeding raised the proportion of 18:1n-9 and decreased 18:2n-6 in lipid fractions. In both tissues and in both strains, the amounts of long-chain n-3 PUFA, as well as the level of total C20-22 PUFA, went up after fish oil feeding. However, the effects were somewhat less pronounced in the HTG rats. The increase in n-3 PUFA occurred mainly at the expense of reduced levels of 18:2n-6 in structural lipids and of 18:1n-9 in triglycerides. These changes were associated, in companion studies reported in this volume, with improved insulin action in HTG rats. In conclusion, the FA composition in lipid subclasses of HTG rats differs significantly from the controls mainly in liver structural lipids, suggesting the impairment of PUFA desaturation. Dietary change effected a similar modulation of FA profile across both strains, with fish oil increasing the levels of long-chain PUFA toward control values in the NTG rats. The HTG rat thus provides an interesting animal model for the study of impaired fatty acid metabolism.

Lipid metabolism in pigs fed beef tallow or high-oleic acid sunflower oil

The objectives of this study were to establish the effects of high-oleic sunflower oil (HOSO) and beef tallow on tissue fatty acids and stearoyl-CoA desaturase activities in swine; and to compare effects of HOSO and tallow on swine plasma triglycerides and lipoprotein-cholesterol fractions. Sixteen gilts were divided into two groups: eight fed a control diet containing 10 g/100 g beef tallow, and eight fed a diet containing 10 g/100 g HOSO. Plasma samples were obtained before feeding began and at 4 weeks and 8 weeks of dietary treatment. Samples were obtained from longissimus dorsi muscle, liver, adipose and duodenal mucosa for the measurement of fatty acid composition and stearoyl-CoA desaturase activity. The HOSO diet increased (P < 0.05) the concentrations (mumol/g wet weight of tissue) of 18:1 and 18:2 (n-6) in adipose tissue. In muscle from pigs fed the HOSO diet, concentrations of 14:0, 16:0, 16:1, 18:0, 18:1, and 18:2 (n-6) decreased (P < 0.05) relative to muscle from pigs fed the beef tallow diet; only 14:0 and 16:1 were reduced in liver by the HOSO diet. Stearoyl-CoA desaturase specific activity [(pmol 7 min-1 mg-1 microsomal protein)] was 40 percent lower, and activity expressed as pmol 7 min-1 g-1 tissue) was 20 percent lower, in adipose tissue of pigs fed HOSO (P < 0.05). No differences due to dietary treatment were observed for desaturase activity from muscle, liver or intestinal mucosa. Plasma triglycerides declined steadily in the tallow-fed pigs, possibly reflecting the lower percentages of liver 18:0 and 18:1 acids, relative to the HOSO-fed pigs. The animals responded similarly to the addition of fat (beef tallow or HOSO) to their diets with increased (P < 0.05) plasma total, LDL- and HDL-cholesterol by 4 weeks of treatment. Total cholesterol, LDL-, VLDL- or HDL-cholesterol were not different between pigs fed beef tallow or HOSO. Thus, differences in fatty acid composition of the diets were sufficient to alter tissue fatty acid composition and adipose tissue desaturase activity, but insufficient to alter plasma lipoprotein cholesterol.

The regulation of stearoyl-CoA desaturase (SCD)

The stearoyl-CoA desaturase gene family encodes stearoyl-CoA desaturase, the key enzyme involved in the biosynthesis of unsaturated fatty acids, as well as in the regulation of this process. Because of the important role that the SCD gene product plays in fat cell metabolism, future studies on SCD1 gene expression could provide new insights into the role of fatty acids in cellular regulation, metabolism, and gene expression both in normal and disease states. In addition, the SCD gene family can be used as a model to study mechanisms of cellular differentiation, tissue-specific gene expression, and dietary and hormonal regulation of gene expression.

Stearoyl-CoA desaturase activity in primary culture of chicken hepatocytes. Influence of insulin, glucocorticoid, fatty acids and cordycepin

1. Stearoyl-CoA desaturase (delta 9-desaturase) activity was measured in chicken primary hepatocytes, as a function of time in culture. 2. When using fasted donor animals, the desaturase activity was low at the beginning of culture and then increased steadily to a maximum value between 30 and 70 hr of culture. When hepatocyte cultures were prepared from fed animals, enzyme activity was high at the beginning of culture and maintained thereafter at similar values to those obtained in cultured hepatocytes from fasted animals after 30 hr of culture. 3. Insulin significantly enhanced enzyme activity when added to the culture medium at a 10(-9) M concentration, and a small stimulating effect was also observed with 10(-6) M dexamethasone. 4. Linoleic acid (0.5 mM) added to the culture medium as albuminic complex partly inhibited delta 9-desaturase activity. 5. Cordycepin (3' deoxyadenosine) decreased enzyme activity when present at a 3 micrograms/ml concentration in the culture medium. 6. Taken together, the induction of enzyme activity in culture, its impairment by cordycepin and response to insulin and linoleic acid strongly suggest that synthesis and translation of the delta 9-desaturase mRNA occur in chicken hepatocytes in primary culture, and that this cellular model may be a useful tool for further studies on delta 9-desaturase regulatory mechanisms.

Influence of linoleic acid on desaturation and uptake of deuterium-labeled palmitic and stearic acids in humans

Objectives of this study were to investigate the desaturation of stearic acid (18:0) and palmitic acid (16:0), to determine if differences in their metabolism provide a reasonable explantation for differences in their effect on serum cholesterol levels, and to investigate the affect of linoleic acid on delta 9-desaturase products in man. Deuterium-labeled 16:0 and 18:0 were used to follow the metabolism of these fatty acids in young adult male subjects that were pre-fed diets containing two different levels of linoleic acid. Results indicate that absorption of 16:0 and 18:0 was similar when all components of the mixture used to formulate the deuterated fat mixture were kept above the melting point of tristearin. The percent of 18:0 desaturated to 9c-18:1 was higher than the percent of 16:0 desaturated to 9c-16:1 (9.2% vs. 3.9%). The subject-to-subject variability suggests that differences in ability to desaturate saturated fatty acids may be related to the variability observed in response of serum cholesterol levels to dietary saturated fatty acids. Data for the distribution of 16:0 and 18:0 between triacylglycerol and phosphatidylcholine (PC) was markedly different. Based on PC data, phospholipid acyltransferase selectivity was about 2-fold higher for 18:0 than for 16:0. A 2-fold difference in the linoleic acid content of the pre-fed diets had little influence on desaturation or distribution of 16:0 and 18:0 between plasma lipid classes. A deuterium isotope effect was estimated to reduce delta 9-desaturase enzyme activity by 30-50%.

[The effect of zinc depletion on the fat content and fatty acid composition of the liver and brain in forcibly fed rats]

In the present work the influence of zinc deficiency on fat content and fatty acid composition of liver and fatty acid composition of brain of rats with a high food intake was investigated. Using the force-feeding technique the rats were fed 14.5 g food daily at days 1 to 4, and then 11.6 g food for later days. After 7 days the zinc-deficient animals had a fatty liver which was characterized by an increase in fat content (68%) and dry matter (23%). The amounts of lauric acid, myristic acid, myristoleic acid, palmitic acid, palmitoleic acid, and oleic acid were also increased by 100 to 200% in the liver of zinc-deficient animals, whereas the amount of arachidonic acid was decreased by 29%. The amounts of phosphatidylcholine and phosphatidylethanolamine in the liver were not changed by zinc deficiency, but the fatty acid composition of these phospholipids was changed. The liver phospholipids of zinc-deficient animals had a decreased proportion of arachidonic acid, but an increased proportion of docosahexaenoic acid. In the zinc-deficient animals there also existed a positive correlation between the fat content in the liver and the ratio between linoleic and arachidonic acid in the liver and a negative correlation between the fat content in the liver and the amount of arachidonic acid in the liver. These correlations as well as the changes in liver fatty acid composition of zinc-deficient animals suggest that the fatty liver might be the result of a disturbed metabolism of linoleic acid. In contrast, zinc deficiency did not influence the fatty acid composition of brain. This means that brain is protected against the effects of short-term zinc deficiency.

Effect of L-triiodothyronine on liver microsomal delta 6 and delta 5 desaturase activity of male rats

The effect of different doses of L-triiodothyronine (T3) on the activity of delta 6 and delta 5 desaturases and lipid fatty acid composition was studied in liver microsomes of male rats. The activity of delta 6 and delta 5 desaturases was decreased 24 and 28%, respectively, in animals administered a daily intraperitoneal dose of 1000 micrograms T3/100 g body wt. for 5 days, whereas with 500 micrograms T3/100 g body wt. only delta 6 desaturase activity was decreased. On the other hand, no enzyme activity changed at a shorter period of hormone treatment. Changes in microsomal fatty acid composition did not seem to be a direct consequence of desaturation activity, since after 1 and 5 days of T3 treatment, the concentrations of 18:2 (n-6) and 20:3 (n-6) decreased and only after 1 day that of 20:4 (n-6) increased in spite of unchanged or decreased delta 6 and delta 5 desaturase activities. Other factors than desaturation activity must be involved in fatty acid composition of thyroid hormone-treated rats, such as diet, membrane lipid synthesis and degradation, fatty acid turn-over and oxidation.

Nonessential fatty acids in formula fat blends influence essential fatty acid metabolism and composition in plasma and organ lipid classes in piglets

The n-6 and n-3 fatty acid status of developing organs is the cumulative result of the diet lipid composition and many complex events of lipid metabolism. Little information is available, however, on the potential effects of the saturated fatty acid chain length (8:0-16:0) or oleic acid (18:1) content of the diet on the subsequent metabolism of the essential fatty acids 18:2n-6 and 18:3n-3 and their elongated/desaturated products. The effects of feeding piglets formulas with fat blends containing either coconut oil (12:0 + 14:0) or medium chain triglycerides (MCT, 8:0 + 10:0) but similar levels of 18:1, 18:2n-6 and 18:3n-3, or MCT with high or low 18:1 but constant 18:2n-6 and 18:3n-3 on the fatty acid composition of plasma, liver and kidney triglycerides, phospholipids and cholesteryl esters, and of brain total lipid, were studied. Diet-induced changes in the fatty acid composition of lipid classes were generally similar for plasma, liver and kidney. Dietary 18:1 content was reflected in tissue lipids and was inversely associated with levels of 18:2n-6. Lower percentage of 18:2n-6, however, was not associated with lower levels of its elongated/desaturated product 20:4n-6 but was associated with higher levels of 22:6n-3. Feeding coconut oil vs. MCT resulted in lower 18:1 levels in all lipids, and higher percentages of 20:4n-6 in tissue phospholipid. Increasing the dietary n-6/n-3 ratio from 5 to 8 significantly increased tissue percentage of 18:2n-6 and decreased phospholipid 22:6n-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in liver microsome lipids and plasma fatty acids induced by dietary orotate in the weanling rat

1. Dietary orotate produced a decrease in total plasma fatty acids which was reflected in low values of saturated, monounsaturated and polyunsaturated fatty acids longer than 18 carbon atoms of the n-6 series. The relative content of saturated fatty acids in microsomes of animals fed orotate was also decreased. 2. Rat liver delta-9 desaturase activity was lower in the group fed orotate. However, delta-6 desaturase activity did not show significant differences between the groups. 3. Microsomal cholesterol content was lower in rats fed orotate than in controls but phospholipid phosphorus contents were similar. These results suggest a direct effect of dietary orotate on the key enzymes which regulates cholesterol liver metabolism.

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.

Effects of dietary purified eicosapentaenoic acid (20:5 (n-3)) and docosahexaenoic acid (22:6(n-3)) on fatty acid desaturation and oxidation in isolated rat liver cells

The effects of dietary supplementation of eicosapentaenoic acid (20:5(n-3), EPA) and docosahexaenoic acid (22:6(n-3), DHA) on the metabolism of polyunsaturated fatty acids were studied in isolated rat liver cells. Both pure EPA and pure DHA and a mixture of the two n-3 fatty acids in different doses were used. The supplementation of moderate amounts of n-3 fatty acids suppressed the activity of delta 6-desaturase (50%) and to a smaller extent of the delta 5-desaturase (60-70%) compared to controls. When higher doses of dietary purified EPA and DHA were used, this inhibitory effect on the delta 6- and delta 5-desaturase activities disappeared. The delta 4-desaturase activity seemed to be unaffected by the feeding conditions used. The supplementation of the n-3 fatty acids in the diet at all dose levels used increased the beta-oxidation of all the polyunsaturated fatty acids, especially of linoleic acid, linolenic acid and eicosapentaenoic acid. The results suggest an increase both in peroxisomal and mitochondrial beta-oxidation. The peroxisomal beta-oxidation of n-3 fatty acids seemed to be particularly increased.

The fatty acid chain elongation system of mammalian endoplasmic reticulum

Much has been learned about FACES of the endoplasmic reticulum since its discovery in the early 1960s. FACES consists of four component reactions, requires the fatty acid to be activated in the form of a CoA derivative, utilizes reducing equivalents in the form of NADH or NADPH, is induced by a fat-free diet, resides on the cytoplasmic surface of the endoplasmic reticulum, appears to function in concert with the desaturase system and appears to exist in multiple forms (either multiple condensing enzymes connected to a single pathway or multiple pathways). FACES has been found in all tissues investigated, namely, liver, brain, kidney, lung, adrenals, retina, testis, small intestine, blood cells (lymphocytes and neutrophils) and fibroblasts, with one exception--the heart has no measurable activity. Yet, much more needs to be learned. The critical, inducible and rate-limiting condensing enzyme has resisted solubilization and purification; the purification of the other components has met with limited success. We know nothing about the site of synthesis of each component of FACES. How is each component enzyme integrated into the endoplasmic reticulum membrane? Is there a single mRNA directing synthesis of all four components or are there four separate mRNAs? How are elongation and desaturation coordinated? What is (are) the physiological regulator(s) of FACES--ADP, AMP, IP3, G-proteins, phosphorylation, CoA, Ca2+, cAMP, none of these? The molecular biology of FACES is only in the fetal stage of development. We are only scratching the surface--it is an undiscovered country.

Influence of testosterone administration on the biosynthesis of unsaturated fatty acids in male and female rats

The in vivo effect of testosterone administration to male or female rats on the biosynthesis of unsaturated fatty acids of liver was studied. Twenty-four hours after injection of testosterone (260 micrograms/kg), delta 9 desaturase activity increased significantly, whereas the activities of delta 5 and delta 6 desaturases were strongly depressed. These effects were more pronounced in female than in male animals. The fatty acid composition of plasma and liver (homogenates, crude microsomes and cytosol) showed differences between the sexes. In males, the percentage of palmitic acid and the 18:1/18:0 ratio were higher whereas the 20:4(n-6)/18:2(n-6) ratio was lower than in female rats. The administration of testosterone significantly modified the fatty acid pattern in all fractions studied. Analytical data correlated with alterations in the fatty acid desaturase activities caused by the hormone. It is suggested that the mechanism by which testosterone exerts its effect on unsaturated fatty acid biosynthesis is different that that previously demonstrated by glucocorticoid action. The effects produced by testosterone may be of biological significance in atherosclerosis pathogenesis.

Effect of L-triiodothyronine on delta 9 desaturase activity in liver microsomes of male rats

Male rats injected with a single saturating dose of L-triiodothyronine (T3) showed, after a lag time of approximately eight hr, a sharp rise in delta 9 desaturase activity. Desaturase activity reached a plateau which was 1-1.2 times above the base line levels of rats which were not hormone-treated. The plateau was maintained for five days in animals which were kept on daily hormone-treatment. The increase in delta 9 desaturase activity by T3 required ongoing protein synthesis, because the increase in enzymatic activity due to hormone treatment was completely abolished in the presence of cycloheximide. These findings suggest that cycloheximide may block the induction of delta 9 desaturase by T3 and/or inhibit the synthesis of protein(s) essential to the desaturation-response to T3. Modifications observed in liver microsomal fatty acid composition in T3 treated rats were independent of the effect on desaturation. It is suggested that other factors, such as diet, membrane lipid synthesis and degradation, as well as fatty acid turnover and oxidation, could be involved in affecting the fatty acid composition of thyroid hormone-treated rats.

The effects of alkylthioacetic acids (3-thia fatty acids) on fatty acid metabolism in isolated hepatocytes

Long-chain alkylthioacetic acids (3-thia fatty acids) inhibit fatty acid synthesis from [1-14C]acetate in isolated hepatocytes, while fatty acid oxidation is nearly unaffected or even stimulated. Desaturation of [1-14C]stearate (delta 9-desaturase) is also unaffected. [1-14C]Dodecylthioacetic acid (a 3-thia fatty acid) is incorporated in triacylglycerol and in phospholipids more efficiently than [1-14C]palmitate in isolated hepatocytes. The metabolism of [1-14C]dodecylthioacetic acid to acid-soluble products (by omega-oxidation) is slow compared to the oxidation of [1-14C]palmitate. In hepatocytes from adapted rats (rats fed tetradecylthioacetic acid for 4 days) the rate of [1-14C]palmitate oxidation is increased and its rate of esterification is decreased. Stearate desaturation is also decreased. The rate of cyanide-insensitive peroxisomal fatty acid beta-oxidation is several-fold increased. The metabolic effects of long-chain 3-thia fatty acids are discussed and it is concluded that they behave essentially like normal fatty acids except for their slow breakdown due to the sulfur atom in the 3 position, which blocks normal beta-oxidation.

Effects of dietary fats on fatty acid composition and delta 5 desaturase in normal and diabetic rats

We have studied the effect of various diets on the phospholipid fatty acid composition and in vitro delta 5 desaturase activity of hepatic microsomes derived either from the normal or streptozotocin-induced diabetic rat. The diets studied were the standard rat chow diet and a basal fat-free diet supplemented either with 20 percent saturated fat, 20 percent unsaturated fat, or 20 percent menhaden oil. Phospholipid fatty acid composition analysis revealed that the normal rat fed the saturated fat or menhaden oil diet had significantly decreased arachidonate levels, consistent with decreased delta 5 desaturase activities and decreased 18:2n-6 intake. On the contrary, the unsaturated fat diet decreased dihomo-gamma-linolenate and increased arachidonate levels, without increased delta 5 desaturase activity. Streptozotocin-induced diabetes resulted in decreased arachidonate and delta 5 desaturase activity. The unsaturated fat diet fed to the diabetic rat also failed to correct this decreased delta 5 desaturase activity. The unsaturated fatty acids in this diet also displaced a substantial amount of n-3 fatty acids in both normal and diabetic microsomes, due to the competition between these two fatty acid families for incorporation into the membrane phospholipids. Conversely, the menhaden oil diet fed to the normal and diabetic rats displaced n-6 fatty acids, reduced delta 5 desaturase activity, and enhanced 22:6n-3 incorporation into diabetic microsomes.

Changes in lipid composition of liver microsomes and fatty acyl-CoA desaturase activities induced by medium chain triglyceride feeding

Changes in fatty acid composition, microsomal delta 9- and delta 6-desaturase activities and liver contents of cholesterol and phospholipids were studied in rats fed medium chain triglyceride-supplemented diets. Weanling rats were divided into four groups and fed for three weeks a basal diet with different 10%-fat supplements: corn oil, medium chain triglyceride-corn oil, olive oil and medium chain triglyceride-olive oil. The highest relative content of saturated fatty acids corresponded to corn oil-fed animals. Both monounsaturated fatty acid content and delta 9-desaturase activity were higher in the animals fed olive oil diets than in corn oil-fed rats. The long chain polyunsaturated fatty acids of the n-3 series were increased in the olive oil and medium chain triglyceride-olive oil-fed groups probably due to the lower linoleic/alpha-linolenic ratios found in these two diets. The cholesterol/phospholipid molar ratio was unaffected by diet and the unsaturation index was only slightly changed in the four groups. Thus, some mechanism may be operative under these conditions to maintain the homeostasis of the membrane.

Delta 6 and delta 5 desaturase activities in liver from obese Zucker rats at different ages

delta 6 Desaturation of linoleic acid (18:2 n-6) and delta 5 desaturation of dihomo-gamma-linolenic acid (20:3 n-6) were measured in liver microsomes from genetically obese Zucker rats (fa/fa) and from their lean littermates (Fa/--). Both groups were fed a balanced commercial diet. The rats were 6, 9 and 12 weeks old, which corresponded to stages in their active growth period. The content of total fatty acids and n-6 polyunsaturated fatty acids in whole liver and liver microsomes was also determined in order to ascertain how the desaturase activities measured in vitro reflected regulation of essential fatty acid metabolism in vivo. Contrary to values obtained for delta 6 desaturation, delta 5 desaturation at nonsaturating substrate levels were lower in obese rats than in lean controls. In contrast, at saturating substrate level, the maximal delta 5 desaturase activities were the same in both phenotypes and they increased with age. Study of delta 5 desaturation kinetics (1/V vs 1/S) showed that Vm did not differ between 12-week-old obese and lean rats, whereas KM in obese rats was much lower than in controls, expressing the very low affinity of the enzyme for the substrate in obese animals. The fatty acid composition of liver lipids reflected the results of desaturase activities in vitro. In particular, the ratios 20:4 n-6/20:3 n-6 were lower in obese rats than in lean rats, which can be explained by the lower conversion of 20:3 n-6 into 20:4 n-6 by delta 5 desaturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary cholesterol and/or n-3 fatty acid modulate delta 9-desaturase activity in rat liver microsomes

delta 9-Desaturase activity and fatty acid composition of liver microsomal phospholipids in rats fed diets enriched with either saturated (hydrogenated beef tallow) or alpha-linolenic (linseed oil) or eicosapentaenoic and docosahexaenoic (fish oil) acids with or without 2% cholesterol supplementation were investigated. Both the linseed oil and the fish oil diets inhibited delta 9-desaturase activity in the rat liver microsomes. The inhibition was greater when feeding fish oil (90%) compared with the linseed oil (60%) diet. Dietary cholesterol feeding accelerated conversion of palmitic (16:0) to palmitoleic (16:1) acid, irrespective of the fatty acid supplement. Feeding the linseed oil diet decreased, while feeding the fish oil diet increased synthesis of the monounsaturated fatty acids of n-7 series (palmitoleic and vaccenic acid) and decreased 18:1(n-9) in microsomal membrane lipids when compared with animals fed beef tallow. Addition of 2% cholesterol to the otherwise low cholesterol diets led to accumulation of 16:1(n-7), and 18:1(n-9) in microsomal membranes. These results suggest that delta 9-desaturase activity is dependent on the cholesterol contents as well as the n-3 fatty acid content of microsomal membranes on which it is localized.

Lipid composition of liver microsomes in rats fed a high monounsaturated fatty acid diet

The fatty acid and cholesterol contents of tissue membranes are the determinants of membrane stability and functionality. This study was designed to evaluate the influence of a high monounsaturated fatty acid diet on the fatty acid composition of rat liver microsomes and on their cholesterol and lipid phosphorus content. Weanling animals were fed for 5 weeks with high fat diets containing olive oil or corn oil. Saturated fatty acids were increased and oleic acid decreased in microsomal total phospholipids and in the three major phosphoglycerides, phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI), of rats fed corn oil as compared to the olive oil group. The percentage of linoleic acid was higher in the corn oil group, but only for total phospholipids and PC. Linoleic and alpha-linolenic metabolites were significantly increased in total phospholipids of olive oil-fed animals with respect to those fed corn oil. These changes were responsible for the low unsaturation index found in microsomal phospholipids of the corn oil group. The diet did not affect the microsome cholesterol or the lipid phosphorus content. These results show that, in olive oil-fed rats, the cholesterol content and the degree of unsaturation of liver microsomes was similar to that observed in weanling animals; this probably suggests an adequate maintenance of functionality of membranes in olive oil-fed animals.

Microsomal desaturation-elongation of linoleic acid following parenteral feeding with lipid emulsions in the rat

The effect of total parenteral nutrition (TPN) with lipid emulsion to supply either 27.5% or 2% total calories on in vitro desaturation-elongation of 18:2(n-6) by liver microsomes was studied in the rat. The emulsion lipid contained ca. 50% 18:2(n-6) plus 8% 18:3(n-3) or 77% 18:2(n-6) plus 0.5% 18:3(n-3). The reaction rate was influenced by the in vitro substrate concentration and inhibited above 50 mumol 18:2(n-6)/mg microsomal protein. At maximum rates of desaturation-elongation, the formation of triene and tetraene (n-6) fatty acids was reduced in rats given either of the two emulsions. The rate of (n-6) pentaene formation was increased in rats given the emulsion with low 18:3(n-3) but not in rats given the emulsion with 8% 18:3(n-3). Analyses of the microsomal lipid indicated increased free cholesterol in all rats that received TPN. Esterified cholesterol was increased only in rats given 27.5% TPN calories as lipid. Microsomal total phospholipids and phospholipid class distributions were not altered by TPN. The data are consistent with reports of reduced levels of long-chain desaturation-elongation metabolites of 18:2(n-6) in tissue phospholipids following infusion of parenteral lipid. The data suggest that the mechanism may include alterations in other metabolic pathways such as cholesterol, in addition to desaturase enzyme activities, and may be influenced by both the quantity and fatty acid composition of the lipid infused.

Lipogenic enzyme activities in primary cultures of adult mouse hepatocytes

The effects of various unsaturated fatty acids such as oleic (18:1n-9), linoleic (18:2n-6) and arachidonic (20:4n-6) on the activities of fatty acid synthetase (FAS), malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) all were determined in primary cultures of mouse hepatocytes. Activities of FAS and ME were found to decrease with time in culture regardless of whether hepatocyte donors were fed diets containing polyunsaturated fatty acid-free hydrogenated cottonseed oil (HCTO) or corn oil (CO). On the other hand, while G6PDH activity also declined in cultured hepatocytes obtained from HCTO-fed mice, the activity of this enzyme increased in cells cultured from CO-fed mice. 6PGDH activity was found to increase in hepatocytes obtained from both diet groups. Neither 18:2 nor 20:4 when added to media could alter FAS or ME activities compared with those observed with either 18:1-containing or fatty acid-free media. Since lactic dehydrogenase activity and the rate of incorporation of [3H] leucine into FAS protein were unaltered with time in hepatocyte cultures, the decreased activities of FAS and ME cannot be attributed to a loss in cell viability during culture but rather appear to be specific for those enzymes which respond to diet hormones in vivo.

Effect of environmental temperature changes on liver delta 6 and delta 5 fatty acid desaturases depressed by hyperglycidic diet on male rats

Male rats maintained on a control diet at 24 C and shifted to 12 C for five days showed no modification in delta 6 or delta 5 desaturase activity. When delta 6 and delta 5 desaturase activities were diminished by a hyperglycidic diet, shifting to cold increased both enzymatic activities. In general, modifications observed in liver microsomal fatty acid composition are consistent with those associated with the enzymatic desaturation activity.

Effect of different acids with delta 9,12-dienoic structures on delta 9 desaturation activity in rat liver microsomes

The effect of oral administration, for 24 or 48 hr, of different octadeca fatty acids containing a 9,12-dienoic structure on the fatty acid composition and delta 9 desaturation activity of liver microsomes of rat fed a fat-free diet was studied. The ethyl esters of linoelaidic and gamma-linolenic acids, the methyl ester of linoleic acid and free columbinic acid were administered to rats maintained on a fat-free diet. The supplementation of the fat-free diet with linoelaidate produced no relevant changes in the fatty acid composition pattern of liver microsomes and did not modify the percentage of conversion of palmitic to palmitoleic acid. The addition of linoleate or gamma-linolenate to the fat-free diet returned liver microsome delta 9 desaturation activity toward the control and partially restored the liver microsome fatty acid spectrum found in the fat-free diet. Columbinic acid (5-trans-9-cis,12-cis-18:3), which cannot be transformed into arachidonic acid, also decreased the delta 9 desaturation activity enhanced by the fat-free diet and evoked changes in the microsomal fatty acid composition similar to those produced by the omega 6 fatty acids. These results suggest that the modulation of delta 9 desaturase activity evoked by dietary administration of unsaturated acids of omega 6 series would depend on the cis double bond configuration of these acids.

Effects of dietary triolein and sunflower oil on insulin release and lipid metabolism in Zucker rats

Obese and lean male Zucker rats were fed ad libitum on diets containing either 50 (L) or 200 (H) g/kg diet of either triolein (T) or sunflowerseed oil (S). The specific activity of the hepatic microsomal delta 9 desaturase enzyme was depressed in both lean and obese rats fed the HS diet compared with the other three diets. The fatty acid composition of liver and subcutaneous white adipose tissue lipids were consistent with a lower delta 9 desaturation activity in rats fed the H diets, particularly for the HS diet. In both genotypes, microsomal delta 9 desaturase activity and the ratio of 16:1/(16:0 + 16:1) fatty acids in liver lipids were inversely related to the proportion of 18:2 in liver lipid. Plasma insulin concentrations and rates of glucose-stimulated insulin release in vivo were higher in obese rats compared with lean rats, and plasma insulin levels were higher in rats fed S compared with T. There was no relationship between delta 9 desaturase activity and either plasma insulin concentration or rates of insulin release in vitro. These findings suggest that hepatic delta 9 desaturase activity of Zucker rats is responsive to changes in the proportion of 18:2 in liver lipids but is not affected by changes in insulin secretion.

The effect of dietary ethanol on the composition of lipids of Drosophila melanogaster larvae

At a moderate concentration (2.5%, v/v) dietary ethanol reduced the chain length of total fatty acids (FA) and increased the desaturation of short-chain FA in Drosophila melanogaster larvae with a functional alcohol dehydrogenase (ADH). The changes in length in total FA were postulated to be due to the modulation of the termination specificity of fatty acid synthetase. Because the ethanol-stimulated reduction in the length of unsaturated FA was blocked by linoleic acid, it was thought to reflect the properties of FA 9-desaturase. Although the ethanol-stimulated reduction in chain length of unsaturated FA was also observed in ADH-null larvae, ethanol promoted an increase in the length of total FA of the mutant larvae. Thus, the ethanol-stimulated change in FA length was ADH dependent but the ethanol effect on FA desaturation was not. Ethanol also stimulated a decrease in the relative amount of phosphatidylcholine and an increase in phosphatidylethanolamine. Because similar ethanol-induced changes have been found in membrane lipids of other animals, ethanol may alter the properties of membranes in larvae. It is proposed that ethanol tolerance in D. melanogaster may be dependent on genes that specify lipids that are resistant to the detrimental effects of ethanol.

Effects of gamma-linolenic acid supplementation on lipogenesis regulation in pregnant zinc-deficient rat and fetus

In the liver of zinc-deficient pregnant rats fatty acid synthetase and delta 9-desaturase activities decreased and diet supplementation with gamma-linolenic acid potentiated this effect. However, in liver microsomes from the foetuses of zinc-deficient mothers, HMG CoA reductase and delta 9-desaturase activities declined but fatty acid activity rose. The same applied to foetuses from mothers whose diet was supplemented with gamma-linolenic acid and here again, the effect of zinc deficiency was potentiated. The fact that delta 9 activity dropped whereas fatty acid synthetase activity rose implied a defect in the mechanism regulating the functioning of these enzymes. In the non zinc-deficient group of pregnant females, gamma-linolenic acid supplementation had no effect on fatty acid synthetase and delta 9-desaturase activities but significantly increased HMG CoA reductase activity. In foetuses from the same group, the activities of MHG CoA reductase, delta 9-desaturase and fatty acid synthetase all increased.

Effects of dietary fat level and aflatoxin B1 treatment on rat hepatic lipid composition

Sprague-Dawley rats were given either ten daily doses of aflatoxin B1 (AFB1) or the solvent tricaprylin intragastrically over a 2-wk period and were fed diets containing either 1.6 or 20% corn oil throughout the study. Hepatic lipid composition was analysed in groups of five rats both 3 and 13 wk after the start of treatment, in order to determine short-term and longer-term alterations. Total lipid and cholesterols (total, free and esterified) increased on the high-fat diet at wk 3. At wk 13 only total and esterified cholesterol were increased by 20% corn oil. AFB1 treatment resulted in large intra-group variations in total lipid and cholesterol at wk 3, but these were no longer apparent by wk 13. AFB1 produced various alterations in the fatty acid composition of hepatic phosphatidylcholine (PC) and phosphatidylethanolamine (PE), apparent at wk 3 but not at wk 13. The unsaturation index decreased but no changes were seen in the saturated fatty acids. Only in animals fed 20% corn oil did AFB1 result in significant changes in 18:2, 20:3 and 22:6 fatty acids, while 20:4 and 22:5 tended to decrease and 18:1 to increase in response to AFB1 treatment with both diets in both phospholipids. The high-corn oil diet was found to increase 18:2, 22:6, and total unsaturation in PC and PE, while the ratio of 20:4 to 18:2 tended to decrease in these phospholipids. gamma-Glutamyltranspeptidase, an indicator of liver damage, was significantly increased in AFB1-treated animals, with the greatest increase over controls in those fed the high-fat diet.

Short term essential fatty acid deficiency in rats. Influence of dietary carbohydrates

The effects of long term (8-14 wk) essential fatty acid (EFA)-deprived diets in rats are well documented. In the present study, we compared, in weanling rats, the effect of a short term (two wk) hydrogenated coconut oil, EFA-deprived, diet (D) with that of a corn oil, EFA-adequate, diet (A), using either sucrose (SU) or starch (ST) as carbohydrate. After two wk, rats fed the sucrose/hydrogenated coconut oil diet developed some characteristic features of EFA deprivation: slower growth rate, decreases in linoleic and arachidonic acid of plasma phospholipids and an increase in n-9 eicosatrienoic acid of plasma phospholipids. When rats ate the starch/hydrogenated coconut oil diet, there was a similar decrease in linoleic acid of plasma phospholipids, but only a small effect on growth rate and no change in the arachidonic acid content of plasma phospholipids. EFA deprivation and sucrose had opposite effects on plasma triglyceride (TG) levels: deprivation induced a decrease, whereas the sucrose induced an increase in very low density lipoprotein (VLDL) triglycerides. The observed decrease in plasma triglyceride during EFA deprivation might result from an activation of lipoprotein lipase during the early stages of deprivation.