Fatty acid specificity of hormone-sensitive lipase. Implication in the selective hydrolysis of triacylglycerols.

The selective mobilization of fatty acids from white fat cells depends on their molecular structure, in particular the degree of unsaturation. The present study was designed to examine if the release of fatty acids by hormone-sensitive lipase (HSL) in vitro i) is influenced by the amount of unsaturation, ii) depends on the temperature, and iii) could explain the selective pattern of fatty acid mobilization and notably the preferential mobilization of certain highly unsaturated fatty acids. Recombinant rat and human HSL were incubated with a lipid emulsion. The hydrolysis of 35 individual fatty acids, ranging in chain length from 12 to 24 carbon atoms and in unsaturation from 0 to 6 double bonds was measured. Fatty acid composition of in vitro released NEFA was compared with that of fat cell triacylglycerols (TAG), the ratio % NEFA/% TAG being defined as the relative hydrolysis. The relative hydrolysis of individual fatty acids differed widely, ranging from 0.44 (24:1n-9) to 1.49 (18:1n-7) with rat HSL, and from 0.38 (24:1n-9) to 1.67 (18:1n-7) with human HSL. No major difference was observed between rat and human HSL. The relative release was dependent on the number of double bonds according to chain length. The amount of fatty acid released by recombinant rat HSL was decreased but remained robust at 4 degrees C compared with 37 degrees C, and the relative hydrolysis of some individual fatty acids was affected. The relative hydrolysis of fatty acids moderately, weakly, and highly mobilized by adipose tissue in vivo was similar and close to unity in vitro. We conclude that i) the release of fatty acids by HSL is only slightly affected by their degree of unsaturation, ii) the ability of HSL to efficiently and selectively release fatty acids at low temperature could reflect a cold adaptability for poikilotherms or hibernators when endogenous lipids are needed, and iii) the selectivity of fatty acid hydrolysis by HSL does not fully account for the selective pattern of fatty acid mobilization, but could contribute to explain the preferential mobilization of some highly unsaturated fatty acids compared with others.     

ATGL and HSL are not coordinately regulated in response to fuel partitioning in fasted rats

Prolonged fasting is characterized by lipid mobilization (Phase 2), followed by protein breakdown (Phase 3). Knowing that body lipids are not exhausted in Phase 3, we investigated whether changes in the metabolic status of prolonged fasted rats are associated with differences in the expression of epididymal adipose tissue proteins involved in lipid mobilization. The final body mass, body lipid content, locomotor activity and metabolite and hormone plasma levels differed between groups. Compared with fed rats, adiposity and epididymal fat mass decreased in Phase 2 (approximately two- to threefold) and Phase 3 (∼4.5-14-fold). Plasma nonesterified fatty acids (NEFA) concentrations were increased in Phase 2 (approximately twofold) and decreased in Phase 3 (approximately twofold). Daily locomotor activity was markedly increased in Phase 3 (∼11-fold). Compared with the fed state, expressions of adipose triglyceride lipase (ATGL; mRNA and protein), hormone-sensitive lipase (HSL; mRNA) and phosphorylated HSL at residue Ser660 (HSL Ser⁶⁶⁰) were increased during Phase 2 (∼1.5-2-fold). HSL (mRNA and protein) and HSL Ser⁶⁶⁰ levels were lowered during Phase 3 (∼3-12-fold). Unlike HSL and HSL Ser⁶⁶⁰, ATGL expression did not correlate with circulating NEFA, mostly due to data from animals in Phase 3. At this stage, ATGL could play an essential role for maintaining a low mobilization rate of NEFA, possibly to sustain muscle performance and hence increased locomotor activity. We conclude that ATGL and HSL are not coordinately regulated in response to changes in fuel partitioning during prolonged food deprivation, ATGL appearing as the major lipase in late fasting.     

Analysis of lipolysis in adipocytes using a fluorescent fatty acid derivative

For facilitation of the experimental analysis of the mechanism and regulation of mobilization of fatty acids from adipose triacylglycerol (TAG) stores, which also represents important targets for pharmacological intervention with the pathogenesis of diabetes and obesity, we developed a convenient and reliable non-radioactive cell-based assay. Isolated rat adipocytes are incubated with the fluorescent fatty acid derivative, 12-((7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoic acid (NBD-FA), in the presence of insulin. The resulting NBD-FA-labeled TAG is efficiently cleaved by hormone-sensitive lipase (HSL) in vitro. After removal of insulin and excess of free NBD-FA, lipolysis is initiated by addition of isoproterenol and/or adenosine deaminase. The amount of NBD-FA generated in total or released into the incubation medium in the presence of modulatory hormones or compounds is then monitored by thin layer chromatography and fluorescence imaging. Release of NBD-FA, glycerol and [3H]oleic acid from TAG follows similar kinetics and concentration dependence in response to various lipolytic and anti-lipolytic stimuli as well as inhibitors of HSL. Release of NBD-FA from adipocytes correlates well to translocation of HSL from the cytosol to TAG droplets. In addition, we found that a cell-free system consisting of NBD-FA-labeled TAG droplets with endogenous associated HSL closely reflects the lipolytic state of the adipocytes used for its preparation. In conclusion, release of NBD-FA from TAG in vivo and in vitro can be used as accurate index for (regulation of) lipolysis in primary and cultured adipocytes.     

Lipolysis: more than just a lipase

Successful adaptation to starvation in mammals depends heavily on the regulated mobilization of fatty acids from triacylglycerols stored in adipose tissue. Although it has long been recognized that cyclic AMP represents the critical second messenger and hormone-sensitive lipase (HSL)**Abbreviations used in this paper: ADRP, adipocyte differentiation-related protein; HSL, hormone-sensitive lipase; PKA, protein kinase A; TAG, triacylglycerol. the rate-determining enzyme for lipolysis, simple activation of the enzyme has failed to account for the robust augmentation of fatty release in response to physiological agonists. In this issue, Sztalryd et al. (2003) provide convincing support to the notion that the subcellular compartmentalization of lipase also regulates lipolysis, and, more importantly, that proteins other than HSL are localized to the lipid droplet and are indispensable for its optimal hydrolysis.     

Might the kinetic behavior of hormone-sensitive lipase reflect the absence of the lid domain?

Hormone-sensitive lipase (HSL) is thought to contribute importantly to the mobilization of fatty acids from the triacylglycerols (TAGs) stored in adipocytes, providing the main source of energy in mammals. To investigate the HSL substrate specificity more closely, we systematically assessed the lipolytic activity of recombinant human HSL on solutions and emulsions of various vinyl esters and TAG substrates, using the pH-stat assay technique. Recombinant human HSL activity on solutions of partly soluble vinyl esters or TAG was found to range from 35 to 90% of the maximum activity measured with the same substrates in the emulsified state. The possible existence of a lipid-water interface due to the formation of small aggregates of vinyl esters or TAG in solution may account for the HSL activity observed below the solubility limit of the substrate. Recombinant human HSL also hydrolyzes insoluble medium- and long-chain acylglycerols such as trioctanoylglycerol, dioleoylglycerol, and olive oil, and can therefore be classified as a true lipase. Preincubation of the recombinant HSL with a serine esterase inhibitor such as diethyl p-nitrophenyl phosphate in 1:100 molar excess leads to complete HSL inhibition within 15 min. This result indicates that the catalytic serine of HSL is highly reactive and that it is readily accessible. Similar behavior was also observed with lipases with no lid domain covering their active site, or with a deletion in the lid domain. The 3-D structure of HSL, which still remains to be determined, may therefore lack the lid domain known to exist in various other lipases.     

Hormone-sensitive lipase deficiency in mice changes the plasma lipid profile by affecting the tissue-specific expression pattern of lipoprotein lipase in adipose tissue and muscle.

Hormone-sensitive lipase (HSL) is believed to play an important role in the mobilization of fatty acids from triglycerides (TG), diglycerides, and cholesteryl esters in various tissues. Because HSL-mediated lipolysis of TG in adipose tissue (AT) directly feeds non-esterified fatty acids (NEFA) into the vascular system, the enzyme is expected to affect many metabolic processes including the metabolism of plasma lipids and lipoproteins. In the present study we examined these metabolic changes in induced mutant mouse lines that lack HSL expression (HSL-ko mice). During fasting, when HSL is normally strongly induced in AT, HSL-ko animals exhibited markedly decreased plasma concentrations of NEFA (-40%) and TG (-63%), whereas total cholesterol and HDL cholesterol levels were increased (+34%). Except for the increased HDL cholesterol concentrations, these differences were not observed in fed animals, in which HSL activity is generally low. Decreased plasma TG levels in fasted HSL-ko mice were mainly caused by decreased hepatic very low density lipid lipoprotein (VLDL) synthesis as a result of decreased NEFA transport from the periphery to the liver. Reduced NEFA transport was also indicated by a depletion of hepatic TG stores (-90%) and strongly decreased ketone body concentrations in plasma (-80%). Decreased plasma NEFA and TG levels in fasted HSL-ko mice were associated with increased fractional catabolic rates of VLDL-TG and an induction of the tissue-specific lipoprotein lipase (LPL) activity in cardiac muscle, skeletal muscle, and white AT. In brown AT, LPL activity was decreased. Both increased VLDL fractional catabolic rates and increased LPL activity in muscle were unable to provide the heart with sufficient NEFA, which led to decreased tissue TG levels in cardiac muscle. Our results demonstrate that HSL deficiency markedly affects the metabolism of TG-rich lipoproteins by the coordinate down-regulation of VLDL synthesis and up-regulation of LPL in muscle and white adipose tissue. These changes result in an "anti-atherogenic" lipoprotein profile.     

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.     

The central role of perilipin a in lipid metabolism and adipocyte lipolysis

The related disorders of obesity and diabetes are increasing to epidemic proportions. The role of neutral lipid storage and hydrolysis, and hence the adipocyte, is central to understanding this phenomenon. The adipocyte holds the major source of stored energy in the body in the form of triacylglycerols (TAG). It has been known for over 35 years that the breakdown of TAG and release of free (unesterified) fatty acids and glycerol from fat tissue can be regulated by a cAMP-mediated process. However, beyond the initial signaling cascade, the mechanistic details of this lipolytic reaction have remained unclear. Work in recent years has revealed that both hormone-sensitive lipase (HSL), generally thought to be the rate-limiting enzyme, and perilipin, a lipid droplet surface protein, are required for optimal lipid storage and fatty acid release. There are multiple perilipin proteins encoded by mRNA splice variants of a single perilipin gene. The perilipin proteins are polyphosphorylated by protein kinase A and phosphorylation is necessary for translocation of HSL to the lipid droplet and enhanced lipolysis. Hence, the surface of the lipid storage droplet has emerged as a central site of regulation of lipolysis. This review will focus on adipocyte lipolysis with emphasis on hormone signal transduction, lipolytic enzymes, the lipid storage droplet, and fatty acid release from the adipocyte.     

Selective mobilization of fatty acids from adipose tissue triacylglycerols

Adipose tissue triacylglycerols represent the main storage of a wide spectrum of fatty acids differing by molecular structure. The release of individual fatty acids from adipose tissue is selective according to carbon chain length and unsaturation degree in vitro and in vivo in animal studies and also in humans. The mechanism of selective fatty acid mobilization from white fat cells is not known. Lipolysis is widely reported to work at a lipid-water interface where only small amounts of substrate are available. A preferential hydrolysis of a small triacylglycerol fraction enriched in certain triacylglycerol molecular species at the lipid-water interface and enzymological properties of hormone-sensitive lipase could explain the selective mobilization of fatty acids from fat cells. This selectivity could affect the individual fatty acid supply to tissues.     

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.     

Significance of lipoidal estradiol in human mammary tumors

Incubations of p-nitrophenyl fatty acyl esters and estradiol-17 beta fatty acyl 17-esters with porcine esterase, human mammary tumor cytosol and rat uterine cytosol leads to ester hydrolysis of compounds with short chain fatty acids. Esters with long chain fatty acids show no hydrolysis except in the presence of Tween 80. Short chain fatty acid esters have a higher binding potency to the estrogen receptor than long chain fatty acid esters. Extraction of the nuclear receptor peak sedimenting at 4.6S and identification of the steroid showed that about 90% of the radioactivity was associated with estradiol and only 10% with estradiol esters. These studies show that estradiol fatty acyl esters act as a storage form from which estradiol is released by enzymatic hydrolysis.     

Hormone-sensitive lipase is necessary for normal mobilization of lipids during submaximal exercise

For the working muscle there are a number of fuels available for oxidative metabolism, including glycogen, glucose, and nonesterified fatty acids. Nonesterified fatty acids originate from lipolysis in white adipose tissue, hydrolysis of VLDL triglycerides, or hydrolysis of intramyocellular triglyceride stores. A key enzyme in the mobilization of fatty acids from intracellular lipid stores is hormone-sensitive lipase (HSL). The aim of the present study was to investigate the metabolic response of HSL-null mice challenged with exercise or fasting and to examine whether other lipases are able to fully compensate for the lack of HSL. The results showed that HSL-null mice have reduced capacity to perform aerobic exercise. The liver glycogen stores were more rapidly depleted in HSL-null mice during treadmill exercise, and HSL-null mice had reduced plasma concentrations of both glycerol and nonesterified fatty acids after exercise and fasting, respectively. The data support the hypothesis that in the absence of HSL, mice are not able to respond to an exercise challenge with increased mobilization of the lipid stores. Consequently, the impact of the lipid-sparing effect on liver glycogen is reduced in the HSL-null mice, resulting in faster depletion of this energy source, contributing to the decreased endurance during submaximal exercise.     

Hepatic triacylglycerol hydrolysis regulates peroxisome proliferator-activated receptor �� activity

Recent evidence suggests that fatty acids generated from intracellular triacylglycerol (TAG) hydrolysis may have important roles in intracellular signaling. This study was conducted to determine if fatty acids liberated from TAG hydrolysis regulate peroxisome proliferator-activated receptor α (PPARα). Primary rat hepatocyte cultures were treated with adenoviruses overexpressing adipose differentiation-related protein (ADRP) or adipose triacylglycerol lipase (ATGL) or treated with short interfering RNA (siRNA) targeted against ADRP. Subsequent effects on TAG metabolism and PPARα activity and target gene expression were determined. Overexpressing ADRP attenuated TAG hydrolysis, whereas siRNA-mediated knockdown of ADRP or ATGL overexpression resulted in enhanced TAG hydrolysis. Results from PPARα reporter activity assays demonstrated that decreasing TAG hydrolysis by ADRP overexpression resulted in a 35–60% reduction in reporter activity under basal conditions or in the presence of fatty acids. As expected, PPARα target genes were also decreased in response to ADRP overexpression. However, the PPARα ligand, WY-14643, was able to restore PPARα activity following ADRP overexpression. Despite its effects on PPARα, overexpressing ADRP did not affect PPARγ activity. Enhancing TAG hydrolysis through ADRP knockdown or ATGL overexpression increased PPARα activity. These results indicate that TAG hydrolysis and the consequential release of fatty acids regulate PPARα activity.     

Spontaneous transfer of monoacyl amphiphiles between lipid and protein surfaces.

The kinetics of transfer of natural and fluorescent nonesterified fatty acids (NEFA) and lysolecithins (lysoPC) from phospholipid and protein surfaces were measured. The kinetics of transfer of 12-(1-pyrenyl)dodecanoic acid, from liquid crystalline and gel phase single unilamellar phospholipid vesicles, very low, low, and high density lipoproteins, human serum albumin, and rat liver fatty acid-binding protein, were first-order and characterized by similar rate constants. The halftimes (t1/2) of NEFA transfer from lipids and proteins were dependent on the acyl chain structure according to log t1/2 = -0.62n + 0.59m + 12.0, where n and m, respectively, are the numbers of carbon atoms and double bonds. The structure of the donor surface had a measurable but smaller effect on transfer rates. The kinetics of NEFA and lysoPC transfer are slow relative to the lipolytic processes that liberate them. Therefore, one would predict a transient accumulation of NEFA and lysoPC during lipolysis and an attendant modulation of many metabolic processes within living cells and within the plasma compartment of blood. These data will be useful in the refinement of current models of membrane and lipoprotein function and in the selection of fluorescent NEFA analogs for studying transport in living cells.     

PKA phosphorylates and inactivates AMPKα to promote efficient lipolysis

The mobilization of metabolic energy from adipocytes depends on a tightly regulated balance between hydrolysis and resynthesis of triacylglycerides (TAGs). Hydrolysis is stimulated by β‐adrenergic signalling to PKA that mediates phosphorylation of lipolytic enzymes, including hormone‐sensitive lipase (HSL). TAG resynthesis is associated with high‐energy consumption, which when inordinate, leads to increased AMPK activity that acts to restrain hydrolysis of TAGs by inhibiting PKA‐mediated activation of HSL. Here, we report that in primary mouse adipocytes, PKA associates with and phosphorylates AMPKα1 at Ser‐173 to impede threonine (Thr‐172) phosphorylation and thus activation of AMPKα1 by LKB1 in response to lipolytic signals. Activation of AMPKα1 by LKB1 is also blocked by PKA‐mediated phosphorylation of AMPKα1 in vitro. Functional analysis of an AMPKα1 species carrying a non‐phosphorylatable mutation at Ser‐173 revealed a critical function of this phosphorylation for efficient release of free fatty acids and glycerol in response to PKA‐activating signals. These results suggest a new mechanism of negative regulation of AMPK activity by PKA that is important for converting a lipolytic signal into an effective lipolytic response.     

Rat long chain acyl-CoA synthetase 5 increases fatty acid uptake and partitioning to cellular triacylglycerol in McArdle-RH7777 cells

Long chain acyl-CoA synthetase (ACSL) catalyzes the initial step in long chain fatty acid metabolism. Of the five mammalian ACSL isoforms cloned and characterized, ACSL5 is the only isoform found to be located, in part, on mitochondria and thus was hypothesized to be involved in fatty acid oxidation. To elucidate the specific roles of ACSL5 in fatty acid metabolism, we used adenoviral-mediated overexpression of ACSL5 (Ad-ACSL5) in rat hepatoma McArdle-RH7777 cells. Confocal microscopy revealed that Ad-ACSL5 colocalized to both mitochondria and endoplasmic reticulum. When compared with cells infected with Ad-GFP, Ad-ACSL5-infected cells at 24 h after infection had 2-fold higher acyl-CoA synthetase activities and 30% higher rates of fatty acid uptake when incubated with 500 microM [1-(14)C]oleic acid. Metabolism of [1-(14)C]oleic acid to cellular triacylglycerol (TAG) increased 42% in Ad-ACSL5-infected cells, but when compared with control cells, metabolism to acid-soluble metabolites, phospholipids, and medium TAG did not differ substantially. The incorporation of [1-(14)C]oleate and [1,2,3-(3)H]glycerol into TAG was similar in Ad-ACSL5-infected cells, thus indicating that Ad-ACSL5 increased TAG synthesis through both de novo and reacylation pathways. However, [1-(14)C]acetic acid incorporation into cellular lipids showed that, when compared with control cells, Ad-ACSL5-infected cells did not increase the metabolism of fatty acids that were derived from de novo synthesis. These results suggest that uptake of fatty acids into cells is regulated by metabolism and that overexpressed ACSL5 partitions exogenously derived fatty acids toward TAG synthesis and storage.     

Purification and properties of a thioesterase from lactating rat mammary gland which modifies the product specificity of fatty acid synthetase

An acyl coenzyme A hydrolase (thioesterase II) has been purified to near homogeneity from lactating rat mammary gland. The enzyme is a monomer of molecular weight 33,000 and contains a single active site residue. The enzyme is specific for acyl groups, as acyl-CoA thioesters, containing eight or more carbon atoms and can also hydrolyze oxygen esters. Thioesterase II is capable of shifting the product specificity of rat mammary gland fatty acid synthetase from predominately long chain fatty acids (C14, C16, and C18) to mainly medium chain fatty acids (C8, C10, and C12). Thioesterase II can restore the capacity for fatty acid synthesis to fatty acid synthetase in which the thioesterase component (thioesterase I) has been inactivated with phenylmethanesulfonyl fluoride or removed by trypsinization. No evidence was found of significant levels of thioesterase II in lactating rat liver. The presence of thioesterase II in the lactating mammary gland and the ability of the enzyme to hydrolyze acyl-fatty acid synthetase thioesters of intermediate chain length, are indicative of a major role for this enzyme in the synthesis of the medium chain fatty acids characteristic of milk fat.     

Investigation of in vivo fatty acid metabolism in AFABP/aP2(-/-) mice

The metabolic impact of the murine adipocyte fatty acid-binding protein (AFABP/aP2) on lipid metabolism was investigated in the AFABP/aP2(-/-) mouse and compared with wild-type C57BL/6J littermates. Mice were weaned on a high-fat diet (59% of energy from fat) and acclimated to meal feeding. Stable isotopes were administered, and indirect calorimetry was performed to quantitate fatty acid flux, dietary fatty acid utilization, and substrate oxidation. Consistent with previous in situ and in vitro studies, fasting serum nonesterified fatty acid (NEFA) release was significantly reduced in AFABP/aP2(-/-) (17.1 +/- 9.0 vs. 51.9 +/- 22.9 mg.kg(-1).min(-1)). AFABP/aP2(-/-) exhibited higher serum NEFA (1.4 +/- 0.6 vs. 0.8 +/- 0.4 mmol/l, AFABP/aP2(-/-) vs. C57BL/6J, respectively) and triacylglycerol (TAG; 0.23 +/- 0.09 vs. 0.13 +/- 0.10 mmol/l) and accumulated more TAG in liver tissue (2.9 +/- 2.3 vs. 1.1 +/- 0.8% wet wt) in the fasted state. For the liver-TAG pool, 16.4 +/- 7.3% of TAG-fatty acids were derived from serum NEFA in AFABP/aP2(-/-). In contrast, a significantly greater portion of C57BL/6J liver-TAG was derived from serum NEFA (42.3 +/- 25.5%) during tracer infusion. For adipose-TAG stores, only 0.29 +/- 0.04% was derived from serum NEFA in AFABP/aP2(-/-), and, in C57BL/6J, 1.85 +/- 0.97% of adipose-TAG was derived from NEFA. In addition, AFABP/aP2(-/-) preferentially oxidized glucose relative to fatty acids in the fed state. These data demonstrate that in vivo disruption of AFABP/aP2(-/-) leads to changes in the following two major metabolic processes: 1) decreased adipose NEFA efflux and 2) preferential utilization of glucose relative to fatty acids.     

Docosahexaenoic acid is selectively enriched in plasma phospholipids during pregnancy in Trinidadian women--results of a pilot study

The fetal demand for docosahexaenoic acid (DHA) has to be satisfied by the mother. We determined the fatty acids in maternal plasma non-esterified fatty acid (NEFA), triacylglycerol (TAG) and phosphatidylcholine (PC), in a cross-sectional study of non-pregnant (n = 10), pregnant (n = 19), and postpartum (n = 9) women. There were lipid class-dependent differences in plasma polyunsaturated fatty acid (PUFA) concentrations between groups. During pregnancy, DHA was most highly enriched in PC, about 230%, with more modest enrichment for linoleic acid (LA) and arachidonic acid (AA), and no enrichment of alpha-linolenic acid (alpha-LNA). There was relative enrichment of LA, AA and alpha-LNA in TAG, but not of DHA. There was no specific enrichment of any PUFA in the NEFA pool. These data accord with the suggestion that the enrichment of alpha-LNA in TAG and of DHA in phospholipids reflects hepatic regulation of n-3 PUFA metabolism which potentially enhances the delivery of DHA to the placenta.