Arachidonic acid metabolic pathway of the rabbit placenta

Prostaglandin production by guinea-pig placenta and other uterine tissues during mid-pregnancy

Prostaglandin (PG) output from cultured placenta, sub-placenta, endometrium and fetal membranes of guinea-pigs was measured on days 22, 29 and 36 of pregnancy to establish the source of increased PGF2alpha production during mid-pregnancy. PGF2alpha and 6-keto-PGF1alpha were produced in larger quantities than PGE2 by the placenta, sub-placenta and endometrium; 6-keto-PGF1alpha was in the major prostaglandin produced by the fetal membranes. The initial outputs of PGF2alpha, PGE2 and 6-keto-PGF1alpha from the sub-placenta, fetal membranes and endometrium either decreased or remained fairly constant between days 22 and 36. In contrast, the initial outputs of PGF2alpha, PGE2 and 6-keto-PGF1alpha from the placenta increased 14.7-, 2.5- and 2.0-fold, respectively, between days 22 and 36, indicating that the placenta is the tissue responsible for the increase in PGF2alpha output from the mid-pregnant guinea-pig uterus. Aristolochic acid (a phospholipase A2 inhibitor) inhibited prostaglandin output from the endometrium, but had a more variable effect in prostaglandin output from the other tissues. Thimerosal (an arachidonic acid uptake inhibitor) inhibited PGF2alpha and PGE2 outputs from the endometrium, but generally potentiated 6-keto-PGF1alpha output and prostaglandin output from the other tissues. Arachidonic acid release for prostaglandin synthesis in the endometrium, but not the placenta, sub-placental or fetal membranes, is apparently dependent upon a constant level of phospholipase A2 activity.

Co-oxidation of all-trans retinol acetate by human term placental lipoxygenase and soybean lipoxygenase

The oxidation of all-trans retinol acetate (t-RAc) mediated by soybean lipoxygenase (SLO) and affinity purified human term placental lipoxygenase (HTPLO) was investigated. Under optimum assay conditions, SLO, which is used as a model enzyme, exhibited a specific activity of 850 nmol of t-RAc depleted/min/nmol of enzyme. For HTPLO, incubation of 100 microM t-RAc, 2 mM linoleic acid, and 50 micrograms of enzyme protein in Tris buffer at pH 9.0 was essential to observe the optimum rate of t-RAc cooxidation (370 nmol of t-RAc depleted/min/mg protein). The linoleate dependent cooxidation of t-RAc by both SLO and HTPLO was significantly inhibited by micromolar concentrations of nordihydroguaiaretic acid, butylated hydroxytoluene, butylated hydroxyanisole, gossypol, and 5,8,11-eicosatriynoic acid. These results suggest that t-RAc is metabolized via the lipoxygenase pathway. Significant rate of t-RAc oxidation by HTPLO observed in this study suggests that it may constitute an important part of the puzzle worthy of consideration in the understanding of retinoid teratogenicity in humans.

Oxidative metabolism of aflatoxin B1 by lipoxygenase purified from human term placenta and intrauterine conceptal tissues

Aflatoxin B1 (AFB1) is a teratogen in rodents and may be a human transplacental carcinogen. Although the presence of DNA adducts of AFB1 in the human placentas has been noted, the enzyme(s) responsible for the bioactivation was not identified. In this investigation, the linoleic acid (LA)-dependent cooxidation of AFB1 catalyzed by lipoxygenase (LO) purified by Con A affinity chromatography from the term placentas of nonsmokers was studied. HPLC chromatograms detected the presence of 5- and 15-HETE as the major metabolites and 12-HETE as a minor metabolite upon incubation of arachidonic acid (AA) with affinity purified human term placental LO. These results suggest that a mixture of LO isozymes is present in the affinity-purified enzyme preparations of term placentas. The optimal assay conditions to observe maximum rate of epoxidation included incubation of 250 microM AFB1 with 80 micrograms LO and 3.5 mM LA at pH 7.2. AFB1-8,9-tris-diol produced in the reaction was estimated spectrofluorimetrically. A Vmax of 432 +/- 26 pmol of AFB1-8,9-tris diol produced/min/mg protein and a Km of 77 microM for AFB1 were observed. The AFB1-8,9-tris-diol formation was dependent on the incubation time, concentration of enzyme protein, AFB1, and LA. LO catalyzed epoxidation of AFB1 was inhibited by NDGA, BHT, BHA, ETI, and gossypol. The evidence presented here clearly demonstrates that placental LO is capable of epoxidation of AFB1. Similar results were observed with LO preparations of human intrauterine conceptal tissues at 8-10 weeks of gestation.

Regiospecific and enantioselective metabolism of 8,9-epoxyeicosatrienoic acid by cyclooxygenase

8(S),9(R)-epoxyeicosatrienoic acid, a major product of the renal cortex, was found to be a substrate for cyclooxygenase from human platelets and ram seminal vesicles. 11(R)-hydroxy-8(S),9(R)-epoxyeicosatrienoic acid was the sole metabolic product. The 8(R),9(S)-enantiomer formed both C-11 and C-15 hydroxylated metabolites. These novel findings suggest that the cyclooxygenase-dependent renal vasoconstrictor activity of 8(S),9(R)-epoxyeicosatrienoic acid may be due to the 11(R)-hydroxy metabolite.

14C-labeled arachidonic acid bioconversion in guinea pig placenta during the last third of gestation

In the present study we investigated the arachidonic acid metabolism in guinea pig placenta during the last third of gestation. Homogenates were incubated with 14C-labeled substrate, and eicosanoid formation was determined using rp HPLC. Arachidonic acid was substantially converted to cyclooxygenase products i.e 6-keto-PGF1 alpha, TxB2, PGF2 alpha, PGE2, PGD2 and 12-HHT. Lipoxygenase activity was also found but of a much lower degree and represented by the mono-hydroxy acids 12-HETE and 15-HETE. The total conversion of arachidonic acid exhibited a progressive rise from day 50 to term, due principally to the increasing part of TxB2, PGE2 and 12-HHT throughout this gestational period and in addition, near term, of 6-keto-PGF1 alpha and PGF2 alpha. These results suggest that there is an increasing concentration and/or activity of cyclooxygenase system enzymes with placental development in guinea pig, which may contribute to the augmented intrauterine availability of prostanoids near parturition. Additional experiments were performed to compare the metabolism of exogenously added 14C-arachidonic acid and endogenously present 12C-arachidonic acid during placental homogenate incubation by means of isotope dilution GC-MS. Although the 14C- and 12C-prostanoid patterns were comparable, the 14C/12C ratios of the prostanoids formed during incubation were significantly different. These data indicate that exogenous arachidonic acid and endogenous arachidonic acid in placental homogenate do not follow up exactly the same metabolic pathway so that the assumption of biochemical identity between exogenous radio-tracer and studied endogenous substrate is not quite true.

Arachidonic acid oxidation by brain and placenta preparations from normal and placental insufficient fetal rabbit

Cytosolic (100,000 g) fractions of fetal rabbit brain and placenta tissue convert [1-14C]arachidonic acid into several oxidation products identified with the lipoxygenase [12-hydroxyeicosatetraenoic acid (12-HETE) and 15-HETE] and cyclooxygenase [prostaglandin E2 (PGE2)] pathways. Formation of 12-HETE and 15-HETE by fetal brain is time-dependent, reaching a plateau after 40 min and is linear with protein concentration. An apparent affinity constant of 0.06 mM and a Vmax of 0.1 mumol/h/g wet weight are presumably responsible for the excessive accumulation of 12-HETE and 15-HETE in comparison to PGE2 (Km = 0.5 mM). The latter is synthesized by the placenta particulate fraction but almost exclusively by the brain cytosol. Compared to brain, the activity of the placenta tissue is exceedingly higher and in addition to 12-HETE and 15-HETE there is a substantial formation of 12-L-hydroxyheptadecatrienic acid. Formation of 12-HETE and 15-HETE at 21 days is as effective as at 31 days gestation and is strongly inhibited by nordihydroguaiaretic acid (93%), BW755c (99%), and AA861 (84%) but not by indomethacin. Placenta and brain tissues of intrauterine growth retarded fetuses after ligation of placental blood vessels fail to convert arachidonic acid into other eicosanoids. Loss of enzymatic activity also observed in normal tissue after prolonged storage cannot be restored by the addition of several SH agents, ascorbate, or ferric iron.