Products of heme-catalyzed transformation of the arachidonate derivative 12-HPETE open S-type K+ channels in Aplysia

In Aplysia mechanosensory neurons, the neuropeptide FMRFamide increases the opening of the background S-K+ channel. This action is mediated by activation of arachidonic acid metabolism. Arachidonic acid in Aplysia nervous tissue is transformed through the 12-lipoxygenase pathway to 12-HPETE, which undergoes further metabolism. In intact sensory cells, 12-HPETE simulates the FMRFamide response, raising the question of whether 12-HPETE is the messenger molecule ultimately acting on the S-K+ channel. Here we show that in cell-free (inside-out) patches from sensory cells, 12-HPETE fails to modulate the S-K+ channel, but in the presence of hematin (which catalyzes 12-HPETE metabolism), it triggers sharp increases in the channel opening probability. We also found that SKF-525A, an inhibitor of the cytochrome P450, reduces the response to FMRFamide, arachidonic acid, and 12-HPETE in intact cells. We conclude that a heme-catalyzed transformation of 12-HPETE is necessary and sufficient to promote the opening of the S-K+ channel and a heme-containing enzyme such as cytochrome P450 might play this key role.     

Formation and biological activity of 12-ketoeicosatetraenoic acid in the nervous system of Aplysia

12-Hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE), a lipoxygenase product, simulates the synaptic responses produced by the modulatory transmitter, histamine, and the neuroactive peptide, Phe-Met-Arg-Phe-amide (FMRFamide), in identified neurons of the marine mollusk, Aplysia californica (Piomelli, D., Shapiro, E., Feinmark, S. J., and Schwartz, J. H. (1987) J. Neurosci. 7, 3675-3886; Shapiro, E., Piomelli, D., Feinmark, S., Vogel, S., Chin, G., and Schwartz, J. H. (1988) Cold Spring Harbor Symp. Quant. Biol. 53, in press). The 12-lipoxygenase pathway has not yet been fully characterized, but 12-HPETE is known to be metabolized further. We therefore began to search for other metabolites in order to investigate whether the actions of 12-HPETE might require its conversion to other active products. Here we report the identification of 12-keto-5,8,10,14-eicosatetraenoic acid (12-KETE), a metabolite of 12-HPETE formed by Aplysia nervous tissue. This product was identified in incubations of the tissue with arachidonic acid using high performance liquid chromatography, UV spectrometry, and gas chromatography/mass spectrometry. [3H]12-KETE was formed from endogenous lipid stores in nervous tissue, labeled by incubation with [3H]arachidonic acid, when stimulated by application of histamine. In L14 and L10 cells, identified neurons in the abdominal ganglion, applications of 12-KETE elicit changes in membrane potential similar to those evoked by histamine. 12(S)-Hydroxy-5,8,10,14-eicosatetraenoic acid, another metabolite of 12-HPETE, is inactive. These results support the hypothesis that 12-HPETE and its metabolite, 12-KETE, participate in transduction of histamine responses in Aplysia neurons.     

Charybdotoxin selectively blocks small Ca-activated K channels in Aplysia neurons

The action of charybdotoxin (ChTX), a peptide component isolated from the venom of the scorpion Leiurus quinquestriatus, was investigated on membrane currents of identified neurons from the marine mollusk, Aplysia californica. Macroscopic current recordings showed that the external application of ChTX blocks the Ca-activated K current in a dose- and voltage-dependent manner. The apparent dissociation constant is 30 nM at V = -30 mV and increases e-fold for a +50- to +70-mV change in membrane potential, which indicates that the toxin molecule is sensitive to approximately 35% of the transmembrane electric field. The toxin is bound to the receptor with a 1:1 stoichiometry and its effect is reversible after washout. The toxin also suppresses the membrane leakage conductance and a resting K conductance activated by internal Ca ions. The toxin has no significant effect on the inward Na or Ca currents, the transient K current, or the delayed rectifier K current. Records from Ca-activated K channels revealed a single channel conductance of 35 +/- 5 pS at V = 0 mV in asymmetrical K solution. The channel open probability increased with the internal Ca concentration and with membrane voltage. The K channels were blocked by submillimolar concentrations of tetraethylammonium ions and by nanomolar concentrations of ChTX, but were not blocked by 4-aminopyridine if applied externally on outside-out patches. From the effects of ChTX on K current and on bursting pacemaker activity, it is concluded that the termination of bursts is in part controlled by a Ca-activated K conductance.     

Effect of curare on responses to different putative neurotransmitters in Aplysia neurons.

We have studied the effects of curare on responses resulting from iontophoretic application of several putative neurotransmitters onto Aplysia neurons. These neurons have specific receptors for acetylcholine (ACh), dopamine, octopamine, phenylethanolamine, histamine, gamma-aminobutyric acid (GABA), aspartic acid, and glutamic acid. Each of these substances may on different specific neurons elicit at least three types of response, caused by a fast depolarizing Na+, a fast hyperpolarizing Cl-, or a slow hyperpolarizing K+ conductance increase. All responses resulting from either Na+ or Cl- conductance increases, irrespective of which putative transmitter activated the response, were sensitive to curare. Most were totally blocked by less than or equal to 10-4 M curare. GABA responses were less sensitive and were often only depressed by 10-3 M curare. K+ conductance responses, irrespective of the transmitter, were not curare sensitive. These results are consistent with a model of receptor organization in which one neurotransmitter receptor may be associated with any of at least three ionophores, mediating conductance increase responses to Na+, Cl-, and K+, respectively. In Aplysia nervous tissue, curare appears not to be a specific antagonist for the nicotinic ACh receptor, but rather to be a specific blocking agent for a class of receptor-activated Na+ and Cl- responses.     

A mammalian two pore domain mechano-gated S-like K+ channel.

Aplysia S-type K+ channels of sensory neurons play a dominant role in presynaptic facilitation and behavioural sensitization. They are closed by serotonin via cAMP-dependent phosphorylation, whereas they are opened by arachidonic acid, volatile general anaesthetics and mechanical stimulation. We have identified a cloned mammalian two P domain K+ channel sharing the properties of the S channel. In addition, the recombinant channel is opened by lipid bilayer amphipathic crenators, while it is closed by cup-formers. The cytoplasmic C-terminus contains a charged region critical for chemical and mechanical activation, as well as a phosphorylation site required for cAMP inhibition.     

Inward rectifier K+-channel kinetics from analysis of the complex conductance of Aplysia neuronal membrane.

Conduction in inward rectifier, K+-channels in Aplysia neuron and Ba++ blockade of these channels were studied by rapid measurement of the membrane complex admittance in the frequency range 0.05 to 200 Hz during voltage clamps to membrane potentials in the range -90 to -40 mV. Complex ionic conductances of K+ and Cl- rectifiers were extracted from complex admittances of other membrane conduction processes and capacitance by vector subtraction of the membrane complex admittance during suppressed inward K+ current (near zero-mean current and in zero [K+]0) from complex admittances determined at other [K+]0 and membrane potentials. The contribution of the K+ rectifier to the admittance is distinguishable in the frequency domain above 1 Hz from the contribution of the Cl- rectifier, which is only apparent at frequencies less than 0.1 Hz. The voltage dependence (-90 to -40 mV) of the chord conductance (0.2 to 0.05 microS) and the relaxation time (4-8 ms) of K+ rectifier channels at [K+]0 = 40 mM were determined by curve fits of admittance data by a membrane admittance model based on the linearized Hodgkin-Huxley equations. The conductance of inward rectifier, K+ channels at a membrane potential of -80 mV had a square-root dependence on external K+ concentration, and the relaxation time increased from 2 to 7.5 ms for [K+]0 = 20 and 100 mM, respectively. The complex conductance of the inward K+ rectifier, affected by Ba++, was obtained by complex vector subtraction of the membrane admittance during blockage of inward rectifier, K+ channels (at -35 mV and [Ba++]0 = 5 mM) from admittances determined at -80 mV and at other Ba++ concentrations. The relaxation time of the blockade process decreased with increases in Ba++ concentration. An open-closed channel state model produces the inductive-like kinetic behavior in the complex conductance of inward rectifier, K+ channels and the addition of a blocked channel state accounts for the capacitive-like kinetic behavior of the Ba++ blockade process.     

Intracellular signalling involved in activation of the volume-sensitive K+ current in Ehrlich ascites tumour cells

The cell swelling-activated K+ channel in Ehrlich ascites tumour cells has a conductance of 5 pS estimated from noise analysis of the volume-sensitive whole-cell K+ current (I(K,vol)). I(K,vol) exhibits Goldman-Hodgkin-Katz type behaviour and is insensitive to clotrimazole, apamin and charybdotoxin (ChTX), but inhibited by clofilium. Its small conductance, lack of intrinsic voltage-dependence and peculiar pharmacological profile are similar to properties described for the two-pore domain background K+ TASK channels. Neither Ca2+ nor ATP work as initiators in the activation of I(K,vol). In contrast, several investigations in Ehrlich cells suggest an important role for leukotriene D4 (LTD4) in the activation of I(K,vol). Under isotonic conditions, LTD4 activates Ca2+-dependent, ChTX-sensitive K+ channels as well as Ca2+-independent. ChTX-insensitive K+ channels. The LTD4-activated, ChTX-insensitive K+ current exhibits a current-voltage relation, pharmacological profile and single channel conductance similar to that of I(K,vol), indicating that LTD4 is the signalling molecule responsible for activation of the volume-sensitive K+ channels in Ehrlich cells. Hypotonic swelling of Ehrlich cells results in translocation of the 85-kDa cytosolic (c) PLA2alpha to the nucleus where it is activated. This activation leads to an increase in arachidonic acid release followed by an increased release of leukotrienes, and is essential in cell swelling-induced activation of I(K,vol) and of the organic osmolyte channels.     

Neuronal background two-P-domain potassium channels: molecular and functional aspects

Background K+ conductances are a major determinant of membrane resting potential and input resistance, two key components of neuronal excitability. Background channels have been cloned and form a K+ channel family structurally different from Kv, KCa and Kir channels. These channels with 2P domains (K2P channels) are voltage- and time-independent. They are relatively insensitive to classical potassium channels blockers such as TEA, 4-AP, Ba2+ and Cs+. TASK and TREK subunits are widely expressed in the nervous system. Open at rest, these channels mainly contribute to the resting potential of somatic motoneurons, brainstem respiratory and chemoreceptor neurones, and cerebellar granule cells. K2P channels are regulated by numerous physical and chemical stimuli including extracellular and intracellular pH, temperature, hypoxia, pressure, bioactive lipids, and neurotransmitters. The regulation of these background K+ channels profoundly alters the neuronal excitability. For example, in Aplysia, regulation of a background potassium conductance by neurotransmitters is involved in synaptic modulation, a simple and primitive form of learning. The recent discovery that clinical compounds such as volatile anaesthetics and other neuroprotective agents including riluzole and unsaturated fatty acids activate K2P channels suggest that neuronal background K+ channels are attractive targets for the development of new drugs.     

Fatty acid-activated K+ channels in autonomic neurons: activation by an endogenous source of free fatty acids

Application of arachidonic acid evoked robust activation of large-conductance K+ channels in cell-attached and excised inside-out patches from acutely isolated chick ciliary ganglion neurons. A similar effect was produced by 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analogue of arachidonic acid. The unitary conductance of fatty acid-activated channels was 35-40 pS at +20 mV with physiological gradients of K+ and 165 pS at +20 mV with an extracellular K+ concentration of 37.5 mM and an intracellular K+ concentration of 150 mM. Gating of these channels in cell-attached patches was potentiated by membrane stretch. Channel gating evoked by both lipids was concentration-dependent, with detectable activation apparent at 4 microM in the majority of patches and maximal activation occurring between 32 and 64 microM. Gating was relatively voltage-independent. Large-conductance K+ channels were also activated in inside-out patches by the monounsaturated fatty acid 11-cis-eicosenoic acid but not by the fully saturated fatty acid arachidic acid. Application of 100 microM H2O2, an agent that activates cytosolic phospholipase A2, also caused activation of large-conductance K+ channels in intact neurons. The stimulatory effects of H2O2 were blocked by pretreatment with 20 microM 4-bromophenacyl bromide, an irreversible inhibitor of phospholipase A2. Therefore, mobilization of endogenous fatty acids can cause activation of large-conductance K+ channels in autonomic neurons.     

Intracellular calcium and the control of neuronal pacemaker activity

Pacemaker activity of the Aplysia bursting pacemaker neuron R-15 was analyzed. It was shown that the free intracellular Ca2+ concentration, as measured by arsenazo III, increases during the depolarizing phase of the pacemaker cycle and declines throughout the hyperpolarizing phase that follows. This increase in Ca2+ results from the activation of voltage-dependent Ca2+ channels that open during the depolarizing phase of the cycle. The extracellular K+ concentration also increases during the depolarizing phase of the cycle and is correlated with an outward K+ current that opposes the inward current carried by Ca2+ ions. The increase in internal Ca2+ is sufficient to activate a K+ conductance that depends on the magnitude of the change in internal Ca2+ and on membrane potential, which is responsible for the hyperpolarizing phase of the cycle. It is proposed that the membrane oscillation depends on three separate but linked systems, which include a voltage-dependent Ca2+ channel, the internal Ca2+ concentration, and a Ca2+-activated K+ channel.     

Apical K+ channels in Necturus taste cells. Modulation by intracellular factors and taste stimuli

The apically restricted, voltage-dependent K+ conductance of Necturus taste receptor cells was studied using cell-attached, inside-out and outside-out configurations of the patch-clamp recording technique. Patches from the apical membrane typically contained many channels with unitary conductances ranging from 30 to 175 pS in symmetrical K+ solutions. Channel density was so high that unitary currents could be resolved only at negative voltages; at positive voltages patch recordings resembled whole-cell recordings. These multi-channel patches had a small but significant resting conductance that was strongly activated by depolarization. Patch current was highly K+ selective, with a PK/PNa ratio of 28. Patches containing single K+ channels were obtained by allowing the apical membrane to redistribute into the basolateral membrane with time. Two types of K+ channels were observed in isolation. Ca(2+)-dependent channels of large conductance (135-175 pS) were activated in cell-attached patches by strong depolarization, with a half-activation voltage of approximately -10 mV. An ATP-blocked K+ channel of 100 pS was activated in cell-attached patches by weak depolarization, with a half-activation voltage of approximately -47 mV. All apical K+ channels were blocked by the sour taste stimulus citric acid directly applied to outside-out and perfused cell-attached patches. The bitter stimulus quinine also blocked all channels when applied directly by altering channel gating to reduce the open probability. When quinine was applied extracellularly only to the membrane outside the patch pipette and also to inside-out patches, it produced a flickery block. Thus, sour and bitter taste stimuli appear to block the same apical K+ channels via different mechanisms to produce depolarizing receptor potentials.     

Time-dependent inhibition of the cyclooxygenase pathway by 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid

We examined effects of small dose (1 microM or less) of exogenous 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) on the formation of cyclooxygenase products from exogenous arachidonic acid (AA) in washed human platelets. With a simultaneous addition of AA, 12-HPETE did not affect the formation of thromboxane (TX)B2 and 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT). However, by being preincubated with platelets before an addition of AA, 0.1 microM or greater of 12-HPETE inhibited the formation of TXB2 and HHT dose-dependently. In addition, the inhibitory effect of 12-HPETE increased as the preincubation time was prolonged. These results suggest that 12-HPETE is a strong inhibitor for the cyclooxygenase pathway.     

G protein control of potassium channel activity in a mast cell line

Using the patch-clamp technique, we studied regulation of potassium channels by G protein activators in the histamine-secreting rat basophilic leukemia (RBL-2H3) cell line. These cells normally express inward rectifier K+ channels, with a macroscopic whole-cell conductance in normal Ringer ranging from 1 to 16 nS/cell. This conductance is stabilized by including ATP or GTP in the pipette solution. Intracellular dialysis with any of three different activators of G proteins (GTP gamma S, GppNHp, or AlF-4) completely inhibited the inward rectifier K+ conductance with a half-time for decline averaging approximately 300 s after "break-in" to achieve whole-cell recording. In addition, with a half-time averaging approximately 200 s, G protein activators induced the appearance of a novel time-independent outwardly rectifying K+ conductance, which reached a maximum of 1-14 nS. The induced K+ channels are distinct from inward rectifier channels, having a smaller single-channel conductance of approximately 8 pS in symmetrical 160 mM K+, and being more sensitive to block by quinidine, but less sensitive to block by Ba2+. The induced K+ channels were also highly permeable to Rb+ but not to Na+ or Cs+. The current was not activated by the second messengers Ca2+, inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, or by cyclic AMP-dependent phosphorylation. Pretreatment of cells with pertussis toxin (0.1 microgram/ml for 12-13 h) prevented this current's induction both by guanine nucleotides and aluminum fluoride, but had no effect on the decrease in inward rectifier conductance. Since GTP gamma S is known to stimulate secretion from patch-clamped rat peritoneal mast cells, it is conceivable that K+ channels become inserted into the plasma membrane from secretory granules. However, total membrane capacitance remained nearly constant during appearance of the K+ channels, suggesting that secretion induced by GTP gamma S was minimal. Furthermore, pertussis toxin had no effect on secretion triggered by antigen, and triggering of secretion before electrical recording failed to induce the outward K+ current. Finally, GTP gamma S activated the K+ channel in excised inside-out patches of membrane. We conclude that two different GTP-binding proteins differentially regulate two subsets of K+ channels, causing the inward rectifier to close and a novel K+ channel to open when activated.     

Mechanism of blocking K+-channels with tetraethylammonium

Using the patch-voltage-clamp method action of tetraethylammonium on the fast (30 pS) and slow K+ channels was investigated. The slow K+ channels were presented by two types: with whole (30 pS) and decreased (20 pS) conductance. In all cases tetraethylammonium decreased the current magnitude and modified the channel kinetic parameters. Apparent blocking constants determined from the current decreasing are as 8-50 and 4-12 mM for the slow K+ channels with whole and decreased conductance, respectively, and 0.05-0.08 mM--for the fast K+ channel. The potential dependency of the blocking constants correlates with that of the channel conductance. Probability of the channel open state for the slow K+ channels decreases, and that for the fast K+ channel increases under application of tetraethylammonium. It is concluded that there are two sites of tetraethylammonium binding: the first site is into the channel pore, and the second one--into the regulatory centre responsible for the channel kinetic behaviour. Blocking of general conductance of the slow channels is accompanied by proportional decrease of the channel substate conductances without change of their number and cooperatively. Block of the fast K+ channel occurs without change of the channel elementary conductance but with decrease of the number of the channel substates and reversible violation of the channel transition cooperativity. The data are discussed from the point of the hypothesis on the channel clustery organization.     

Comparative analysis defines a broader FMRFamide-gated sodium channel family and determinants of neuropeptide sensitivity

FMRFamide (Phe-Met-Arg-Phe-amide, FMRFa) and similar neuropeptides are important physiological modulators in most invertebrates, but the molecular basis of FMRFa activity at its receptors is unknown. We therefore sought to identify the molecular determinants of FMRFa potency against one of its native targets, the excitatory FMRFa-gated sodium channel (FaNaC) from gastropod mollusks. Using molecular phylogenetics and electrophysiological measurement of neuropeptide activity, we identified a broad FaNaC family that includes mollusk and annelid channels gated by FMRFa, FVRIamides, and/or Wamides (or myoinhibitory peptides). A comparative analysis of this broader FaNaC family and other channels from the overarching degenerin (DEG)/epithelial sodium channel (ENaC) superfamily, incorporating mutagenesis and experimental dissection of channel function, identified a pocket of amino acid residues that determines activation of FaNaCs by neuropeptides. Although this pocket has diverged in distantly related DEG/ENaC channels that are activated by other ligands but enhanced by FMRFa, such as mammalian acid-sensing ion channels, we show that it nonetheless contains residues that determine enhancement of those channels by similar peptides. This study thus identifies amino acid residues that determine FMRFa neuropeptide activity at FaNaC receptor channels and illuminates the evolution of ligand recognition in one branch of the DEG/ENaC superfamily of ion channels.     

Voltage-sensitive potassium channels in Limulus ventral photoreceptors

The steady-state slope conductance of Limulus ventral photoreceptors increases markedly when the membrane is depolarized from rest. The ionic basis of this rectification has been examined with a voltage-clamp technique. Tail currents that occur when membrane potential is repolarized after having been depolarized have been identified. The tail currents reverse direction at a voltage that becomes more positive when Ko is increased. Rectification is reduced by extracellular 4-aminopyridine and by intracellular injection of tetra-ethyl-ammonium (TEA). These results indicate that the membrane rectification around resting potential is due primarily to voltage-sensitive K+ channels. The increase in gK caused by depolarization is not mediated by a voltage-dependent rise in in Cai++, since intracellular injection of Ca++ causes a decrease rather than an increase in slope conductance. TEA can be used to examine the functional role of the K+ channels because it blocks them without substantially affecting the light-activated Na+ conductance. The effect of TEA on response-intensity curves shows that the K+ channels serve to compress the voltage range of receptor potentials.     

Requirement of free arachidonic acid for leukotriene B4 biosynthesis by 12-hydroperoxyeicosatetraenoic acid-stimulated neutrophils

Stimulation of human neutrophils with 12-hydroperoxyeicosatetraenoic acid (12-HPETE) led to formation of 5S, 12S-dihydroxyeicosatetraenoic acid (DiHETE), but leukotriene B4 (LTB4) or 5-hydroxyeicosatetraenoic acid (5-HETE) was not detectable by reversed-phase high-performance liquid chromatography analysis. N-formylmethionylleucylphenylalanine (FMLP) induced the additional synthesis of small amounts of LTB4 in 12-HPETE-stimulated neutrophils. The addition of arachidonic acid greatly increased the synthesis of LTB4 and 5-HETE by neutrophils incubated with 12-HPETE. In experiments using [1-14C]arachidonate-labeled neutrophils, little radioactivity was released by 12-HPETE alone or by 12-HPETE plus FMLP, while several radiolabeled compounds, including LTB4 and 5-HETE, were released by A23187. These findings demonstrate that LTB4 biosynthesis by 12-HPETE-stimulated neutrophils requires free arachidonic acid which may be endogenous or exogenous.     

Stretch activation of the Aplysia S-channel

The S-channel, a receptor-mediated K+ channel of Aplysia sensory neurons which functions in neuromodulation, bears a strong resemblance to the ubiquitous stretch-activated channels of snail neurons. Snail neuron stretch channels are stretch sensitive only in the patch, not at the macroscopic level, a situation which leaves open the question of their physiological role. If S-channels resemble snail stretch channels because both belong to the same general class of channels, the S-channel, too, should display stretch sensitivity in the patch. We show, using single-channel recording, that the S-channel can be activated by stretch. Furthermore, we show that Aplysia neurons in general have stretch-activated K+ channels. We suggest that the stretch-sensitive K+ channels of molluscan neurons and other preparations (e.g., Drosophila muscle, snail heart) are S-like channels, i.e., receptor-mediated channels which adventitiously exhibit mechanosensitivity in the patch.     

Intracellular injection of guanyl nucleotides alters the serotonin- induced increase in potassium conductance in Aplysia neuron R15

The effects of the adenylate cyclase inhibitor GDP beta S on the response of Aplysia neuron R15 to serotonin (5HT) were investigated. Previous studies have demonstrated that 5HT causes an increase in K+ conductance in R15 and that the response is mediated by cAMP. At concentrations in the micromolar range, GDP beta S inhibits the stimulation of adenylate cyclase by 5HT in particulate fractions from Aplysia ganglia. When micromolar concentrations of GDP beta S are injected into neuron R15, there is no effect on the resting membrane conductance, but the increase in K+ conductance normally elicited by 5HT is completely inhibited. Furthermore, the decrease in inward current normally elicited by dopamine (DA), which does not appear to involve cAMP, is not affected by micromolar concentrations of GDP beta S. In addition, application of 8-benzylthio cAMP to R15 can evoke an increase in K+ conductance even after the injection of GDP beta S, which indicates that events subsequent to the activation of adenylate cyclase are not inhibited by the GDP analogue. In contrast, when millimolar concentrations of GDP beta S are injected into R15, direct effects on membrane conductance are observed and the response of R15 to 5HT is enhanced. Although these effects of high concentrations of GDP beta S are only poorly understood, the results with micromolar concentrations are consistent with the hypothesis that stimulation of adenylate cyclase is necessary for the 5HT-induced increase in K+ conductance in neuron R15.     

Resolution by DEAE-cellulose chromatography of the enzymatic steps in the transformation of arachidonic acid into 8, 11, 12- and 10, 11, 12-trihydroxy-eicosatrienoic acid by the rat lung

The 30-50% ammonium sulfate fraction of the high speed supernatant (100,000 xg) of a rat lung homogenate is capable of catalysing the conversion of arachidonic acid into 8,11,12- and 10,11, 12-trihydroxyeicosatrienoic acids. This enzyme preparation was resolved through DEAE cellulose chromatography into three stages which were assayed with precursors specific for each stage. Thus in the first stage arachidonic acid is converted by 12-lipoxygenase into 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) detected as the corresponding 12-hydroxy product (12-HETE). 12-HPETE in turn is converted into 8-hydroxy-11,12-epoxy-5,9,14-eicosatrienoic acid and 10-hydroxy-11,12-epoxy-5,8,14-eicosatrienoic acid. These epoxides are in turn selectively converted through an epoxide hydrase into the respective triols. While the first and third stages were carried out by distinct fractions from the DEAE columns, the second i.e. conversion of 12-HPETE into epoxides, was detected in all fractions as was the reduction of 12-HPETE into 12-HETE.