The physiological role and control of mammalian fatty acyl-coenzyme A desaturases

Copper deficiency in ruminants

Copper deficiency can reduce the productivity of livestock. The effect of copper deficiency on a number of copper enzymes and copper-dependent systems is discussed, to highlight the areas where their role needs to be clarified. Special reference is made to cytochrome c oxidase, lysyl oxidase, superoxide dismutase and endoplasmic reticulum enzymes and to their role in the expression of disease. The modification of microbiological insult by a change in superoxide dismutase activity without any other direct metabolic consequences is discussed, to introduce the concept of an external challenge being necessary before any effect of an otherwise sub-clinical copper deficiency is observed. The changes in activity of the various copper enzymes are described in clinical and experimentally induced copper deficiency in sheep and cattle, two species in which copper deficiency can have economic consequences. The diagnostic value of various blood markers, such as copper, caeruloplasmin and erythrocyte superoxide dismutase is discussed. The measured degree of hypocupraemia is related to different types of sampling (e.g. plasma or serum), physiological status (e.g. in the pre- and postpartum cow), changes that occur in the neonate, and also to the effect of the acute-phase reaction. The use of erythrocyte superoxide dismutase as a marker for the copper status of sheep and cattle is compared with more conventional markers such as plasma concentration of copper. The use of blood markers to map the extent and location of hypocupraemia (due to reduced copper intake or availability) among suckler (beef) herds in Northern Ireland is also discussed.

Fatty acid delta9-desaturation in the Triatoma infestans fat body: response to food and trehalose administrations

The effects of food intake and carbohydrate administration on fatty acid delta9-desaturation were investigated in isolated microsomes from Triatoma infestans fat body. Fifth instar nymphs, which were solely blood-fed just after the molt and then fasted, were used as controls and for determination of optimal assay conditions. Both [1-14C]palmitic and [1-14C]stearic acids in a medium containing ATP, CoA, MgCl2, NADH, NaF, and O2 were tested. For the control group, optimal conditions were a pH of 6.8-7.2, an incubation temperature of 29 degrees C, and an incubation time of 10 min. delta9-Desaturation depended on the presence of reduced pyridine dinucleotides; NADH and NADPH were equally efficient. Stearic acid showed a higher apparent Vmax than palmitic acid, but the apparent Km were very similar. When fifth instar nymphs were blood-fed weekly, a marked increase of delta9-desaturation was observed for both acids. Higher desaturation activity was also induced by injection of the carbohydrate trehalose into the hemolymph of fasted nymphs. These results indicate that insect delta9-desaturation, like the mammalian counterpart, is sensitive to dietary changes and carbohydrate administration.

Structure and expression of fatty acid desaturases

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. They are present in all groups of organisms, i.e., bacteria, fungi, plants and animals, and play a key role in the maintenance of the proper structure and functioning of biological membranes. The desaturases are characterized by the presence of three conserved histidine tracks which are presumed to compose the Fe-binding active centers of the enzymes. Recent findings on the structure and expression of different types of fatty acid desaturase in cyanobacteria, plants and animals are reviewed in this article. Roles of individual desaturases in temperature acclimation and principles of regulation of the desaturase genes are discussed.

Delta 6-desaturase: improved methodology and analysis of the kinetics in a multi-enzyme system

A new method of assay for the delta 6-desaturation of linoleic acid was developed. This method, which uses HPLC for separation of the fatty acid substrate and product, exhibited a lower coefficient of variation (0.3%) than the reported TLC method (3.5%), and avoided the step of methylation of the saponified fatty acid substrate and product. Using this new method of assay, the kinetics of the delta 6-desaturase in a multi-enzyme system were analysed. A number of factors that could have striking effects on desaturase kinetics were investigated, including the effect of (i) endogenous microsomal linoleic acid on total substrate concentration, and (ii) the pre-reaction catalysed by acyl-CoA synthetase and competing reactions catalysed by lysophospholipid acyltransferase and acyl-CoA hydrolase. Endogenous free linoleate in the hepatic microsomes was found to be 2.9 +/- 1.0 microM (0.5 mg microsomal protein/ml), which was comparable to added substrate concentrations (1.8 to 7.9 microM). The kinetics of the delta 6-desaturase were dissected from the kinetics of the above mentioned pre-reaction and competing reactions through a combination of experimental approaches and computer modeling. From computer modeling, a Km and Vmax of 1.5 microM and 0.63 nmol/min were calculated for the delta 6-desaturase, compared to Km and Vmax of 10.7 microM and 0.08 nmol/min calculated directly from data uncorrected for endogenous substrate. It was concluded that lysophospholipid acyltransferase, acyl-CoA synthetase and endogenous linoleic acid significantly affect the kinetic measurements of hepatic microsomal delta 6-desaturase. These results have implications for kinetic analyses of all desaturates in microsomal systems.

Arachidonic acid regulates unsaturated fatty acid synthesis in lymphocytes by inhibiting stearoyl-CoA desaturase gene expression

This work was based upon the observation that a reduction in the level of serum, provided to murine lymphocytes in culture, augmented endogenous unsaturated fatty acid (UFA) synthesis. Since the phospholipids of BW5147 cells grown in 1% serum were especially deficient in arachidonic acid (20:4), and given the findings of previous workers, we suspected that the availability of exogenous 20:4 in serum might correlate with the squelching of UFA synthesis. Indeed, after a 5 h exposure to 4-28 microM 20:4, the 20:4 content of BW5147 cell phospholipids increased from 1% to 15% of the total fatty acids with a coincident reduction in 18:1 synthesis to approx. 30% of starting values. Subsequent studies were done to define the mechanism by which 20:4 down-regulates 18:1 synthesis. The results indicated that 20:4 inhibited endogenous 18:1 synthesis by reducing stearoyl-CoA desaturase (SCD) enzyme activity. Moreover, as determined by Northern blot analyses, the inhibitory effect of 20:4 on stearoyl-CoA desaturase activity coincided with decreased stearoyl-CoA desaturase mRNA levels. Exposure of BW5147 cells to either 20:4, actinomycin D, or both, resulted in a temporal decay of stearoyl-CoA desaturase mRNAs with half-lives ranging from 4.0 h to 4.4 h. Such a similarity in decay times implied that 20:4 regulates stearoyl-CoA desaturase expression by inhibiting transcription. This was confirmed by nuclear run-on studies in which 20:4 was found to inhibit transcription of nascent stearoyl-CoA desaturase mRNA. Collectively, these findings implicate 20:4 as an important regulator of stearoyl-CoA desaturase gene expression, and hence UFA synthesis, in lymphoid cells.

Stearoyl-CoA desaturase gene expression in lymphocytes

B lymphocytes from the spleens of normal (BALB/c) and autoimmune (MRL/lpr) strains of mice express the SCD-2 form of stearoyl-CoA desaturase as opposed to the SCD-1 form of the gene which is expressed in liver. However, whereas BALB/c T cells did not express SCD-1 or SCD-2, both BALB/c thymocytes and MRL/lpr T cells expressed SCD-2, suggesting a developmental down-regulation of SCD-2 within the T cell lineage. Northern analyses also revealed the expression of SCD-2 in the T cell lines BW5147, CTLL-2 and HT-2 and in BCL1, a B cell line. SCD-1 expression was not detected in any of the lymphoid cells tested. Finally, we show that SCD-2 gene expression is inhibited by arachidonic acid (20:4). These results demonstrate the complexity of SCD-2 regulation in lymphoid cells.

[Effect of exclusive enteral feeding with Enteronutril on serum lipids]

Ten patients aged between 18 and 73 years admitted to our intensive care unit were placed on exclusive artificial nutrition with Enteronutril receiving on average 2.855 +/- 496 kcal/day (715 +/- 125 g). This solution is poor in lipids (1.5%) and contains no linoleic acid (C18:2). Its effects on serum lipids were analysed after 5, 10 and 15 days of treatment. Total cholesterol (TC) and phospholipids (PL) showed no significant change. However, triglycerides (TG) rose on d15 and pre-beta-lipoproteins were significantly higher on d5. The latter rose further from d10 to d15. The C18:2 fraction of total lipids (TL) fell precipitously on d5 (-50%). Although C18:2 remained stable from d5 to d10, a new, slower decrease occurred from d10 to d15. This decrease was accompanied by a specific rise in oleic (C18:1) and palmitoleic (C16:1) acids. Palmitic (C16:0) and stearic (C18:0) acids were unaffected. Arachidonic acid (C20:4) remained within normal limits up to d15. Eicosatrienoic acid which was measurable on d0 presented no significant rise during the course of the study. The changes in C18:2 were inversely proportional to those of C18:1 or C16:1. This translated a lack of C18:2 relative to delta 9 desaturase (freed of the inhibition usually exerted by C18:2). These results showed that the body could maintain a sufficient level of linoleic acid to synthesize arachidonic acid by drawing on its C18:2 reserves from fatty tissue. Nevertheless, these reserves were progressively exhausted beyond d10, thereby causing a shortage of arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective effects of isomeric cis and trans fatty acids on fatty acyl delta 9 and delta 6 desaturation by human skin fibroblasts

Human skin fibroblasts incorporate and actively desaturate long-chain fatty acids. Growth of these cells in lipid-free medium can be used to enhance delta 9 and delta 6 desaturation of [14C]stearate and [14C]linoleate, respectively. Medium supplementation with cis fatty acids inhibits delta 9 desaturation; effectiveness as inhibitors is linoleate (9c,12c-18:2) greater than oleate (9c-18:1) greater than vaccenate (11c-18:1). Linoelaidate (9t,12t-18:2), trans-vaccenate (11t-18:1) and saturated fatty acids are without effect; elaidate (9t-18:1) appears stimulatory. By contrast, the trans fatty acids elaidate and linoelaidate are potent inhibitors of delta 6 desaturation; inhibition by trans-vaccenate is 50% of that of elaidate. Desaturation of [14C]linoleate is only slightly inhibited by oleate, cis-vaccenate, or (6c,9c,12c)-linolenate. The relative effectiveness of isomeric cis- and trans-octadecenoic acids as inhibitors of delta 9 and delta 6 desaturation in intact human cells is different from that found in microsomal studies. The cell culture system can thus be important in evaluating physiological effects of isomeric fatty acids on cellular metabolic processes.

Thyroid control over biomembranes. Liver-microsomal cytochrome b5 in hypothyroidism

Hypothyroid rats were prepared by thyroidectomy and maintenance on a low-iodine diet (group A); Group B was additionally pretreated with 0.5 mCi of 131I as NaI, given intraperitoneally. Liver microsomes obtained from hypothyroid and normal rats were compared. After fasting and refeeding on 20% sucrose solution, high levels of microsomal fatty-acyl-CoA delta 9-desaturase (as measured spectrophotometrically by the rate constants for cytochrome b5 reoxidation) were induced in all the normal animals, half of the group A hypothyroid rats, and none of the group B hypothyroid rats. Hypothyroidism did not change desaturase Arrhenius profiles or V and Km for NADH-cytochrome c reductase, but increased content of cytochrome b5. The inability of adequately hypothyroid rats to induce the delta 9-desaturase seems to be specific, in that injection of methylcholanthrene successfully induced microsomal benzpyrene monooxygenase activity and increased cytochrome b5 contents in hypothyroid animals. The defects in overall fatty acyl desaturation reported in hypothyroid animals [Landriscina, C., Gnoni, G. V. & Quagliariello, E. (1976) Eur. J. Biochem. 71, 135-143] are suggested to be due to deficiencies in the specific desaturase(s).

The acyl-CoA desaturases of microsomes from rat liver and the Morris 7777 hepatoma

We have investigated the role of the microsomal oxidative desaturase in defining the aberrant phosphoglyceride fatty acid composition of hepatomas. The microsomal delta 9-stearoyl-CoA, delta 6-oleoyl(linolenoyl)-CoA, and delta 5-eicosatrienoyl-CA desaturase activities were studied in control and host liver and in the poorly differentiated Morris 7777 hepatoma. The delta 9-stearoyl-CoA desaturase of the hepatoma was significantly decreased (42%) relative to control liver, yet the hepatoma specific activity was twice that of host liver. Additionally, the specific activity of the delta 9-stearoyl-CoA desaturase of the tumor was found to decrease with increasing tumor weight. Also this desaturase was inactivated by freezing and thawing. The delta 6-oleoyl(linolenoyl)-CoA and delta 5-eicosatrienoyl-CoA desaturases of the hepatoma were 39% and 4% of control, respectively. The electron transport components involved in the desaturase system were reduced, although this did not appear to be rate-limiting. In addition, two competing metabolic reactions which could lower the observed desaturase activities, hydrolysis of the thioester and incorporation of substrate acyl-CoA molecules into glycerides, did not appear to be responsible for the lowered desaturase activities of the tumor. Thus, it appears that reduced levels of the desaturases themselves may be responsible for the observed activities. These results indicate that the capacity of the hepatoma to biosynthesize polyunsaturated fatty acids is greatly reduced and this is consistent with the decreased polyene content observed in many neoplasms.

The influence of diet and diabetes on stearoyl Conenzyme A desaturase (EC 1.14.99.5) activity and fatty acid composition in rat tissues

1. Rats were given low-fat diets for 3 d in which the carbohydrate source was starch. The livers of animals given the fructose or sucrose had increased hepatic activities of the fatty acid synthetase and stearoyl CoA desaturase (EC 1.14.99.5) enzyme complexes: in those given fructose there was a lower activity of the enzymes in adipose tissue. 2. Similar results were obtained in rats given fructose diets for 30 d, but in animals which had previously been made diabetic with streptozotocin, the activities were lower. The dietary treatment made little difference to the fatty acid profiles of the tissue lipids. The diabetic condition on the other hand produced considerable changes in fatty acid profile. 3. With diets containing approximately 200 g fat/kg in the form of butter or of polyunsaturated margarine, the tissue lipids from rats given sucrose had less linoleic acid than those from rats given starch. In addition, there was the expected difference between the rats given butter or margarine. The results are discussed in relation to the current literature.