Effects of a fish oil diet on the metabolism of endogenous (n-6) and (n-3) fatty acids in rat neutrophils

The present study was carried out to define more precisely how dietary (n-3) fatty acids mediate leukotriene and phospholipid metabolism in neutrophils. Neutrophils from chow-fed rats did not synthesize detectable levels of LTB5 or 5-hydroxyeicosapentanenoic acid. The ratio of esterified 20:4(n-6)/20:5(n-3) in the phospholipids of neutrophils from rats fed a corn oil/fish oil diet was 4.6. The ratio of LTB4/LTB5 made by these cells was 2.6, thus suggesting that 20:5(n-3) release and/or subsequent metabolism was somewhat more efficient than for 20:4(n-6). When tritiated lyso-platelet activating factor was added to neutrophils from chow-fed rats, it was deactivated primarily by acylation with arachidonic acid. With the fish oil-fed animals both arachidonic acid and 20:5(n-3) were transferred to deactivate lyso-platelet activating factor. Molecular species analysis of the resulting radioactive 1-O-alkyl-2-acyl-sn-glycerol-3-phosphocholine showed that 20:5(n-3) pairs with the same 1-O-alkyl groups and in approximately the same ratio as does arachidonate. Collectively, these studies show that once 20:5(n-3) is incorporated into neutrophil lipids it is metabolized in a similar way as is arachidonate.

On singular or dual positional specificity of lipoxygenases. The number of chiral products varies with alignment of methylene groups at the active site of the enzyme

We tested a simple model which explains the singular or dual specificity of lipoxygenases. The dual specificity considered here is typified by the oxygenation of arachidonic acid by the reticulocyte lipoxygenase: two chiral products are formed (12S- and 15S-hydroperoxides, ratio approximately 1:9) via hydrogen abstraction from two separate methylene groups (C-10 and C-13). The rate-limiting step is known to involve this hydrogen abstraction, and we assumed that alignment of the methylenes with the hydrogen acceptor on the enzyme is critical in terms of reaction rate and positional specificity. Optimal alignment will be associated with a fast rate of reaction and formation of a single chiral product. A shift in position of the double bonds (and hence of the methylene groups) should be associated with a slower rate of reaction and formation of two chiral products; two methylenes are now able to react, although neither has perfect alignment. We tested this idea using two lipoxygenases and polyenoic fatty acids differing in the number and position of the double bonds. Optimal substrates for the soybean lipoxygenase had a doubly allylic methylene in the n-8 position, while the reticulocyte enzyme preferred substrates with a n-9 methylene. These substrates were converted to a single chiral product. With both enzymes, the other series of substrates reacted more slowly and were converted to two chiral products. We conclude that alignment of methylene groups of the substrate at the active site is a major determinant of the reaction rate and the singular or dual specificity of lipoxygenases.

Metabolism of 13-hydroxy-9,11-octadecadienoic acid by MOLT-4 lymphocytes

MOLT-4 lymphocytes metabolize 13-hydroxy-9,11-octadecadienoic acid, via the beta-oxidation pathway with retention of the omega 6 hydroxyl group and the conjugated diene system. The products which accumulate include 11-hydroxy-7,9-hexadecadienoic acid and 9-hydroxy-5,7-tetradecadienoic acid. In addition, it was possible to isolate two beta-hydroxy acids which were shown to be 3,13-dihydroxy-9,11-octadecadienoic acid and 3,11-dihydroxy-7,9-hexadecadienoic acid. The odd chain aldehyde, 12-hydroxy-8,10-heptadecadien-1-al, also was detected. However, neither the pathway nor the immediate precursor for the synthesis of this compound was established.

2,4-Dienoyl-coenzyme A reductase deficiency: a possible new disorder of fatty acid oxidation

Several inherited disorders of fatty acid beta-oxidation have been described that relate mainly to saturated precursors. This study is the first report of an enzyme defect related only to unsaturated fatty acid oxidation and provides the first in vivo evidence that fat oxidation in humans proceeds by the reductase-dependent pathway. The patient was a black female, presenting in the neonatal period with persistent hypotonia. Biochemical studies revealed hyperlysinemia, hypocarnitinemia, normal organic acid profile, and an unusual acylcarnitine species in both urine and blood. The new metabolite was positively identified by mass spectrometry as 2-trans,4-cis-decadienoylcarnitine, derived from incomplete oxidation of linoleic acid. In spite of dietary therapy, the patient died of respiratory acidosis at four months of age. Samples of liver and muscle from the autopsy were assayed for 2,4-dienoyl-coenzyme A reductase activity. Using the substrate 2-trans,4-cis-decadienoylcoenzyme A, the reductase activity was 40% of the control value in liver and only 17% of that found in normal muscle. It is suggested that unsaturated substrates should be used for in vitro testing to cover the full range of potential beta-oxidation defects and that acylcarnitine species identification be used for in vivo detection of this disorder.

Chain elongation of polyunsaturated fatty acids by vascular endothelial cells: studies with arachidonate analogues

This study has utilized radiolabeled analogues of arachidonic acid to study the substrate specificity of elongation of long-chain polyunsaturated fatty acids. Human umbilical vein endothelial cells were incubated for 2-72 hr in medium supplemented with 0.9-2.6 microM [14C]fatty acid, and cellular glycerolipids were analyzed by gas-liquid chromatography with radioactivity detection. Elongation of naturally occurring C20 polyunsaturated fatty acids occurred with eicosapentaenoate (20:5(n-3] greater than Mead acid (20:3(n-9] greater than arachidonate (20:4(n-6]. Chain length markedly influenced the extent of elongation of 5,8,11,14-tetraenoates (18:4 greater than 19:4 greater than 20:4 greater than 21:4); effects of initial double bond position were also observed (6,9,12,15-20:4 greater than 4,7,10,13-20:4. Neither 5,8,14- nor 5,11,14-20:3 was elongated to the extent of 5,8,11-20:3. Differences between polyunsaturated fatty acids were observed both in the initial rates and in the maximal percentages of elongation, suggesting that the content of cellular C20 and C22 fatty acids may represent a balance between chain elongation and retroconversion. Umbilical vein endothelial cells do not exhibit significant desaturation of either 22:4(n-6) or 22:5(n-3). By contrast, incubation with 5,8,11,14-[14C]18:4(n-4) resulted in formation of both [14C]20:5(n-4) and [14C]22:5(n-4). The respective time courses for the appearances of [14C]22:5(n-4) and [14C]20:5(n-5) suggests delta 6 desaturation of [14C]22:4(n-4) rather than delta 4 desaturation of [14C]20:4(n-4).

Metabolism of 15-hydroxy-5,8,11,13-eicosatetraenoic acid by MOLT-4 cells and blood T-lymphocytes

MOLT-4 lymphocytes metabolize 15-hydroxy-5,8,11,13-eicosatetraenoic acid (15-HETE) via beta-oxidation with retention of the hydroxyl group at the omega 6-carbon atom. 15-HETE oxidation is accompanied by the time-dependent accumulation of both beta-hydroxy acids and metabolites produced by repetitive cycles of the beta-oxidation spiral. Detection of 7-hydroxy-5-dodecenoic acid shows that these cells continue to beta-oxidize the substrate when the conjugated diene is allylic to a hydroxyl group. When 15-HETE was the substrate, it was also possible to detect 12-hydroxy-5,8,10-heptadecatrien-1-al and 3,15-dihydroxy-8,11,13-eicosatrienoic acid. The former product may be produced by alpha-oxidation of 13-hydroxy-6,9,11-octadecatrienoic acid followed by its decarboxylation. Detection of a 20-carbon metabolite, lacking a double bond at position 5, suggests that an intermediate of beta-oxidation was used as a substrate for chain elongation. When 13-hydroxy-6,9,11-octadecatrienoic acid was used as a substrate, it was indeed possible to detect 3,15-dihydroxy-8,11,13-eicosatrienoic acid as well as 15-hydroxy-8,11,13-eicosatrienoic acid. In addition, 13-hydroxy-6,9,11-octadecatrienoic acid was a precursor for the biosynthesis of both 14-hydroxy-7,10,12-nonadecatrien-1-al and 1,14-dihydroxy-7,10,12-nonadecatriene. These studies with MOLT-4 cells as well as with T-lymphocytes isolated from blood show that products of the 15-lipoxygenase pathway are metabolized with the accumulation of a variety of compounds. Since 15-HETE has been implicated as a modulator of T-cell function, these findings raise the possibility that the newly described metabolites may be involved in regulating lymphocyte function.

The metabolism of platelet-activating factor in human T-lymphocytes

The metabolism of 1-[3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-[3H]alkyl-2-acetyl-GPC; platelet-activating factor; PAF) was investigated in purified human peripheral blood T-lymphocytes and a human leukemia cell line of T-cell origin (MOLT-4). The major metabolic products of T-lymphocyte PAF metabolism are 1-alkyl-2-acyl-GPC, 1-alkyl-2-lyso-GPC and neutral lipid. The pattern of PAF metabolism in peripheral blood T-lymphocytes and MOLT-4 lymphoblasts was similar, although MOLT-4 lymphoblasts transformed PAF to 1-alkyl-2-acyl-GPC faster than peripheral blood T-lymphocytes (67% vs. 21% of added label after 64 min at 37 degrees C, respectively). Pre-exposure of MOLT-4 lymphoblasts to 1 mM of the serine hydrolase inhibitor phenylmethylsulfonyl fluoride resulted in an inhibition of PAF metabolism. Our results indicate that intact T-lymphocytes actively metabolize this biologically active phospholipid by the deacetylation-transacylation pathway.

Interactions between the metabolism of n-3 and n-6 fatty acids

Dietary n-3 fatty acids modify the fatty acid composition of phospholipids of different cells and tissues from rats in diverse ways. Neutrophil and platelet phospholipids contain elevated amounts of 20:5n-3, but only relatively small changes occur in the levels of 22-carbon n-3 fatty acids. Conversely, dietary n-3 acids result primarily in an increase in 22-carbon n-3 acids in heart, liver and kidney phospholipids. Platelets metabolize exogenous n-6 and n-3 fatty acids into a variety of different autocoids. However, it appears that only arachidonate and 20:5n-3 are released from phospholipids upon agonist-induced stimulation of phospholipases. Neutrophils metabolize arachidonate and 20:5n-3 in similar ways, both relative to phospholipid biosynthesis and the subsequent release of these acids for metabolism into leukotrienes.

Substrate specificity of the agonist-stimulated release of polyunsaturated fatty acids from vascular endothelial cells

Stimulation of vascular endothelial cells with agonists such as histamine and thrombin results in release of arachidonic acid from membrane lipids and subsequent eicosanoid synthesis. As shown previously, the agonist-stimulated deacylation is specific for arachidonate, eicosapentaenoate, and 5,8,11-eicosatrienoate. This study has utilized radiolabeled fatty acids differing in chain length and position of double bonds to further elucidate the fatty acyl specificity of agonist-stimulated deacylation. Replicate wells of confluent human umbilical vein endothelial cells were incubated with 14C-labeled fatty acids and then challenged with histamine, thrombin, or the calcium ionophore A23187. Comparison of the results obtained with isomeric eicosatetraenoic fatty acids with initial double bonds at carbons 4, 5, or 6 indicated that the deacylation induced by all three agonists exhibited marked specificity for the cis-5 double bond. Lack of stringent chain length specificity was indicated by agonist-stimulated release of 5,8,11,14- tetraenoic fatty acids with 18, 19, 20, and 21 carbons. Release of 5,8,14-[14C]eicosatrienoate was two-to threefold that of 5,11,14-[14C]eicosatrienoate, thus indicating that the cis-8 double bond may also contribute to the stringent recognition by the agonist-sensitive phospholipase. The present study has also demonstrated that histamine, thrombin, and A23187 do not stimulate release of docosahexaenoate from endothelial cells.

Distribution of arachidonic acid in choline- and ethanolamine-containing phosphoglycerides in subfractionated human neutrophils

Human neutrophils were fractionated on Percoll gradients and the various subcellular fractions were analyzed for phospholipid and fatty acid composition. The results showed that plasma membranes and azurophilic granules were enriched with ethanolamine-(PE) relative to choline-(PC) containing phosphoglycerides. A remarkable degree of uniformity existed throughout the gradient with respect to the subclass composition of the subcellular PC and PE components. In each fraction 50-60% of the PC was diacyl, 40-45% was 1-O-alkyl-2-acyl (ether linked), and 2-5% was 1-O-alk-1'-enyl-2-acyl (plasmalogenic). For PE, 20-25% was diacyl, 7-12% ether linked, and 64-76% plasmalogenic. When neutrophils were incubated for 15 min with [1-14C]arachidonic acid and subfractionated most of the PC-associated label was intracellularly localized. A similar result was observed in PE, however, when the cells were allowed to stand for 2 h in fatty acid-free buffer following the 15 min of labeling and then subfractionated there was a sizable migration of [14C]arachidonate into plasma membrane PE. In all cases the diacyl subclass was labeled most heavily after 15 min but after an additional 2 h of incubation in fatty acid-free buffer there was a direct transfer of label to the ether- and plasmalogenic-linked PC and PE subclasses. It was also found that arachidonoyl-coenzyme A 1-acyl-lysophosphatide acyltransferase activity was inherent in all three major membrane types but was enriched in the endoplasmic reticulum/secondary granule fraction. Arachidonate consistently accounted for roughly 5% of the PC and 17% of the PE fatty chain composition in each subcellular fraction. These findings demonstrate that, despite the uniform arachidonate and PC and PE subclass composition within the various neutrophil subcellular fractions, the bulk of the PC- and PE-associated arachidonate is localized in intracellular membranes.

Selective acyl transfer in the reacylation of brain glycerophospholipids. Comparison of three acylation systems for 1-alk-1'-enylglycero-3-phosphoethanolamine, 1-acylglycero-3-phosphoethanolamine and 1-acylglycero-3-phosphocholine in rat brain microsomes

The activities of three acylation systems for 1-alkenylglycerophosphoethanolamine (1-alkenyl-GPE), 1-acyl-GPE and 1-acylglycerophosphocholine (1-acyl-GPC) were compared in rat brain microsomes and the acyl selectivity of each system was clarified. The rate of CoA-independent transacylation of 1-[3H]alkenyl-GPE (approx. 4.5 nmol/10 min per mg protein) was about twice as high as in the case of 1-[3H]acyl-GPE and 1-[14C]acyl-GPC. On the other hand, the rates of CoA-dependent transacylation and CoA + ATP-dependent acylation (acylation of free fatty acids by acyl-CoA synthetase and acyl-CoA acyltransferase) of lysophospholipids were in the order 1-acyl-GPC greater than 1-acyl-GPE much greater than 1-alkenyl-GPE. HPLC analysis of newly synthesized molecular species revealed that the CoA-independent transacylation system exclusively esterified docosahexaenoate and arachidonate, regardless of the lysophospholipid class. The CoA-dependent transacylation and CoA + ATP-dependent acylation systems were almost the same with respect to the selectivities for unsaturated fatty acids when the same acceptor lysophospholipid was used, but some distinctive acyl selectivities were observed with different acceptor lysophospholipids. 1-Alkenyl-GPE selectively acquired only oleate in these two systems. 1-Acyl-GPE and 1-acyl-GPC showed selectivities for both arachidonate and oleate. In addition, an appreciable amount of palmitate was transferred to 1-acyl-GPC, not to 1-acyl-GPE, in CoA- or CoA + ATP-dependent manner. The acylation of exogenously added acyl-CoA revealed that the acyl selectivities of the CoA-dependent transacylation and CoA + ATP-dependent acylation systems may be mainly governed through the selective action of acyl-CoA acyltransferase. The preferential utilization of oleoyl-CoA by all acceptors and the different utilization of arachidonoyl-CoA between alkenyl and acyllysophospholipids indicated that there might be two distinct acyl-CoA:lysophospholipid acyltransferases that discriminate between oleoyl-CoA and arachidonoyl-CoA, respectively. Our present results clearly show that all three microsomal acylation systems can be active in the reacylation of three major brain glycerophospholipids and that the higher contribution of the CoA-independent system in the reacylation of ethanolamine glycerophospholipids, especially alkenylacyl-GPE, may tend to enrich docosahexaenoate in these phospholipids, as compared with in the case of diacyl-GPC.

Studies on the incorporation and transacylation of various fatty acids in choline and ethanolamine-containing phosphoacylglycerol subclasses in human neutrophils

The incorporation of eight 14C-labeled fatty acids into human neutrophil phospholipids was investigated and the results were expressed as percent of the total phospholipid associated 14C-labeled substrate incorporated after an initial 15 min labeling and a subsequent 2 h reincubation in fatty acid-free buffer. In all cases, the PC fraction accounted for more than 40% of the total phospholipid radioactivity. The inositol-containing phosphoacylglycerols were also labeled well by all the fatty acids except 22:6(n - 3) and 16:0; however, a greater percentage of [14C]22:6(n - 3) was found in PE than that of any other labeled fatty acid substrate. In all cases, most of the radioactivity in PC after 15 min was in the diacyl subclass. After 2 h, there was a shift of [14C]20:4(n - 6), [14C]20:5(n - 3), [14C]22:6(n - 3) and [14C]18:4(n - 4) into the ether-linked subclass. No such shift was observed for [14C]16:0 or [14C]18:2(n - 6) and, although there was an increase in the percent radioactive 20:3(n - 6) and 20:3(n - 9) in ether-linked PC after 2 h, the total radioactivity in this fraction remained low by comparison. A similar shift in label occurred in the plasmalogenic-linked PE subspecies in cells labeled with [14C]20:4(n - 6), [14C]20:5(n - 3) and [14C]22:6(n - 3).

The effect of a fish oil diet on the fatty acid composition of individual phospholipids and eicosanoid production by rat platelets

When rats were fed a diet containing chow or fish oil for six weeks, the platelet phospholipid content and percent distribution were similar. In the fish oil fed animals there was a 54, 40, 41, and 24% reduction, respectively, in the levels of 20:4(n-6) in the choline-, ethanolamine-, inositol- and serine-containing glycerophospholipids. Dietary fish oil increased the total (n-3) polyunsaturated fatty acid content in all lipids. This effect was most pronounced in the ethanolamine glycerophospholipids which now contained 26, 11, and 4 nmols of 20:5(n-3), 22:5(n-3), and 22:6(n-3) in 10(9) cells. Ionophore A23187 stimulation of platelets from the chow fed rats resulted in the synthesis of 7, 64, and 3.5 nmols of 12-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxy-5,8,10,14-eicosatetraenoic acid and 12-hydroxy-5,8,10,14,17-eicosapentaenoic acid, respectively, from 1 X 10(9) cells. The values from animals fed fish oil were 4, 18, and 27 nmol/10(9) platelets. It was not possible to detect any lipoxygenase products from 22:5(n-3) or 22:6(n-3), even though both acids are readily metabolized by lipoxygenase when added directly to platelets. These findings suggest that 22-carbon (n-3) fatty acids are not liberated when phospholipases are activated by calcium mobilization.

Metabolism of polyunsaturated fatty acids by an (n - 6)-lipoxygenase associated with human ejaculates

Washed cells of normal human ejaculates were incubated with [14C]arachidonic acid (20:4(n - 6] at 37 degrees C for 30-40 min and the main product was characterized as 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid by reverse phase, straight phase and chiral phase high performance liquid chromatography (HPLC) and by capillary gas chromatography-mass spectrometry. The biosynthesis of 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid from exogenous 20:4(n - 6) was inhibited by nordihydroguaiaretic acid and abolished by heat inactivation, but it appeared to be unaffected by the ionophore A23187 and Ca2+. Human spermatozoa were partly purified from contaminating material by the swim-up procedure and incubated with 14C-labelled 18:2(n - 6), 20:4(n - 6), 22:5(n - 6) and 22:6(n - 3) for 30-40 min at 37 degrees C. The main radiolabelled products, which were obtained in low yields, co-chromatographed with the Ls (n - 6)-hydroxy fatty acid of each substrate on reverse phase, straight phase and chiral phase HPLC. The (n - 6)-lipoxygenase was also present in ejaculates with oligozoospermia or azoospermia. The seminal fluid contains membrane-surrounded organelles (e.g., 'prostasomes' secreted by the prostate gland) and the (n - 6)-lipoxygenase was present and appeared to be relatively prominent in almost cell-free preparations of organelles of seminal fluid. The (n - 6)-lipoxygenase activity associated with the spermatozoa may thus be explained by the presence of prostasomes or other organelles, which may conceivably bind to the spermatozoon through hydrophobic interactions.

Metabolism of long-chain polyunsaturated fatty acids in isolated cardiac myocytes

The uptake and integrated intracellular metabolism of (n - 6) and (n - 3) polyunsaturated fatty acids was studied in isolated rat cardiac myocytes and in the perfused heart. Labeled linolenic acid (18:3(n - 3)) uptake and its subsequent metabolism into carbon dioxide as well as acylation into lipids was nonsaturable over a substrate range of 0.02 to 0.4 mM. [1-14C]Linoleic acid (18:2(n - 6)), dihomo-gamma-linolenic acid (20:3(n - 6)) and arachidonic acid (20:4(n - 6)) were transported into myocytes at rates similar to those for linolenic acid. Conversely both [1-14C]-gamma-linolenic acid (18:3(n - 6)) and eicosapentaenoic acid (20:5(n - 3)) were taken up at a slower rate. Oxidation of 18:3(n - 6) was 4-5-fold greater when compared with C18-C20 polyunsaturated fatty acids. When myocytes were incubated with labeled 18:2(n - 6), 18:3(n - 6), 18:3(n - 3), 20:4(n - 6) or 20:5(n - 3), it was not possible to detect any desaturation or chain-elongation products. Identical results were obtained when hearts were perfused with 1-14C-labeled linoleic acid.

Effect of a fish oil diet on the composition of rat neutrophil lipids and the molecular species of choline and ethanolamine glycerophospholipids

When rats were fed a corn oil versus a corn oil-fish oil diet the overall phospholipid content and composition as well as the subclass distribution of the choline- and ethanolamine-containing glycerophospholipids from neutrophils were not altered. The serine-containing glycerophospholipids were characterized by high levels of stearic and oleic acids. When fish oil was added to the diet it replaced some of the arachidonate in both the inositol- and the serine-containing glycerophospholipids. In the corn oil-fed animals, 25.2 and 33.6 mole %, respectively, of the molecular species of 1,2-diacyl- and 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine contained arachidonate. The values for 1,2-diacyl and 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine were, respectively, 41 and 55.8 mole %. When half of the 5% corn oil in the diet was replaced by fish oil, there was a 53, 38, 27, and 25% reduction, respectively, in the level of arachidonate in these four lipid subclasses. The amount of 5,8,11,14,17-eicosapentaenoic acid incorporated into these four subclasses was always less than the decline in arachidonic acid. This was due, in part, to the acylation of small amounts of 22-carbon (n-3) acids into these lipids. Molecular species analysis demonstrated that 5,8,11,14,17-eicosapentaenoic acid paired with the same components at the sn-1 position, and in the same ratio, as did arachidonic acid. The amounts of 16- and 18-carbon saturated and unsaturated fatty acid at the sn-2 position were not altered by dietary change. Collectively, these findings suggest that 5,8,11,14,17-eicosapentaenoic and arachidonic acids are metabolized in a similar way by neutrophils. These studies also support the concept that neutrophils contain two metabolic pools of phospholipids. One pool is altered by dietary fat change while the pool containing 16- and 18-carbon acids is resistant to change when fish oil is included in the diet.

Regulation of the metabolism of linoleic acid to arachidonic acid in rat hepatocytes

When 5 X 10(6) hepatocytes were incubated for 40 min with from 0.15 to 0.60 mM [1-14C]linoleic acid, [1-14C]6,9,12-octadecatrienoic acid, or [1-14C]8,11,14-eicosatrienoic acid, there was a concentration-dependent acylation of radioactive metabolites into both triglycerides and phospholipids. When the concentration of either [1-14C]linoleic acid or [1-14C]8,11,14-eicosatrienoic acid exceeded 0.3 mM, there was no further increase in the metabolism of either fatty acid to other (n-6) metabolites. When the concentration of [1-14C]6,9,12-octadecatrienoic acid exceeded 0.15 mM, there was an apparent substrate-induced inhibition in its metabolism to 8,11,14-eicosatrienoic acid. With all three substrates (0.3 mM), there was time-dependent metabolism to other (n-6) acids. Cells then were incubated simultaneously with 0.3 mM [1-14C]linoleic acid along with 0.15 to 0.45 mM 6,9,12-octadecatrienoic acid or 8,11,14-eicosatrienoic acid. These exogenous nonradioactive (n-6) acids suppressed but did not abolish the conversion of [1-14C]linoleate to radioactive arachidonate. These findings suggest that some linoleate is converted to arachidonate without intracellular mixing of 6,8,12-octadecatrienoic or 8,11,14-eicosatrienoic acids. This hypothesis is supported by the finding that exogenous linoleate did not markedly affect the metabolism of [1-14C]6,9,12-octadecatrienoic or [1-14C]8,11,14-eicosatrienoic acid by microsomal chain elongating or desaturating enzymes.

Metabolism of 6,9,12-octadecatrienoic acid and 6,9,12,15-octadecatetraenoic acid by rat hepatocytes

When 5.10(6) hepatocytes were incubated for 40 min with 0.015-0.3 mM (1-14C)-labeled 6,9,12-octadecatrienoic acid or (1-14C)-labeled 6,9,12,15-octadecatetraenoic acid there was a concentration-dependent acylation of radioactive metabolites into both phospholipids and triacylglycerol. However, when the concentration of either substrate exceeded 60-150 microM there was no further increase in the metabolism of either substrate to longer-chain (n-6) and (n-3) acids. When cells were then incubated for various periods of time with 60 microM substrate there was initial rapid removal of the substrate which was accompanied by its acylation into lipids. Over time, the amount of both substrates in lipids declined without an overall drop in specific activity. This decline was accompanied by an increase in long-chain (n-6) and (n-3) fatty acids. Similar results were obtained when the time-dependent metabolism of the two substrates was examined in individual hepatocyte phospholipids. Collectively, these findings suggest that when these two 18-carbon acids are produced by desaturation of dietary linoleate and linolenate that they are in part initially acylated into a labile phospholipid pool. Rapid release and subsequent further metabolism to longer-chain (n-6) and (n-3) acids may explain why these products of the 6-desaturase do not accumulate in membrane lipids.

Formation of 8-hydroxyhexadecatrienoic acid by vascular smooth muscle cells

Smooth muscle cells derived from the human umbilical vein produce four radioactive metabolites when they are incubated in culture with [3H]-12-hydroxyeicosatetraenoic acid. This conversion does not require the addition of an agonist for eicosanoid formation. The main product, which accounts for 60% of the radioactivity converted to these metabolites, has been identified as 8-hydroxyhexadecatrienoic acid.

On the mechanism of action of xanthine oxidase. Evidence in support of an oxo transfer mechanism in the molybdenum-containing hydroxylases

The mechanism of action of xanthine oxidase has been investigated using single-turnover experiments in an effort to determine the primary source of the oxygen atom incorporated into product in the course of catalysis. It is found from mass spectroscopic analysis of the uric acid generated in these experiments that when 16O-labeled enzyme in [18O]H2O is reacted with substoichiometric amounts of xanthine (under conditions where no enzyme molecule is likely to react with more than one substrate molecule), the uric acid isolated from the reaction mixture contains 16O at position 8 of the purine ring. Conversely, when 18O-labeled enzyme in [16O]H2O is exposed to substoichiometric xanthine, 18O is incorporated into the product uric acid. These results strongly support a variety of chemical studies with model molybdenum complexes suggesting that the oxygen atom of the Mo = O group known to be present at the active site of xanthine oxidase is transferred to product in the course of catalysis. The mechanistic implications of the present work are discussed.