Synthesis of arachidonoyl coenzyme A and docosahexaenoyl coenzyme A in retina

Docosahexaenoic acid signalolipidomics in nutrition: significance in aging, neuroinflammation, macular degeneration, Alzheimer's, and other neurodegenerative diseases

Essential polyunsaturated fatty acids (PUFAs) are critical nutritional lipids that must be obtained from the diet to sustain homeostasis. Omega-3 and -6 PUFAs are key components of biomembranes and play important roles in cell integrity, development, maintenance, and function. The essential omega-3 fatty acid family member docosahexaenoic acid (DHA) is avidly retained and uniquely concentrated in the nervous system, particularly in photoreceptors and synaptic membranes. DHA plays a key role in vision, neuroprotection, successful aging, memory, and other functions. In addition, DHA displays anti-inflammatory and inflammatory resolving properties in contrast to the proinflammatory actions of several members of the omega-6 PUFAs family. This review discusses DHA signalolipidomics, comprising the cellular/tissue organization of DHA uptake, its distribution among cellular compartments, the organization and function of membrane domains rich in DHA-containing phospholipids, and the cellular and molecular events revealed by the uncovering of signaling pathways regulated by DHA and docosanoids, the DHA-derived bioactive lipids, which include neuroprotectin D1 (NPD1), a novel DHA-derived stereoselective mediator. NPD1 synthesis agonists include neurotrophins and oxidative stress; NPD1 elicits potent anti-inflammatory actions and prohomeostatic bioactivity, is anti-angiogenic, promotes corneal nerve regeneration, and induces cell survival. In the context of DHA signalolipidomics, this review highlights aging and the evolving studies on the significance of DHA in Alzheimer's disease, macular degeneration, Parkinson's disease, and other brain disorders. DHA signalolipidomics in the nervous system offers emerging targets for pharmaceutical intervention and clinical translation.

Rescue and repair during photoreceptor cell renewal mediated by docosahexaenoic acid-derived neuroprotectin D1

Retinal degenerative diseases result in retinal pigment epithelial (RPE) and photoreceptor cell loss. These cells are continuously exposed to the environment (light) and to potentially pro-oxidative conditions, as the retina's oxygen consumption is very high. There is also a high flux of docosahexaenoic acid (DHA), a PUFA that moves through the blood stream toward photoreceptors and between them and RPE cells. Photoreceptor outer segment shedding and phagocytosis intermittently renews photoreceptor membranes. DHA is converted through 15-lipoxygenase-1 into neuroprotectin D1 (NPD1), a potent mediator that evokes counteracting cell-protective, anti-inflammatory, pro-survival repair signaling, including the induction of anti-apoptotic proteins and inhibition of pro-apoptotic proteins. Thus, NPD1 triggers activation of signaling pathway/s that modulate/s pro-apoptotic signals, promoting cell survival. This review provides an overview of DHA in photoreceptors and describes the ability of RPE cells to synthesize NPD1 from DHA. It also describes the role of neurotrophins as agonists of NPD1 synthesis and how photoreceptor phagocytosis induces refractoriness to oxidative stress in RPE cells, with concomitant NPD1 synthesis.

Survival signaling in retinal pigment epithelial cells in response to oxidative stress: significance in retinal degenerations

Photoreceptor survival depends on the integrity of retinal pigment epithelial (RPE) cells. The pathophysiology of several retinal degenerations involves oxidative stress-mediated injury and RPE cell death; in some instances it has been shown that this event is mediated by A2E and its epoxides. Photoreceptor outer segments display the highest DHA content of any cell type. RPE cells are active in DHA uptake, conservation, and delivery. Delivery of DHA to photoreceptor inner segments is mediated by the interphotoreceptor matrix. DHA is necessary for photoreceptor function and at the same time is a target of oxidative stress-mediated lipid peroxidation. It has not been clear whether specific mediators generated from DHA contribute to its biological properties. Using ARPE-19 cells, we demonstrated the synthesis of 10,17S-docosatriene [neuroprotectin Dl (NPDI)]. This synthesis was enhanced by the calcium ionophore A-23187, by IL-1 3P, or by supplying DHA. Added NPD1 (50nM) potently counteracted H2O2/tumor necrosis factor-alpha oxidative stress-triggered apoptotic DNA damage in RPE. NPD1 also up-regulated the anti-apoptotic proteins Bcl-2 and Bcl-xL and decreased pro-apoptotic Bax and Bad expression. Moreover, NPD1 (50nM) inhibited oxidative stress-induced caspase-3 activation. NPD1 also inhibited IL-1beta-stimulated expression of COX-2. Furthermore, A2E-triggered oxidative stress induction of RPE cell apoptosis was also attenuated by NPD1. Overall, NPD1 protected RPE cells from oxidative stress-induced apoptosis. In conclusion, we have demonstrated an additional function of the RPE: its capacity to synthesize NPD1. This new survival signaling is potentially of interest in the understanding of the pathophysiology of retinal degenerations and in exploration of new therapeutic modalities.

Cell survival matters: docosahexaenoic acid signaling, neuroprotection and photoreceptors

Recent data have provided important clues about the molecular mechanisms underlying certain retinal degenerative diseases, including retinitis pigmentosa and age-related macular degeneration. Photoreceptor cell degeneration is a feature common to these diseases, and the death of these cells in many instances seems to involve the closely associated retinal pigment epithelial (RPE) cells. Under normal circumstances, both cell types are subject to potentially damaging stimuli (e.g. sunlight and high oxygen tension). However, the mechanism or mechanisms by which homeostasis is maintained in this part of the eye, which is crucial for sight, are an unsolved riddle. The omega-3 fatty acid family member docosahexaenoic acid (DHA), which is enriched in these cells, is the precursor of neuroprotectin D1 (NPD1). NPD1 inhibits oxidative-stress-mediated proinflammatory gene induction and apoptosis, and consequently promotes RPE cell survival. This enhanced understanding of the molecular basis of endogenous anti-inflammatory and neuroprotective signaling in the RPE presents an opportunity for the development of therapies for retinal degenerative diseases.

Neuroprotectin D1 (NPD1): a DHA-derived mediator that protects brain and retina against cell injury-induced oxidative stress

The biosynthesis of oxygenated arachidonic acid messengers triggered by cerebral ischemia-reperfusion is preceded by an early and rapid phospholipase A2 activation reflected in free arachidonic and docosahexaenoic acid (DHA) accumulation. These fatty acids are released from membrane phospholipids. Both fatty acids are derived from dietary essential fatty acids; however, only DHA, the omega-3 polyunsaturated fatty acyl chain, is concentrated in phospholipids of various cells of brain and retina. Synaptic membranes and photoreceptors share the highest content of DHA of all cell membranes. DHA is involved in memory formation, excitable membrane function, photoreceptor cell biogenesis and function, and neuronal signaling, and has been implicated in neuroprotection. In addition, this fatty acid is required for retinal pigment epithelium cell (RPE) functional integrity. Here we provide an overview of the recent elucidation of a specific mediator generated from DHA that contributes at least in part to its biological significance. In oxidative stress-challenged human RPE cells and rat brain undergoing ischemia-reperfusion, 10,17S-docosatriene (neuroprotectin D1, NPD1) synthesis evolves. In addition, calcium ionophore A23187, IL-1beta, or the supply of DHA enhances NPD1 synthesis. A time-dependent release of endogenous free DHA followed by NPD1 formation occurs, suggesting that a phospholipase A2 releases the mediator's precursor. When NPD1 is infused during ischemia-reperfusion or added to RPE cells during oxidative stress, apoptotic DNA damage is down-regulated. NPD1 also up-regulates the anti-apoptotic Bcl-2 proteins Bcl-2 and BclxL and decreases pro-apoptotic Bax and Bad expression. Moreover, NPD1 inhibits oxidative stress-induced caspase-3 activation. NPD1 also inhibits IL-1beta-stimulated expression of COX-2. Overall, NPD1 protects cells from oxidative stress-induced apoptosis. Because photoreceptors are progressively impaired after RPE cell damage in retinal degenerative diseases, understanding of how these signals contribute to retinal cell survival may lead to the development of new therapeutic strategies. Moreover, NPD1 bioactivity demonstrates that DHA is not only a target of lipid peroxidation, but rather is the precursor to a neuroprotective signaling response to ischemia-reperfusion, thus opening newer avenues of therapeutic exploration in stroke, neurotrauma, spinal cord injury, and neurodegenerative diseases, such as Alzheimer disease, aiming to up-regulate this novel cell-survival signaling.

Neuroprotectin D1: a docosahexaenoic acid-derived docosatriene protects human retinal pigment epithelial cells from oxidative stress

Docosahexaenoic acid (DHA) is a lipid peroxidation target in oxidative injury to retinal pigment epithelium (RPE) and retina. Photoreceptor and synaptic membranes share the highest content of DHA of all cell membranes. This fatty acid is required for RPE functional integrity; however, it is not known whether specific mediators generated from DHA contribute to its biological significance. We used human ARPE-19 cells and demonstrated the synthesis of 10,17S-docosatriene [neuroprotectin D1 (NPD1)]. This synthesis was enhanced by the calcium ionophore A-23187, by IL-1beta, or by supplying DHA. Under these conditions, there is a time-dependent release of endogenous free DHA followed by NPD1 formation, suggesting that phospholipase A(2) releases the mediator's precursor. Added NPD1 potently counteracted H(2)O(2)/tumor necrosis factor alpha oxidative-stress-triggered apoptotic RPE DNA damage. NPD1 also up-regulated the antiapoptotic proteins Bcl-2 and Bcl-x(L) and decreased proapoptotic Bax and Bad expression. Moreover, NPD1 (50 nM) inhibited oxidative-stress-induced caspase-3 activation. NPD1 also inhibited IL-1beta-stimulated expression of cyclooxygenase 2 promoter transfected into ARPE-19 cells. Overall, NPD1 protected RPE cells from oxidative-stress-induced apoptosis, and we predict that it will similarly protect neurons. This lipid mediator therefore may indirectly contribute to photoreceptor cell survival as well. Because both RPE and photoreceptor cells die in retinal degenerations, our findings contribute to the understanding of retinal cell survival signaling and potentially to the development of new therapeutic strategies.

Identification and quantitation of the fatty acids composing the CoA ester pool of bovine retina, heart, and liver

Several proteins found in retinal photoreceptor cells (guanylate cyclase activating protein, protein kinase A, recoverin, and transducin) are N-terminally modified with the fatty acids 12:0, 14:0, 14:1n-9, and 14:2n-6, whereas similar proteins in other tissues contain only 14:0. It has been hypothesized that the acyl-CoA pool of the retina contains amounts of 12:0, 14:1n-9, and 14:2n-6 elevated over 14:0, in comparison to other tissues, and this accounts for the specificity of N-terminal fatty acylation. To test this hypothesis, we performed fatty acid analysis on total acyl-CoAs purified from bovine retina (light-adapted), heart, and liver. We also examined the N- and S-linked fatty acid composition of the total protein pools from these tissues. Acyl-CoAs were prepared from heart, liver, and retina and separated by high performance liquid chromatography (HPLC). Identities of peaks were based on HPLC of standard 12:0, 14:0, 14:1n-9, and 14:2n-6 CoAs. Total protein was subjected to base hydrolysis followed by acidic methanolysis to release S- and N-linked fatty acids, respectively, and fatty acid phenacyl esters were prepared for HPLC analysis. Retina had levels of 12:0 (2.7 +/- 2.1%), 14:1n-9 (2.9 +/- 2.2%), and 14:2n-6 (1.6 +/- 0.7%) CoAs below that of 14:0 CoA (7.0 +/- 1.8%). Likewise, heart levels of 14:2n-6 CoA (3.7 +/- 0.1%) were near and 12:0 (2.6 +/- 0. 6%) and 14:1n-9 (0.7 +/- 0.3%) CoAs were below that of 14:0 CoA (3.8 +/- 1.0%). Liver had levels of 12:0 (16.1 +/- 5.7%) and 14:2n-6 (8.1 +/- 1.2%) CoAs above and 14:1n-9 CoA (1.2 +/- 0.6%) below that of 14:0 CoA (5.9 +/- 0.8%). Fatty acid analysis of total protein showed that all tissues contained S-linked 16:0, 18:0, and 18:1n-9. Retina proteins contained N-linked 14:0, 14:1n-9, and 14:2n-6, whereas heart and liver had only 14:0. Our findings do not support the hypothesis that the CoA ester pool of the retina is enriched with 12:0, 14:1n-9, and 14:2n-6 over 14:0, in comparison to other tissues. This suggests that alternative models must be considered for the regulation of N-terminal fatty acylation of proteins in photoreceptor cells.

Post-Golgi vesicles cotransport docosahexaenoyl-phospholipids and rhodopsin during frog photoreceptor membrane biogenesis

Post-Golgi vesicles budding from the trans-Golgi network (TGN) are involved in the vectorial transport and delivery of rhodopsin to photoreceptor rod outer segments (ROS). We report here that newly synthesized docosahexaenoyl (DHA) phospholipids are sequestered and cotransported by rhodopsin-bearing post-Golgi vesicles to ROS. Frog retinas were pulse-labeled with [35S]methionine/cysteine and [3H]DHA prior to ROS isolation and subcellular fractionation. After a 1-h pulse, relatively uniform [3H]DHA-lipid labeling (DPM/microg protein) was observed in all fractions enriched in post-Golgi vesicles, TGN, Golgi, and endoplasmic reticulum (ER) membranes. During the subsequent 2-h chase translocation of free [3H]DHA from ROS to the photoreceptor inner segment contributed to an additional overall increase in labeling of lipids. The specific activity (dpm/nmol DHA) in ER-enriched fraction was similar or higher than in other subcellular fractions after both the pulse and the chase, indicating that the bulk of [3H]DHA-lipids was synthesized in the ER. After the chase a 2-fold increase in labeling of lipids in the ER and Golgi and a 2.6-fold in lighter TGN-enriched fractions was observed. The highest labeling was in the post-Golgi vesicle fraction (4-fold increase), with [3H]DHA-phosphatidylcholine and [3H]DHA-phosphatidylethanolamine showing the greatest increase. At the same time, newly synthesized [35S]rhodopsin shifted from the ER and Golgi toward TGN and post-Golgi fractions. Therefore, sequestration and association of [35S]rhodopsin and [3H]DHA-lipids in a TGN membrane domain occurs prior to their exit and subsequent vectorial cotransport on post-Golgi vesicles to ROS. Labeling of ROS lipids was very low, with phosphatidylinositol and diacylglycerols displaying the highest labeling. This indicates that other mechanisms by-passing Golgi, i.e. facilitated by lipid carrier proteins, may also contribute to molecular replacement of disc membrane DHA-phospholipids, particularly phosphatidylinositol.

Acyl-CoA:lysophosphatidylcholine acyltransferase activity in bovine retina rod outer segments

In the present paper the properties of acyl-CoA:lysophosphatidylcholine acyltransferase activity associated with rod outer segments (ROS) have been studied. Under adequate experimental conditions, ROS acyl-CoA:lysophosphatidylcholine acyltransferase activity presented a maximum at pH 7.0. The enzyme was able to incorporate as much as 60% of the label offered as [1-14C]oleoyl-CoA into phosphatidylcholine after 5 min of incubation. The use of varying concentrations of oleoyl-CoA and 46 microM lysophosphatidylcholine gave an apparent K(m) value for oleoyl-CoA of 100 microM and a Vmax value of 153 nmol x h-1 x (mg protein)-1. The use of varying concentrations of lysophosphatidylcholine and 100 microM oleoyl-CoA gave an apparent K(m) value for lysophosphatidylcholine of 27 microM and a Vmax value of 155 nmol x h-1 x (mg protein)-1. The enzyme was inhibited by 25% when ROS membranes were incubated in the presence of 10 mM MgCl2. The acyltransferase was able to incorporate other acyl-CoAs (palmitoyl-CoA and arachidonoyl-CoA) into ROS phospholipids and to acylate other lysophospholipids but less efficiently than lysophosphatidylcholine. Lysophoshatidylcholine was preferentially acylated with arachidonic acid followed by oleic acid and, less efficiently, with palmitic acid. The high specific activity of acyl-CoA lysophosphatidylcholine acyltransferase found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to the ROS.

Alterations in rabbit retina lipid metabolism induced by detachment. Decreased incorporation of [3H]DHA into phospholipids

BACKGROUND

Docosahexaenoic acid (22:6n-3, DHA) is found in high concentration in phospholipids from retinal membranes, and is essential for their function. This study investigated the effect of in vivo retinal detachment on in vitro lipid metabolism using [3H]DHA.

METHODS

Rabbit retina was detached from the retinal pigment epithelium by injecting physiological saline into the subretinal space of the eye. Retinal samples from control (non-operated) and sham (operated, no detachment) animals, and from attached and detached retinal areas from the same eye, were incubated in vitro with [3H]DHA for 4 hours, and then prepared for biochemical and autoradiographic analysis.

RESULTS

In control and sham retinas, [3H]DHA was preferentially esterified into phospholipids (82%) with low labeling of free fatty acids (FFA) (5%). In samples from detached areas of the retina, a higher proportion of [3H]DHA was recovered in the FFA pool (up to 30%) and its esterification was shunted into triacylglycerol, thereby reducing the formation of [3H]DHA-phospholipids. Changes were sustained through 48 hours of postdetachment. High labeling of inner segments and synaptic terminals was observed autoradiographically in control retinas, while in detached retinas, clusters of labeling were detected in the neural retina, and eventually within the photoreceptor layer.

CONCLUSION

Retinal detachment induces longlasting changes in lipid metabolism which are reflected in lower labeling of [3H]DHA-phospholipids. Metabolic changes, sustained through 48 hours, may lead to inadequate synthesis/turnover of phospholipids, among them, those containing DHA, possibly resulting in defective disc membrane assembly with subsequent deterioration of visual cells.

Phospholipid synthesis by chick retinal microsomes: fatty acid preference and effect of fatty acid binding protein

The acylation of 1-palmitoyl-sn-glycerophosphocholine (1-16:0-GPC) or 1-palmitoyl-sn-glycerophosphoethanolamine (1-16:0-GPE) was measured using the microsomal fraction prepared from retinas of 14-15-day-old chick embryos. Rates of incorporation of exogenously supplied fatty acids into diacyl-GPC were generally 5-7 times greater than into diacyl-GPE. Substrate preferences for incorporation into diacyl-GPC and diacyl-GPE were, respectively, 18:2 greater than 18:3 = 20:5 greater than 20:4 greater than 18:1 greater than 22:6 = 18:0 and 18:2 greater than 22:6 greater than or equal to 18:3 = 18:0 greater than or equal to 20:4 = 18:1 greater than 20:5. The apparent selectivities were not consistent with the reported fatty acid compositions of these lipid classes. The addition of partially purified fatty acid binding protein (FABP) to the reaction had no effect either on overall rates of incorporation or on the substrate preference. When fatty acyl-CoA substrates were used, rates of incorporation of the 18:0 derivative were much higher than with the fatty acid, while rates with other fatty acyl-CoA were similar to those with the respective fatty acid. Substrate preferences for CoA derivatives incorporated into diacyl-GPC were: 18:0 greater than 20:4 greater than 18:2 greater than or equal to 22:6, and into diacyl-GPE: 20:4 = 22:6 greater than 18:0 greater than 18:2. Polyunsaturated fatty acyl CoA (PUFA-CoA) were thus favored for incorporation into diacyl-GPE, and to a lesser extent into diacyl-GPC, a result that is consistent with composition data.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential uptake and metabolism of docosahexaenoic acid in membrane phospholipids from rod and cone photoreceptor cells of human and monkey retinas

The uptake, metabolism, and cellular distribution of [3H]docosahexaenoic acid (DHA) in human and monkey retinas were studied with biochemical and autoradiographic techniques. In specimens from two human retina biopsies, incubated for 4 hr or 6 hr with [3H]docosahexaenoic acid (110 nM), 80% of the esterified [3H]fatty acid was recovered in phospholipids and the remainder in triacylglycerols and diacylglycerols. The distribution of [3H]DHA in individual phospholipids (PL) was similar in both retinas, with phosphatidylcholine (PC) and phosphatidylethanolamine (PE) accounting for most of the label. A similar labeling profile was observed in glycerolipids from monkey retina, and after 1 hr of incubation, high labeling of phosphatidic acid (PA, 11%) and phosphatidylinositol (PI, 20%) was observed. In both human and monkey retinas, a preferential uptake of [3H]DHA by photoreceptor cells was revealed by autoradiography. Cone photoreceptors showed a slightly higher density of silver grains in their inner segments than did rod photoreceptors. Photoreceptors accounted for 59% and 79% of the total [3H]DHA taken up by the human and monkey retinas, respectively, the remainder being distributed throughout the neural retina. In conclusion, this study shows for the first time that in human and monkey retinas, DHA is taken up with a high degree of selectivity by photoreceptor cells, and then becomes esterified mainly into phospholipids that will be subsequently utilized for the synthesis of new disc membranes.

Membrane docosahexaenoate is supplied to the developing brain and retina by the liver

Docosahexaenoic acid [22:6 omega 3; 22:6(4, 7, 10, 13, 16, 19)] is concentrated in phospholipids of cellular membranes from brain and retina. Although linolenic acid [18:3 omega 3; 18:3(9, 12, 15)] is the major omega 3 fatty acid of mouse dams' milk, 22:6 is the prevalent omega 3 fatty acid in serum and tissues. Intraperitoneal injection of [1-14C]18:3 into 3-day-old mouse pups resulted in liver and serum lipid labeling that was initially high, followed by a rapid decline. In contrast, labeling of brain and retinal lipids were initially low and increased with time. Labeled 22:6 first appeared in liver 2 hr after injection and later in brain and retina. We suggest that 22:6 synthesized from 18:3 by the liver is secreted into the bloodstream in lipoproteins, taken up by brain and retina, and incorporated into cell membranes. We hypothesize that the 22:6 requirements of membranes (e.g., during synaptogenesis, photoreceptor membrane biogenesis, or repair after ischemic injury or neurodegenerative disorders) are met by a signal that is sent by the appropriate tissues to the liver to evoke the secretion of 22:6-containing lipoproteins.

Light exposure stimulates arachidonic acid metabolism in intact rat retina and isolated rod outer segments

This paper presents evidence that short-term exposure to light increases synthesis of hydroxyeicosatetraenoic acid (HETE) and prostaglandin D2 (PGD2) and stimulates the uptake and metabolism of 20:4 in phospholipids and triacylglycerols in rat retina. There was a time-dependent increase in incorporation of 1-14C-20:4 into glycerolipids in both dark-adapted and light-exposed groups. Exposure to light for 15 or 30 min enhanced the acylation of 20:4 into phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine and triacylglycerols. In the light-exposed groups there was a large increase in the conversion of 20:4 to leukotriene B4, two diHETEs, 5-HETE, 15-HETE, and PGD2. The stimulation of HETE synthesis by light could be due to early stages of light-induced lipid peroxidation in visual cells. To examine this, we studied peroxidation of 20:4 in isolated rod outer segments (ROS). There was more oxidation of 20:4 in light-exposed ROS, as compared to ROS incubated in the dark. Vitamin E and nordihydroguaiaretic acid inhibited the light-induced formation of some of these products. The data indicate that photo-oxidation of 20:4 in ROS is accompanied by enzymatic oxygenation that is stimulated by light. Increased production of HETEs an PGD2 may be a consequence of the light-induced stimulation of the metabolism of 20:4 in membrane phospholipids in the retina.

Acylation and deacylation of phospholipids in isolated bovine rod outer segments

Isolated bovine rod outer segments (ROS) were incubated under different conditions with radiolabeled fatty acid-Coenzyme A (CoA) compounds, fatty acids and phospholipids in order to further investigate the rates, mechanisms and function of phospholipid metabolism within that organelle. ROS contain acyl CoA synthetase, acyl transferase, acyl CoA hydrolase, and phospholipase A activities. Although different radiolabeled fatty acid CoAs were esterified to the major ROS phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine) at the same rate, different free fatty acids were esterified at different rates. There was no correlation between these estimates of in vitro rates of incorporation of fatty acids and the fatty acid composition of ROS phospholipids. Both the deacylation of radiolabeled phospholipids (phospholipase A activity) and the acylation of endogenous phospholipids (acyl transferase activity) were maximally stimulated when ATP, CoA, Mg2+ and Ca2+ were present, and both processes were stimulated by pro-oxidizing conditions and exposure to light. Under phospholipase A-stimulatory conditions, there was preferential hydrolysis of polyenoic fatty acids from endogenous ROS phospholipids. Both the acylation and deacylation reactions were primarily at the sn-2 position of ROS phospholipids.

Arachidonoyl-coenzyme A synthetase and nonspecific acyl-coenzyme A synthetase activities in purified rat brain microvessels

Purified rat brain microvessels were prepared to demonstrate the occurrence of acyl-CoA (EC 6.2.1.3) synthesis activity in the microvasculature of rat brain. Both arachidonoyl-CoA and palmitoyl-CoA synthesis activities showed an absolute requirement for ATP and CoA. This activity was strongly enhanced by magnesium chloride and inhibited by EDTA. The apparent Km values for acyl-CoA synthesis by purified rat brain microvessels were 4.0 microM and 5.8 microM for palmitic acid and arachidonic acid, respectively. The apparent Vmax values were 1.0 and 1.5 nmol X min-1 X mg protein-1 for palmitic acid and arachidonic acid, respectively. Cross-competition experiments showed inhibition of radiolabelled arachidonoyl-CoA formation by 15 microM unlabelled arachidonic acid, with a Ki of 7.1 microM, as well as by unlabelled docosahexaenoic acid, with a Ki of 8.0 microM. Unlabelled palmitic acid and arachidic acid had no inhibitory effect on arachidonoyl-CoA synthesis. In comparison, radiolabelled palmitoyl-CoA formation was inhibited competitively by 15 microM unlabelled palmitic acid, with a Ki of 5.0 microM and to a much lesser extent by arachidonic acid (Ki, 23 microM). The Vmax of palmitoyl-CoA formation obtained on incubation in the presence of the latter fatty acids was not changed. Unlabelled arachidic acid and docosahexaenoic acid had no inhibitory effect on palmitoyl-CoA synthesis. Both arachidonoyl-CoA and palmitoyl-CoA synthesis activities were thermolabile. Arachidonoyl-CoA formation was inhibited by 75% after 7 min at 40 degrees C whereas a 3-min heating treatment was sufficient to produce the same relative inhibition of palmitoyl-CoA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Docosahexaenoate metabolism and fatty-acid composition in developing retinas of normal and rd mutant mice

The fatty-acid composition of retinal lipids in developing control and rd mice (C57BL/6J) was determined. In addition, fatty-acid composition in brain and retina of normal and rd adult animals was compared. At 11 days of postnatal age, rd retinas contained proportionally less docosahexaenoate than controls, whereas the reverse relationship held for oleate at 20 days of age. In contrast, no differences in the fatty-acid composition of brain lipids were observed between rd and control animals. We also examined docosahexaenoate metabolism in rd and control retinas of different postnatal ages in vitro. These studies of [1-14C]docosahexaenoic-acid incorporation into retinal phospholipids and neutral lipids demonstrated significantly higher incorporation into triacylglycerols of the rd retina, beginning at 14-15 days of postnatal age. Incorporation into diacylglycerols and phospholipids was also higher in rd retinas than in controls at 14 days of age and older. Moreover, the concentration of ganglioside (a glycolipid class probably enriched within inner retinal layers) was higher in adult rd retinas. Degeneration of the rd retina can be detected histologically as early as 8-9 days. Therefore, the alterations of fatty-acid composition and docosahexaenoate metabolism described here are probably an effect, rather than a cause, of retinal degeneration in the rd mouse.

Fatty acid composition and arachidonic acid metabolism in vitreous lipids from canine and human eyes

About 55% of the acyl groups of dog and human vitreous are unsaturated fatty acids. The major components are oleate (18:1, n-9) and arachidonate (20:4, n-6) with moderate amounts of linoleate (18:2, n-6) and docosahexaenoate (22:6, n-3). Palmitate (16:0) and stearate (18:0) are the major saturated fatty acids. There are no significant changes between ages 37-82 years in the fatty acyl group content and composition of human vitreous. In vitreous from Irish setters with hereditary rod-cone dysplasia (RCD) the levels of oleate are decreased with a concomitant increase in arachidonate. [1-14C]Arachidonic acid was actively incorporated into canine vitreous glycerolipids both in vitro and in vivo. The incorporation was mainly into phosphatidylinositol, triacylglycerol, phosphatidylcholine and phosphatidylethanolamine. There were some differences in the pattern of incorporation between human and dog and between in vivo and in vitro incubations of canine vitreous. Glycerolipid acylation was significantly increased in phosphatidylinositol and phosphatidylcholine in RCD canine vitreous. The pattern of incorporation of [U-14C]docosahexaenoic acid into vitreous glycerolipids was different from arachidonic acid incorporation. Although vitreous did not produce any measurable enzymatic synthesis of cyclooxygenase and lipoxygenase products from [1-14C]-arachidonic acid in vitro, there was significant generation of autooxidation products. These results suggest an active lipid metabolism in vitreous.

Synthesis of docosahexaenoyl-, arachidonoyl- and palmitoyl-coenzyme A in ocular tissues

The synthesis of long-chain acyl coenzyme A (CoA) was studied in the cornea, lens, vitreous, retina and pigment epithelium (PE) in the rat using [14C]-labeled palmitic, arachidonic and docosahexaenoic acids as substrates. Except for retina and PE, the ocular tissues studied showed relatively little enzyme activity with the fatty acid substrates. In addition, the enzyme activities were studied in homogenates and microsomal fractions from retina, pigment epithelial cells and choroid of frog, bovine and human eyes. Long-chain acyl CoA synthetase from the microsomal fraction exhibited three- to fivefold greater activity than homogenates in retina and PE. The enzyme activity was highest with palmitic acid, followed by arachidonic acid and docosahexaenoic acid. There were significant differences in enzyme activity between the species. The apparent Km (microM) and Vmax [nmol min-1 (mg protein)-1] values for the enzyme in bovine retinal microsomes were 7.91 +/- 0.39 (S.E.) and 21.6 +/- 1.04, respectively, for palmitic acid substrate and 5.88 +/- 0.25 and 4.58 +/- 0.21, respectively, for docosahexaenoic acid substrate. These values for bovine pigment epithelial microsomes were 13.0 +/- 0.27 and 36.9 +/- 1.18, respectively, for palmitic acid and 15.8 +/- 0.40 and 13.2 +/- 0.56, respectively, for docosahexaenoic acid. The synthesis of acyl CoA may play a central role in controlling the availability of free arachidonic acid for eicosanoid formation and in the retention of polyunsaturated fatty acid families (18:2, n-6 and 18:3, n-3) within cells of ocular tissues, particularly retina and retinal PE.

Activation of polyunsaturated fatty acids by rat tissues in vitro

The conversion of labeled palmitic, linoleic, arachidonic and docosahexaenoic acids to their respective acyl CoA's was studied in homogenates and microsomes of rat tissues. The highest activity, both in homogenates and microsomes, was seen in liver and heart. There was moderate activity in retina, brain, lung, kidney and testes and the lowest activity was found in spleen. Docosahexaenoic acid was activated much less actively in heart tissue than the other fatty acids. In all tissues examined, the highest activation was observed with arachidonic acid and the lowest with docosahexaenoic acid. Except for liver, those tissues that contained high levels of docosahexaenoic acid also had the highest activation capacity for this fatty acid.