A pertussis toxin-sensitive 8-lipoxygenase pathway is activated by a nicotinic acetylcholine receptor in aplysia neurons.

Acetylcholine (ACh) activates two types of chloride conductances in Aplysia neurons that can be distinguished by their kinetics and pharmacology. One is a rapidly desensitizing current that is blocked by alpha-conotoxin-ImI and the other is a sustained current that is insensitive to the toxin. These currents are differentially expressed in Aplysia neurons. We report here that neurons that respond to ACh with a sustained chloride conductance also generate 8-lipoxygenase metabolites. The sustained chloride conductance and the activation of 8-lipoxygenase have similar pharmacological profiles. Both are stimulated by suberyldicholine and nicotine, and both are inhibited by alpha-bungarotoxin. Like the sustained chloride conductance, the activation of 8-lipoxygenase is not blocked by alpha-conotoxin-ImI. In spite of the similarities between the metabolic and electrophysiological responses, the generation of 8-lipoxygenase metabolites does not appear to depend on the ion current since an influx of chloride ions is neither necessary nor sufficient for the formation of the lipid metabolites. In addition, the application of pertussis toxin blocked the ACh-activated release of arachidonic acid and the subsequent production of 8-lipoxygenase metabolites, yet the ACh-induced activation of the chloride conductance is not dependent on a G protein. Our results are consistent with the idea that the nicotinic ACh receptor that activates the sustained chloride conductance can, independent of the chloride ion influx, initiate lipid messenger synthesis.

Interactions between antiarrhythmic drugs and cardiac memory.

OBJECTIVE: Ventricular pacing or arrhythmias can induce cardiac memory (CM). We hypothesized that clinically administered antiarrhythmic drugs alter the expression of CM, and that the repolarization changes characteristic of CM can modulate the effects of antiarrhythmic drugs. METHODS: We studied conscious, chronically-instrumented dogs paced for two 1-h periods to study the effects of drugs on the evolution of memory (protocol 1) or for 21 days (protocol 2) to observe the effects of steady-state memory on drug actions. Dogs were treated in both settings with quinidine, lidocaine or E4031, in random order, and within therapeutic serum concentration ranges. RESULTS: Pacing, alone, for 2 h significantly prolonged ERP only near the left ventricular pacing site, whereas pacing alone for 21 days prolonged ERP at all sites (P<0.05). Quinidine and E4031, but not lidocaine, prolonged repolarization and ERP and suppressed evolution of CM in protocol 1. However, quinidine's effect in prolonging repolarization was diminished in both protocols, while its effect in prolonging ERP was diminished in the 21-day protocol only. In contrast, the effects of E4031 were additive to those of CM, prolonging repolarization and ERP in both protocols, while lidocaine showed no changes in effect at all. CONCLUSIONS: Pacing to induce CM significantly affects ventricular repolarization and refractoriness, and there are interactions between CM, quinidine and E4031. Depending on the specific drug, these interactions have the potential to be anti- or proarrhythmic, and may impact importantly on the clinical efficacy of drugs as well as on electrophysiologic testing of drug actions.

Alteration of human leukotriene A4 hydrolase activity after site-directed mutagenesis: serine-415 is a regulatory residue.

Leukotriene A4 hydrolase (LTA-H) is a bifunctional protein that has aminopeptidase activity, but also contains an epoxide hydrolase activity that converts leukotriene (LT)A4 to LTB4. The lipid metabolic activity of this enzyme plays a central role in the control of polymorphonuclear leukocyte function and in the development of inflammation. LTA-H is widely spread in many mammalian tissues, although it appears to be inactive in many cases. Regulation of this enzyme's activity by phosphorylation of a serine at residue 415 has recently been described. Since the activation of LTA-H in the presence of activated PMNL would likely lead to a substantial increase in the production of inflammatory lipids, regulation of LTA-H presents a novel potential target for anti-inflammatory therapy. We have now made a series of site-directed mutants at this site to test the importance of this residue to the activity of LTA-H. Replacement of the critical serine with threonine or glutamine has little effect on either the epoxide hydrolase or aminopeptidase activities. However, replacing serine with a negatively charged amino acid (either aspartate or glutamate), intended to mimic phosphorylation at that site, causes significant reduction in epoxide hydrolase activity (50-70%). These mutations have little effect on the aminopeptidase activity of the LTA-H, suggesting that the mutation models the regulatory event and is not simply due to improper folding of the protein.

Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium: II. In vivo study.

BACKGROUND: In the companion article, we report a significant difference in quinidine effects on the action potential duration between surface (epicardial and endocardial) cells and midmyocardial cells (M cells) of canine left ventricle in vitro. This article considers two questions raised by the previous study: (1) Are the complex quinidine effects in vitro reflected in its actions on the heart in situ? (2) What are the cellular determinants of quinidine effects on QT interval in ECG? METHODS AND RESULTS: We used plunge and surface electrodes to measure activation-recovery intervals (ARIs) of bipolar electrograms obtained from epicardium, endocardium, and midmyocardium (3, 5, and 9 mm from epicardium) of canine left ventricle in conditions of AV block and right ventricular pacing. Quinidine was infused continuously; its plasma level increased from 1.6+/-0.1 microg/mL at 30 minutes to 7.6+/-0.7 microg/mL at 180 minutes. At cycle lengths (CLs) from 300 to 1500 ms, there was no ARI gradient across the ventricular wall before and during quinidine infusion. At a CL of 300 ms, therapeutic concentrations of quinidine prolonged ARIs and QT intervals. At a CL of 1500 ms, ARIs were significantly prolonged at low quinidine concentrations. With an increase of quinidine concentration, this effect subsided and disappeared. CONCLUSIONS: In situ, quinidine-induced prolongation of repolarization is uniform in all myocardial layers and follows the pattern observed in M cells in vitro. The ability of quinidine in therapeutic concentrations to prolong repolarization at rapid heart rates can contribute to its antiarrhythmic efficacy.

Is there a role for 15-lipoxygenase in atherogenesis?

15-Lipoxygenase has been suggested to play a role in atherogenesis. The proposed action of this enzyme is to oxidize low density lipoprotein (LDL) to the extent that LDL becomes a ligand for the macrophage scavenger receptor. 15-Lipoxygenase and oxidized LDL are co-localized in atherosclerotic lesions; antioxidant drugs that block the lipoxygenase also block oxidation of LDL and the progression of experimental atherosclerosis. Biochemical experiments have demonstrated that the lipoxygenase can be induced by cytokines and/or another factor(s) associated with hypercholesterolemia. However, molecular biological work has shown that induction of the enzyme alone is not sufficient to induce lesion formation. Furthermore, the mechanism of action of 15-lipoxygenase in atherogenesis remains unclear. Predictions of the stereochemistry of enzyme-oxidized linoleate products appear to conflict with the available data. In fact, most studies have discovered substantial levels of racemic 13-hydroxyoctadecadienoic acid (13-HODE) in arterial lesions rather than the stereochemically pure 13(S)-HODE expected from purified enzyme. The possibility that the generation of products of 15-lipoxygenase metabolism must occur in a specific cellular location and during a brief time window in the development of the disease has been discussed. It is also possible that the true function of the linoleate metabolites is to modulate gene expression and regulate mitogenesis, and that oxidation of LDL may play a secondary role. The advent of transgenic species that both develop atherosclerosis and either fail to express or overexpress the lipoxygenase presents an opportunity to clarify some of these issues in the near future.

Identification of an 8-lipoxygenase pathway in nervous tissue of Aplysia californica.

Arachidonic acid is converted to (8R)-hydroperoxyeicosa-5,9,11, 14-tetraenoic acid (8-HPETE) during incubations with homogenates of the central nervous system of the marine mollusc, Aplysia californica. 8-HPETE can be reduced to the corresponding hydroxy acid or be enzymatically converted to a newly identified metabolite, 8-ketoeicosa-5,9,11,14-tetraenoic acid (8-KETE). These metabolites were identified by high performance liquid chromatography, UV absorbance, and gas chromatography/mass spectrometry. Stereochemical analysis of the products demonstrate that the neuronal enzyme is an (8R)-lipoxygenase. Previously we have shown that the neurotransmitters, histamine and Phe-Met-Arg-Phe-amide, activate 12-lipoxygenase metabolism in isolated identified Aplysia neurons. We now show that acetylcholine activates the (8R)-lipoxygenase pathway within intact nerve cells. Thus, both (12S)- and (8R)-lipoxygenase co-exist in intact Aplysia nervous tissue but are differentially activated by several neurotransmitters. The precise physiological role of the 8-lipoxygenase products is currently under investigation, but by analogy to the well-described 12-lipoxygenase pathway, we suggest that (8R)-HPETE and 8-KETE may serve as second messengers in Aplysia cholinoceptive neurons.

Electrophysiologic effects of interactions between activated canine neutrophils and cardiac myocytes.

INTRODUCTION: Myocardial ischemia causes neutrophils to bind to activated myocytes and liberate platelet-activating factor (PAF). PAF causes delayed repolarization, early afterdepolarizations (EADs), and arrest of repolarization. We studied the effect of activation of neutrophils bound to canine cardiac myocytes to determine if such activation causes PAF generation and similar changes in transmembrane potentials. METHODS AND RESULTS: Myocytes from canine left ventricle and neutrophils from the same dog were superfused with Tyrode's solution and transmembrane potentials recorded from the former. Neutrophils (100 microL, 10(6)/mL) were added and allowed to bind to the myocytes. Neutrophils were activated with 1% zymosan-activated serum (ZAS). CV-6209 (100 nM) was used to block receptors for PAF. Liberation of PAF by activated neutrophils was quantified with a commercial radioimmunoassay kit. Neutrophils activated with ZAS caused changes in myocyte transmembrane potentials like those induced by PAF: action potential prolongation, runs of EAD, and periods of plateau arrest. PAF receptor blockade prevented neutrophil activation from altering transmembrane potentials. Neutrophils activated with 1% ZAS liberated significant amounts of PAF. CONCLUSIONS: When neutrophils bound to cardiac myocytes are activated by exposure to 1% ZAS, they cause prompt and consistent changes in myocyte electrical activity that could be arrhythmogenic for the in situ heart. These changes are similar to those caused by PAF in pharmacologic studies. Neutrophils activated in this manner generate PAF, and the effects of their activation are prevented by blockade of PAF receptors. We conclude that, during reperfusion of ischemic myocardium, PAF generated by activated neutrophils most likely is a cause of some arrhythmias.

Attenuation of diet-induced atherosclerosis in rabbits with a highly selective 15-lipoxygenase inhibitor lacking significant antioxidant properties.

1. 15-Lipoxygenase (15-LO) has been implicated in the pathogenesis of atherosclerosis because of its localization in lesions and the many biological activities exhibited by its products. To provide further evidence for a role of 15-LO, the effects of PD 146176 on the development of atherosclerosis in cholesterol-fed rabbits were assessed. This novel drug is a specific inhibitor of the enzyme in vitro and lacks significant non specific antioxidant properties. 2. PD 146176 inhibited rabbit reticulocyte 15-LO through a mixed noncompetitive mode with a Ki of 197 nM. The drug had minimal effects on either copper or 2,2'-azobis(2-amidinopropane)hydrochloride (ABAP) induced oxidation of LDL except at concentrations 2 orders higher than the Ki. 3. Control New Zealand rabbits were fed a high-fat diet containing 0.25% wt./wt. cholesterol; treated animals received inhibitor in this diet (175 mg kg-1, b.i.d.). Plasma concentrations of inhibitor were similar to the estimated Ki (197 nM). During the 12 week study, there were no significant differences in weight gain haematocrit, plasma total cholesterol concentrations, or distribution of lipoprotein cholesterol. 4. The drug plasma concentrations achieved in vivo did not inhibit low-density lipoprotein (LDL) oxidation in vitro. Furthermore, LDL isolated from PD 146176-treated animals was as susceptible as that from controls to oxidation ex vivo by either copper or ABAP. 5. PD 146176 was very effective in suppressing atherogenesis, especially in the aortic arch where lesion coverage diminished from 15 +/- 4 to 0% (P < 0.02); esterified cholesterol content was reduced from 2.1 +/- 0.7 to 0 micrograms mg-1 (P < 0.02) in this region. Immunostainable lipid-laden macrophages present in aortic intima of control animals were totally absent in the drug-treated group. 6. Results of these studies are consistent with a role for 15-LO in atherogenesis.

Thromboxane-mediated alveolar responses to acute obstruction of the pulmonary vasculature.

To investigate the mechanisms responsible for the accelerated radioaerosol transalveolar clearance rates caused by acute balloon occlusion of the pulmonary arterial tree in dogs, the effects of vagal tone (n = 5) and prostaglandins (n = 21) including thromboxane were evaluated. In 15 animals, serial pulmonary arterial and aortic blood samples were acquired to evaluate pertinent metabolic products. Balloon occlusion of a localized arterial territory caused significant acceleration of technetium-99m diethylenetriamine pentaacetic acid (Tc-99m DTPA) clearance in the zone immediately distal to the occlusion (baseline clearance half-time 23.1 +/- 0/7 min versus 19.3 +/- 0.4 min, mean +/- SEM, p < 0.05). Vagotomy had no effect on occlusion-accelerated clearance. However, significant (p < 0.05) normalization did occur in the presence of indomethacin (21.9 +/- 0.4 min) and meclofenamic acid (20.4 +/- 0.5 min). Plasma values of thromboxane rose dramatically (pulmonary blood baseline 119 pg/ml to > 40,000 pg/ml) and transiently immediately after pulmonary vascular occlusion, and this rise was blunted significantly (peak pulmonary thromboxane B2 [TXB2] concentration = 668 pg/ml, p < 0.05) by meclofenamic acid. Significant normalization of local DTPA clearance rates also occurred in the presence of a thromboxane receptor blocker (n = 4), even when blood levels of thromboxane were elevated. Changes in transalveolar DTPA clearance rates after balloon occlusion of pulmonary arteries seem to a significant extent to be thromboxane-mediated.

Regulation of leukotriene A4 hydrolase activity in endothelial cells by phosphorylation.

Endothelial cells contain leukotriene (LT) A4 hydrolase (LTA-H) as detected by Northern and Western blotting, but several studies have been unable to detect the activity of this enzyme. Since LTA-H could play a key role in determining what biologically active lipids are generated by activated endothelium during the inflammatory process, we studied possible mechanisms by which this enzyme may be regulated. We find that LTA-H is phosphorylated under basal conditions in human endothelial cells and in this state does not exhibit epoxide hydrolase activity (i.e. conversion of LTA4 to LTB4). LTA-H purified from endothelial cells is efficiently dephosphorylated by incubation with protein phosphatase-1 in the presence of an LTA-H peptide substrate and not at all in the absence of substrate. Under conditions that lead to dephosphorylation, protein phosphatase-1 activates the epoxide hydrolase activity of LTA-H. Using peptide mapping and site-directed mutagenesis, we have identified serine 415 as the site of phosphorylation of LTA-H by a kinase found in endothelial cell cytosol. In parallel, we have studied a human lung carcinoma cell line that expresses active LTA-H. Although these cells have cytosolic kinases that phosphorylate recombinant LTA-H, they do not target serine 415 and thus do not inhibit LTA-H activity. We believe that LTA-H is regulated in intact cells by a kinase/phosphatase cycle and further that the kinase in endothelial cells specifically recognizes and phosphorylates a regulatory site in the LTA-H.

Mouse peritoneal macrophages contain abundant omega-6 lipoxygenase activity that is independent of interleukin-4.

The action of an omega-6 lipoxygenase (LO) has been implicated in the development of atherosclerosis through a mechanism involving oxidation of LDL, and its regulation in macrophages may have important implications for the disease process. Human monocyte-derived macrophages (HMDMs) showed no demonstrable LO protein or activity unless they were incubated with interleukin-4 (IL-4). In contrast, mouse peritoneal macrophages (MPMs) possessed significant basal levels of LO activity and protein that were augmented by IL-4 treatment. Interferon gamma prevented the induction of LO in both HMDMs and MPMs. Whereas interferon gamma could completely block the IL-4 induction of LO in human cells, it did not suppress basal LO activity in MPMs. Both HMDMs and MPMs exhibited similar concentration-response relationships for stimulation of LO activity and protein, with maximal induction at 1 ng/mL IL-4. The time course of IL-4 induction of LO activity was markedly different in human and murine cells. IL-4 induced LO activity and protein in human cells by 48 hours that were maximal by 72 hours; there was a decline to a new baseline by 96 hours. MPMs have a significant amount of LO activity at baseline, which declined with time by nearly 10-fold in the absence of IL-4, IL-4 blunted the decline of LO activity with time and restored activity to that found at baseline by 48 hours. IL-4 was not responsible for the LO activity present in freshly isolated MPMs since both activity and protein content were similar in cells harvested from IL-4+/+ and IL-/- mice. Therefore, whereas IL-4 may be an important modulator of LO production in vitro, it is not essential for the in vivo expression of this protein. Further, these studies demonstrate that significant differences exist between monocyte-derived macrophages matured in vitro and tissue macrophages that have matured in vivo.

Phospholipase D activity regulates the binding of leukotriene B4 to human polymorphonuclear leukocytes.

Unsaturated fatty acids, such as arachidonic acid, have been implicated as second messengers of various cellular responses. In this paper, we demonstrate that the release of arachidonate from human polymorphonuclear leukocytes (PMNL) via a pathway initiated by phospholipase D (PLD) mediates phorbol myristate acetate (PMA)-induced desensitization of leukotriene B4 (LTB4) receptors. PMA caused a delayed release of arachidonic acid from PMNL, which occurred within minutes of the generation of the PLD products, phosphatidic acid and diglyceride, from [3H]lysophosphatidylcholine prelabeled PMNL. Moreover, stimulating PMNL with PMA in the presence of ethanol resulted in the formation of phosphatidylethanol at the expenses of phosphatidic acid, diglyceride and arachidonic acid. The PMA-induced generation of these three PLD products was inhibited by mepacrine which, in parallel, significantly blocked PMA-induced desensitization of [3H]LTB4 binding, which suggested that elevation of one or more of these products played a role in desensitization. In contrast, under basal conditions mepacrine reduced levels of these three lipids in a dose-related manner and, in parallel, increased basal [3H]LTB4 binding to PMNL. The reduction of PMA-induced LTB4 receptor desensitization by mepacrine could be overcome to various degrees by adding back PLD-derived lipids such as arachidonic acid. These data demonstrate that [3H]LTB4 binding to PMNL is decreased by PLD-derived lipids and suggests that intracellular arachidonic acid may modulate PMNL responsiveness to LTB4 activation.

Arrhythmias caused by platelet activating factor.

INTRODUCTION: Both ischemia and reperfusion are associated with ventricular arrhythmias. In both instances, neutrophils migrate into the ischemic zone, are activated by locally released factors, and bind to myocytes. The activated neutrophils liberate platelet activating factor (PAF). We have studied the arrhythmogenic actions of PAF on transmembrane potentials of isolated canine cardiac myocytes. METHODS AND RESULTS: Cardiac myocytes were prepared from normal canine hearts by standard methods and studied in vitro by recording transmembrane potentials under control conditions and during exposure to graded doses of PAF, usually 0.25 to 1.25 micrograms (0.25 to 1.2 microM). Myocytes were superfused with Tyrode's solution (2.0 mL/min), paced at a cycle length of 1000 msec, and maintained at a temperature between 36 degrees and 38 degrees C. PAF caused a consistent and dose-dependent set of alterations in the transmembrane potential, including increased action potential duration, runs of early afterdepolarizations (EADs), and transient arrest of repolarization (PA). In addition, in some myocytes PAF caused intermittent small depolarizations both at the plateau voltage and resting potential. The effects of PAF were transient: only some residual action potential prolongation was noted after Tyrode's washout for 5 minutes. Effects of PAF were blocked in a dose-dependent manner by the PAF receptor antagonist, CV-6209. Both tetrodotoxin (1.2 x 10(-6) M) and xylocaine (5 x 10(-5) M) antagonized the ability of PAF to cause EADs and PA. CONCLUSIONS: PAF consistently exerts arrhythmogenic effects on the membrane of ventricular myocytes. Since PAF is liberated by activated neutrophils and since activated neutrophils migrate into ischemic myocardium on reperfusion, we judge that PAF liberated by such neutrophils is an important arrhythmogenic factor for reperfusion arrhythmias. The same mechanism may be a cause of arrhythmias during the evolution of infarction.

Stereochemistry of the Aplysia neuronal 12-lipoxygenase: specific potentiation of FMRFamide action by 12(S)-HPETE.

Nervous tissue of the marine mollusc, Aplysia californica, generates arachidonic acid metabolites in response to neurotransmitters such as histamine or FMRFamide. In addition, identified neurons of Aplysia respond to the pharmacologic application of some of these products, particularly those of the 12-lipoxygenase pathway. We investigated the chirality of the initial Aplysia 12-lipoxygenase product, 12-HPETE, in preparation for more detailed metabolic studies and for the analysis of the physiological activity of the endogenous lipid. Neural homogenates and intact ganglia exclusively generate 12(S)-HPETE as do the better characterized mammalian lipoxygenases. The direct application of 12(S)-HPETE to cultured sensory neurons induced a hyperpolarization which averaged 2.6 mV. We did not find any difference between the response to the naturally-occurring 12(S)-HPETE and its diastereomer, 12(R)-HPETE which is not generated in Aplysia. Both isomers were significantly more effective than 15(S)-HPETE. In contrast, 12(S)-HPETE, but not 12(R)-HPETE, was a potent modulator of the action of the molluscan neuropeptide, FMRFamide. Prior application of 12(S)-HPETE to cultured sensory neurons increased the subsequent response to a submaximal dose of FMRFamide by 60%. On the other hand, 12(R)-HPETE reduced the subsequent response to the peptide by 30%. The lack of stereospecificity in the direct effect of the lipids differs markedly from their stereospecific effects as modulators of FMRFamide action. This suggests that there may be an important neurophysiologic role for these lipid modulators which is distinct from their direct effects, and also indicates that there are multiple sites and mechanisms by which lipid hydroperoxides act on neurons in Aplysia.

The role of the endothelial cell in leukotriene biosynthesis.

Endothelial cells do not contain 5-lipoxygenase and thus are unable to generate LTA4 from arachidonate. Nonetheless, endothelial cells may play an important role in leukotriene synthesis by virtue of their ability to metabolize LTA4 derived from activated polymorphonuclear leukocytes (PMNL) and to modulate PMNL 5-lipoxygenase activity. Porcine aortic endothelial cells were found to metabolize exogenous LTA4 to LTC4, and under some conditions human umbilical vein endothelial cells have been found to generate LTB4. Production of LTB4 by these cells appears to be under poorly understood cellular control, and it remains a controversial area of research. Under physiologic conditions, endothelial cells are in constant contact with circulating PMNL, which are known to generate substantial amounts of LTA4. When these two cell types are coincubated in vitro, clear evidence of transcellular metabolism of PMNL-derived LTA4 to LTC4 by endothelial cells is found. Coincubations produce from two to greater than 10 times more LTC4 than either cell alone. In contrast to these findings, when these cells were activated by the receptor-mediated agonist fMLP, evidence for an endothelial cell inhibition of PMNL 5-lipoxygenase was obtained. Rather than augmentation of LTC4 production, as seen with A23187 activation, coincubation activated by fMLP generated significantly less LTC4 (0.23 +/- 0.08 versus 0.75 +/- 0.39 pmol/10(7) cells). The endothelial cell inhibition was removed when these cells were pretreated with aspirin, suggesting that their major cyclooxygenase product, prostacyclin, acts as a feedback regulator of LT synthesis. When cyclooxygenase was blocked, significant transcellular LTC4 synthesis was once again apparent (1.66 +/- 0.44 pmol/10(7) cells).

Aplysia californica contains a novel 12-lipoxygenase which generates biologically active products from arachidonic acid.

Physiologic stimulation of identified neurons in ganglia of the marine mollusk, Aplysia californica, leads to the generation of arachidonic acid metabolites. Using various preparations of Aplysia nervous tissue, we have identified 12-lipoxygenase products including the inactive 12-hydroxyeicosatetraenoic acid (12-HETE) and the biologically active 12-ketoeicosatetraenoic acid (12-KETE) and 8-hydroxy-11(12)-epoxyeicosatrienoic acid (8-HEpETE). Each of these metabolites can be derived from the intermediate 12-hydroperoxyeicosatetraenoic acid (12-HPETE), which can itself activate several identified neurons in Aplysia. In spite of conflicting results in studies of mammalian brain 12-lipoxygenase, Aplysia nervous tissue clearly contains an enzymatic activity which generates stereochemically pure 12(S)-HETE. This activity is destroyed by boiling and is sensitive to nonspecific lipoxygenase inhibitors but not to agents specific for other lipoxygenases or the cyclooxygenase enzyme. The Aplysia 12-lipoxygenase is highly enriched in neural tissue and is almost completely absent in the neural sheath, which is composed primarily of connective tissue and muscle. Preliminary purification has shown that, in contrast to the previously characterized 12-lipoxygenases, the Aplysia enzyme is associated with membrane fractions and is not found in the cytosol. Further studies are in progress to determine the kinetic properties and to define the cellular and subcellular distribution of this novel lipoxygenase.

Rabbit and human liver contain a novel pentacyclic triterpene ester with acyl-CoA: cholesterol acyltransferase inhibitory activity.

Acyl-coenzyme A (CoA):cholesterol acyltransferase (ACAT) catalyzes the intracellular fatty acid esterification of cholesterol and is thought to play a key role in lipoprotein metabolism and atherogenesis. Herein we describe the purification and characterization of a novel pentacyclic triterpene ester from rabbit liver that has ACAT inhibitory activity. The inhibitor was purified by a combination of silicic acid chromatography and preparative thin layer chromatography. The compound inhibited both rabbit and rat liver microsomal ACAT activity with an IC50 = 20 microM. The lipid did not inhibit fatty acid incorporation into triglycerides, diglycerides, monoglycerides, or phospholipids nor did it inhibit plasma lecithin:cholesterol acyltransferase activity. However, rat liver microsomal acyl-CoA:retinol acyltransferase activity was inhibited by the terpene ester. Kinetic data are consistent with a mechanism in which ACAT is inhibited by the compound in an irreversible manner. The subcellular fractionation pattern of both ACAT activity and the ACAT inhibitor were similar in rabbit liver (both were approximately equally distributed in membranes that pelleted at 10,000 X g and 100,000 X g). A lipid with similar properties to the rabbit liver inhibitor was found in many other rabbit tissues, including adrenal and spleen, as well as in human liver. Rat liver did not contain this lipid. Structural analysis by NMR, mass spectrometry, and x-ray crystallography indicated that the rabbit liver inhibitor was a fatty acid ester (mostly stearate) of a pentacyclic triterpene acid. The carbon skeleton of the triterpene moiety is a new member of the olean-12-ene triterpene family. Both the negatively charged carboxylic acid group of the triterpene moiety and the esterified fatty acid group were necessary for the ACAT-inhibitory activity of the triterpene ester. Lastly, we present preliminary data which, together with the structural homology of the rabbit triterpene with known plant compounds, suggest the hypothesis that the triterpene moiety of the rabbit ACAT inhibitor arises from dietary absorption of a plant triterpene.