Arachidonic acid metabolism in starfish oocytes

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.

The effect of prostaglandin E2 and prostaglandin F2 alpha on ovarian tissue in the Florida crayfish Procambarus paeninsulanus

Prostaglandins are oxygenated fatty acid derivatives of arachidonic acid involved in a number of vertebrate and invertebrate reproductive processes. While the role of prostaglandins in vertebrate reproduction has been well established, their function in the invertebrate has not been investigated extensively. The purpose of this study was to investigate the effect prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) on ovarian tissue in the crayfish Procambarus paeninsulanus. PGF2 alpha induced contraction of ovarian tissue in a dose-dependent manner, while PGE2 alpha had little effect. Incubation of ovarian tissue with PGF2 alpha also produced a dose-dependent increase in cAMP. In addition, the experimental technique of back-phosphorylation, in which exogenously added cAMP-dependent protein kinase is able to transfer phosphate to previously non-phosphorylated proteins, revealed that PGF2 alpha-induced increases in cAMP resulted in the specific phosphorylation of a 45 kDa protein. These data give evidence that PGF2 alpha may be involved in crustacean ovulation by causing the cAMP-mediated contraction of ovarian tissue and that this contraction may involve the phosphorylation of proteins associated with the cytoskeleton.

Prostaglandin biosynthesis by fat body from the tobacco hornworm, Manduca sexta

We describe prostaglandin (PG) biosynthesis by microsomal-enriched preparations of fat body from larvae of the tobacco hornworm Manduca sexta. Four major PGs were synthesized under most experimental conditions, PGA2, PGE2, PGD2 and PGF2 alpha. PGA2, was the predominant product under most conditions. Unlike mammals, in which PGA2, is generally thought to arise from non-enzymatic rearrangements of PGE2, the fat body preparations did not convert exogenous PGE2 into PGA2. These findings suggest that PGA2 is an important fat body product that is synthesized by a route that does not involve PGE2. The PG synthase activity and the overall profile of PG synthesis were sensitive to experimental conditions, including incubation time, temperature, and protein concentration. Optimal PG biosynthesis was observed with 1 mg of microsomal-rich protein, incubated at 30 degrees C for 1-2 min. The fat body preparations is sensitive to two non-steroidal anti-inflammatory drugs, indomethacin and naproxen, both of which inhibited PG synthesis at low dosages.

Lipoxygenase metabolism of polyunsaturated fatty acids in oocytes of the frog Xenopus laevis

Recently, oocytes or eggs of two marine invertebrates have been found to metabolize arachidonic acid to specific monohydroxy products. These studies have prompted our examination of the oocytes of higher organisms. In the present study, oocytes of an amphibian, Xenopus laevis, were examined for their capacity to biosynthesize hydroxyeicosatetraenoic acids (HETEs) and related hydroxy fatty acids. Two hydroxyeicosanoids were formed during incubations of oocyte homogenates with [14C]arachidonic acid; their structures and stereochemistry were determined by high-pressure liquid chromatography, uv spectroscopy, and gas chromatography-mass spectrometry. The compounds were identified as 15(S)- and 12(S)-hydroxyeicosatetraenoic acids. The synthesis of the two HETEs was not blocked by a cyclooxygenase inhibitor, indomethacin (10 microM), or by prior exposure of the oocyte homogenates to carbon monoxide, an inhibitor of cytochrome P450. Furthermore, 12(S)- and 15(S)-hydroperoxyeicosatetraenoic acids were isolated from brief incubations of gel-filtered ammonium sulfate fraction of frog oocyte homogenates; isolation of the hydroperoxide is further support for the existence of 12(S)- and 15(S)-lipoxygenase activities in the oocytes of X. laevis. Other polyunsaturated acids, including C18.2, C18.3, C20.3, C20.5, and C22.6 were also substrates for the lipoxygenase, and in each case the major product was formed by omega 6 oxygenation.

Biosynthesis, catabolism, and biological properties of HPETEs, hydroperoxide derivatives of arachidonic acid

The oxygenation of arachidonic acid by lipoxygenases results in the formation of HPETEs (hydroperoxyeicosatetraenoic acids), the first products of the LOX pathway. These compounds are short lived and are catabolised into various families of more stable compounds of which the HETEs, hepoxilins, lipoxins and leukotrienes have been identified so far. The development of new techniques have helped to identify and understand the structures of various HPETEs and only recently the biological effects of HPETEs and their various catabolites are being unraveled. Although lipoxygenases are ubiquitous, not all tissues possess the same spectrum of lipoxygenase enzymes. Hence different HPETEs can be formed in different tissues. Recent studies have revealed that HPETEs or products derived from them possess a diversity of important biological properties including the regulation of electrolyte flux and eicosanoid and corticosterone syntheses, release of histamine, regulation of oocyte maturation and release of various reproductive hormones. HPETEs appear to be involved in some pathological conditions viz, skin psoriasis, Clarkson's disease, nerve injury and spinal cord ischemia. These novel eicosanoids are associated with the release of insulin as well as renin. Recently HPETEs have been suggested to act as second messengers in the Aplysia sensory neurons and its catabolite, hepoxilin, has been demonstrated to have effects on mammalian hippocampal neurons. The purpose of this review is to provide a brief summary of the formation of the HPETEs and the various families of compounds derived from them as well as the various types of biological activities for these products described so far.

Arachidonic acid metabolism in Ascidia ceratodes eggs: role of lipid peroxidation

1. Addition of arachidonic acid (AA) to Ascidia ceratodes oocyte homogenates results in its rapid oxidation to several polar products. 2. AA oxidation in homogenates has both calcium independent and calcium stimulated components. 3. Calcium or AA addition to an oocyte homogenate stimulates O2-consumption. 4. Stimulation of homogenate O2-consumption by AA and calcium is additive. 5. Intact eggs oxidize AA to products similar to those detected in vitro. 6. Quantitatively total AA oxidation was similar for unfertilized and fertilizing eggs and dividing embryos, while qualitative differences were detected for the three stages. 7. These results demonstrate the presence of lipoxygenase-like, peroxidizing activity, in Ascidia eggs that is capable of producing products potentially important to the control of early metabolic events during development.

Starfish oocyte maturation: 1-methyladenine triggers a drop of cAMP concentration related to the hormone-dependent period

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Oocytes of Evasterias troschelii contain 0.43 pmole cyclic AMP/mg protein and 0.47 pmole cyclic GMP/mg protein. Upon stimulation by 1-MeAde the oocytes undergo a moderate (10-30%) decrease in their cAMP concentration. The concentration of cGMP remains unaltered. Oocytes treated with forskolin, an activator of adenylate cyclase, increase their cAMP concentration over 35-fold, up to 16 pmole cAMP/mg protein. When stimulated by 1-MeAde these forskolin-pretreated oocytes undergo a major (50-70%) decrease in their cAMP concentration. A similar decrease is triggered by mimetics of 1-MeAde, such as dithiothreitol, arachidonic acid (AA), and 8-hydroxyeicosatetraenoic acid (8-HETE), but not by adenine which is inactive. 1-MeAde-stimulated oocytes of Pisaster ochraceus also undergo a decrease in cAMP content, the size of which is increased by forskolin. Although a decrease in cAMP begins at sub-threshold 1-MeAde concentrations, the maximal decrease occurs at the same concentration of 1-MeAde needed for maturation induction and a further 1000-fold increase of the 1-MeAde concentration has no further effect. Upon removal of 1-MeAde, the cAMP concentration immediately increases to its original level. Sequential addition and removal of 1-MeAde triggers a sequential decrease and increase of the cAMP concentration, illustrating the continuous requirement for 1-MeAde for eliciting the decrease. Successive additions of 1-MeAde, however, do not trigger further decreases of the cAMP concentration. The temperature dependences of the cAMP concentration decrease and of the hormone-dependent period (HDP; the time of contact with 1-MeAde required for induction of maturation) are closely related. Forskolin, which increases the cAMP concentration, also increases the duration of the HDP (2.5-fold), delays the time course of protein phosphorylation burst and germinal vesicle breakdown, and inhibits AA- and 8-HETE-induced maturation. We conclude that 1-MeAde triggers a drop in cAMP concentration, which is tightly associated with the hormone-dependent period of oocyte maturation.

Contrasting effects of fatty acids on oocyte maturation in several starfish species

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine. In some species (group 2) oocyte maturation can be induced by micromolar concentrations of a few fatty acids such as arachidonic and eicosapentaenoic acids or by nanomolar concentrations of hydroxyeicosatetraenoic acid. Complete maturation is triggered: increased protein phosphorylation, appearance of the cytoplasmic "maturation-promoting factor", germinal vesicle breakdown, emission of the two polar bodies and formation of the female pronucleus. In other species (group 1), however, no maturation can be induced by the fatty acids active in the species of group 2, despite a large variety of experimental conditions.