Metabolism of PGI2 by 15-hydroxyprostaglandin-dehydrogenase and Δ13-reductase in different vascular beds of the cat in vivo

The metabolism of endogenous PGI₂ (released by angiotensin II or bradykinin) and exogenous PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was studied in five different vascular beds of the anaesthetized cat. Plasma concentrations of 6-keto-PGF_1α (the product of spontaneous hydrolysis of PGI₂) and 6,15-diketo-13,14-dihydro-PGF_1α (the metabolite formed from PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase) were determined in the efferent vessels of the respective vascular beds by specific radioimmunoassays.No major metabolism of PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was detected in the head and the hindlimbs of the cat. In the lung exogenous (circulating) PGI₂ was not metabolized, whereas PGI₂ synthetized in the lung itself was converted to 6,15-diketo-13,14-dihydor-PGF_1α. No significant amounts of 6,15-diketo-13,14-dihydro-PGF_1α-immunoreactivity were detected in hepatic venous blood after infusion of PGI₂ into the portal vein. However as also no 6-keto-PGF_1α was found, the liver seems to efficiently extract PGI₂ from the circulation. The cat kidney had the highest capacity of all vascular beds investigated to release endogenous and exogenous PGI₂ as 6-15-diketo-13,14-dihydro-PGF_1α. In other organs (vascular beds) investigated PGI₂ is either metabolized less efficiently by the 15-hydroxy-PG-dehydrogenase or further transformed to other metabolites.     

Thromboxane synthase inhibition: Implications for prostagland endoperoxide metabolism II testing the ‘redirection hypothesis’ in an acute intravenous challenge model

In the preceding paper we described the characterisation of an acute intravenous challenge model for the evaluation of the effects of thromboxane synthase inhibition (TXSI) on eicosanoid metabolism (1). Herein we describe the biochemical pharmacology of two TXSI and aspirin in this model. Both TXSI caused significant inhibition of plasma TXB₂ without elevation of 6-oxo-PGF_1α levels. Similar results were obtained when combined levels of 6-oxo-PGF_1α,13,14 dihydro 6-oxo-PGF_1α, 13,14 dihydro 6,15-dioxo-PGF_1α and 6-oxo-PGE₁ were measured as an index of PGI₂ biosynthesis (PGI₂m). Thus no evidence of redirection of PGH₂ to PGI₂ was found. experiments performed in serum gave an apparent stimulation of immunoreactive 6-oxo-PGF_1α following TXSI but RPHPLC analysis of extracted serum showed that this stimulation was accounted for by increase in a product co-eluting with [³H]PGF_2α. The implications of these findings in relation to TXSI and receptor antagonists are discussed.     

Metabolism of PGI2 by 15-hydroxyprostaglandin-dehydrogenase and Δ-reductase in different vascular beds of the cat in vivo

The metabolism of endogenous PGI₂ (released by angiotensin II or bradykinin) and exogenous PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was studied in five different vascular beds of the anaesthetized cat. Plasma concentrations of 6-keto-PGF_1α (the product of spontaneous hydrolysis of PGI₂) and 6,15-diketo-13,14-dihydro-PGF_1α (the metabolite formed from PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase) were determined in the efferent vessels of the respective vascular beds by specific radioimmunoassays.No major metabolism of PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was detected in the head and the hindlimbs of the cat. In the lung exogenous (circulating) PGI₂ was not metabolized, whereas PGI₂ synthetized in the lung itself was converted to 6,15-diketo-13,14-dihydor-PGF_1α. No significant amounts of 6,15-diketo-13,14-dihydro-PGF_1α-immunoreactivity were detected in hepatic venous blood after infusion of PGI₂ into the portal vein. However as also no 6-keto-PGF_1α was found, the liver seems to efficiently extract PGI₂ from the circulation. The cat kidney had the highest capacity of all vascular beds investigated to release endogenous and exogenous PGI₂ as 6-15-diketo-13,14-dihydro-PGF_1α. In other organs (vascular beds) investigated PGI₂ is either metabolized less efficiently by the 15-hydroxy-PG-dehydrogenase or further transformed to other metabolites.     

Concerning the biosynthesis of prostaglandin I2

Two diastereoisomers, 5R,6R-5-hydroxy-6(9α)-oxido-11α,15S-dihydroxyprost-13-enoic acid (7) and 5S,6S-5-hydroxy-6(9α)-oxido-11α,15S-dihydroxyprost-13-enoic acid (10) were synthesized for evaluation as possible biosynthetic intermediates in the enzymatic transformation of PGH₂ or PGG₂ into PGI₂. The synthetic sequence entails the stereospecific reduction of the 9-keto function in PGE₂ methyl ester after protecting the C-11 and C-15 hydroxyls as tbutyldimethylsilyl ethers. The resulting PGF_2α derivative was epoxidized exclusively at the C-5 (6) double bond to yield a mixture of epoxides, which underwent facile rearrangement with SiO₂ to yield the 5S,6S and 5R,6R-5-hydroxy-6(9α)-oxido cyclic ethers. It was found that dog aortic microsomes were unable to transform radioactive 9β-5S,6S[³H] or 9β-5R,6R[³H]-5-hydroxy-6(9α)-oxido cyclic ethers into PGI₂. Also, when either diastereoisomer was included in the incubation mixture, neither isomer diluted the conversion of [1-¹⁴C]arachidonic acid into [1-¹⁴C]PGI₂.     

Lung surfactant synthesis: a Ca++-dependent microsomal phospholipase A2 in the lung.

We present the first direct evidence for a highly active, Ca⁺⁺-dependent phospholipase A₂ in the microsomal fraction of rat lung homogenate. Several previously reported studies from other laboratories strongly implicate this enzyme as a key metabolic step in the biosynthesis of dipalmitoyl lecithin, the primary component of pulmonary surfactant. In the present study, stoichiometric amounts of [³H]lysophosphatidylethanolamine and [¹⁴C]fatty acid were released during incubation of 1-[9, 10-³H]palmitoyl-2-sn-[1′-¹⁴C]linoleoyl phosphatidylethanolamine with the lung microsomal fraction. Marker enzyme measurements showed that the microsomal activity cannot be due to contamination with mitochondria, which also show phospholipase A₂ in both lung and liver. In contrast, liver microsomes show predominantly a phospholipase A₁ activity.     

Hepatic metabolism of prostacyclin (PGI2) in the rabbit: Formation of a potent novel inhibitor of platelet aggregation

Metabolism of [9-³H]-PGI₂ was studied in the isolated Tyrode's perfused rabbit liver. Five products, four radioactive and one non-radioactive, were identified in the perfusate: 19-hydroxy-6-keto-PGF_1α, 6-keto-PGF_1α, dinor-6-keto-PGF_1α, pentanor PGF_1α and a 6-keto-PGE₁-like substance. The first two, 19-hydroxy-6-keto-PGF_1α and 6-keto-PGF_1α, represented 5% and 45% respectively, of the total radioactivity; the last two accounted for 39%. The presence of dinor and pentanor derivatives of 6-keto-PGF_1α indicated that β -oxidation and oxidative-decarboxylation occurs in the liver as the major metabolic pathway of PGI₂. One non-radioactive metabolite which co-migrated with authentic 6-keto-PGE₁ was found to inhibit platelet aggregation, having a potency similar to authentic 6-keto-PGE₁, and its effect can be eliminated by boiling and by alkali treatment. This metabolite, having similar Rf value on TLC and biological behavior as 6-keto-PGE₁, may arise from oxidation of 6-keto-PGF_1α via the 9-hydroxyprostaglandin dehydrogenase pathway, as suggested by recovery of tritiated water in the aqueous phase of the perfusate. This material, a potent inhibitor of platelet aggregation, may arise from PGI₂ or its hydrolysis product, 6-keto-PGF_1α.     

Prostaglandin F2α antagonizes thromboxane A2-induced human platelet aggregation

The effects of prostaglandin F_2α on human blood platelet function were investigated. PGF_2α at 15 μM completely blocked platelet aggregation induced by 500 μM arachidonic acid or 3 μM U46619 but had no effect on aggregatin induced by 7.5 μM ADP. A similar specificity of action was not obtained with either PGI₂ or PGE₂. Thus concentrations of PGI₂ (3 nM) or PGE₂ (20 μ M) which inhibited U46619-induced aggregation by 100% also blocked ADP-stimulated aggregation.The inhibitory properties of PGF_2α were not related to increases in platelet cAMP, since direct measurement of intracellular cAMP revealed that 15 μ M PGF_2α produced no substantial change in cAMP levels. This finding was in direct contrast to results obtained using either PGI₂ or PGE₂. Both PGI₂ (3 nM) and PGE₂ (20 μ M) induced significant increases in platelet cAMP levels.The possibility that PGF_2α directly interacts at the platelet TXA₂/PGH₂ receptor was investigated by measuring [³H]PGF_2α binding to isolated platelet membranes. It was found that [³H] PGF_2α binding reached equilibrium within 30 min at room temperature and could be 90% displaced by addition of 1000 fold excess of unlabelled PGF_2α. Furthermore, when 1000 fold excess of either the TXA₂/PGH₂ “mimetic” U46619 or the TXA₂/PGH₂ antagonist 13-azaprostanoic acid was added, specific [³H] PGF_2α binding was displaced by 95% and 85% respectively. In contrast, the same molar excess of 6-keto-PGF_1α, azo analog 1, or TXB₂, caused displacement of only 15%, 20% or 25% of the [³H] PGF_2α binding. Scatchard analysis indicated that [³H] PGF_2α has two binding sites; i.e., a high affinity binding site with an apparent Kd of 50 nM and a low affinity binding site with apparent Kd of 320 nM. These results suggest that the selective inhibition by PGF_2α of AA or U46619-induced aggregation may be mediated through interaction at the platelet TXA₂/PGH₂ receptor.     

A simple and sensitive assay for 9-hydroxyprostaglandin dehydrogenase

The tritium recovery assay of 9-hydroxyprostaglandin dehydrogenase [Pace-Asciak, C. (1975) J. Biol. Chem.250, 2789] has been modified to ensure its applicability to both crude and purified enzyme preparations. The stereospecificity of NAD⁺-dependent 9-hydroxyprostaglandin dehydrogenase with respect to NAD⁺ was determined first and found to be A-side specific. Based on the stereospecificity of the enzyme, a simple and sensitive assay method for 9-hydroxyprostaglandin dehydrogenase has been developed. The assay is able to detect picomole quantities of substrate conversion. When 15-keto-13,14-dihydro-[9β-³H]PGF_2α is employed as substrate, the tritium label of the tritiated prostaglandin is effected to transfer to lactate stereospecifically by coupling 9-hydroxyprostaglandin dehydrogenase with a saturating level of lactate dehydrogenase. The amount of prostaglandin oxidized is quantitated by the radioactivity of the labeled lactate produced, which is separated from labeled prostaglandin by charcoal precipitation. Simultaneous assays with the current tritium-release and thin-layer chromatography methods indicated excellent correlation. Using this method we have found that rat kidney possesses the highest enzyme activity among those tissues examined. Rat kidney enzyme activity is linear for the first 10 min it is studied and is nonlinear with increasing amounts of crude enzyme extract, indicating the possible presence of endogenous inhibitor(s). The apparent K_m for 15-keto-13,14-dihydro-PGF_2α is 0.66 μm. The enzyme is activated by imipramine, inhibited by indomethacin, but not affected by furosemide and ethacrynic acid. These results confirm previous findings reported in the literature.     

Effect of 7-fluoro prostacyclin,a stable prostacyclin analogue,on cAMP accumulation and prostaglandin binding in mastocytoma P-815 cells

The effect of 7-fluoro proscyclilin (PGI₂-F), a chemically stable analogue of prostacyclin, on cAMP accumulation in and [³H]PGE binding to mastocytoma P-815 cells was compared with those of the Na salt and methyl ester of prostacyclin (PGI₂Na or PGI₂Me), which are rapidly inactivated in aqueous solution or metabolized in the tissue.PGIF was as effective as PGI₂Me, and slightly less effective than PGI₂Na in stimulating cAMP accumulation in mastocytoma cells and rabbit platelets. PGI₂F was also more stable than PGI₂Me or PGI₂Na, and retained its original cAMP elevating activity even after incubation with or without cells for 4 h at 37°C. Cells which had been exposed to PGI₂F and then washed free of unbound reagent continued to produced cAMP for more than 3 h. PGI₂F was also as effective as PGE₁ or PGE₂ in displacing [³H]PGE₂ bound to the cells. Non-competitive inhibition by PGI₂F or PGI₂Me of [³H]PGE₂ binding to the cells, with apparent K_is of 1.29 μM and 1.13 μM, respectively, indicates the presence of different receptors for PGE₂ and for PGI₂F or PGI₂Me in mastocytoma P-815 cells.     

Intracellular distributian and substrate specificity of the 16 alpha-hydroxylase in the adrenal of human fetus

When [7α-³H] dehydroepiandrosterone was incubated with the adrenal homogenates of human fetus at 22 to 26 weeks gestational age, 16α-hydroxydehydroepiandrosterone and/or its sulfate was formed as the only detectable metabolite. The 16α-hydroxylase activity was concentrated in the microsomal fraction of the adrenal homogenate.[1,2-³H]Androstenedione, [4-¹⁴C] pregnenolone and [7α-³H] progesterone were also 16α-hydroxylated by incubation with the microsomal fraction. Amoung these substrates, progesterone gave the highest yield of 16α-hydroxylated products. By incubation with the microsomal fraction, formation of following steroids were also established: 6β-hydroxyandrostenedione from androstenedione; 17-hydroxypregnenolone, 17,21-dihydroxypregnenolone and dehydroepiandrosterone from pregnenolone; 17-hydroxy-progesterone, deoxycorticosterone, 11-deoxycortisol and androstenedione from progesterone.     

Preparation of two dinor-PGI2 metabolites from 6-keto-PGF1α by mycobacterium rhodochrous

The transformation of 6-keto-PGF_1α to two prostacyclin metabolites, 2,3-dinor-6-keto-PGF_1α (I) and 2,3-dinor-6,15-diketo-13,14-dihydro-PGF_1α (II) by $\text{Mycobacterium rhodochrous}$ UC-6176 is described. The finding that the bacterium oxidized 6-keto-PGF_1α to the 6,15-diketo metabolite II shows that it contains 15-hydroxy prostaglandin dehydrogenase and Δ¹³ reductase enzyme systems.     

Arachidonic acid metabolic pathways in the rabbit pericardium

Minced rabbit pericardium actively converts [1-14C]arachidonic acid into the known prostaglandins (6-[1-14C]ketoprostaglandin F1 alpha, [1-14C]prostaglandin E2 and [1-14C]prostaglandin F2 alpha) and into several unidentified metabolites. The major metabolite was separated by C18 reverse-phase high-pressure liquid chromatography (HPLC) and identified by gas chromatography-mass spectrometry (GC-MS) to be 6,15-[1-14C]diketo-13,14-dihydroprostaglandin F1 alpha. The other nonpolar metabolites were 15-[1-14C]hydroxy-5,8,11,13-eicosa-tetraenoic acid (15-HETE), 11-[1-14C]hydroxy-5,8,12,14-eicosatetraenoic acid (11-HETE) and 12-[1-14C]hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE). Arachidonic acid metabolites actively produced by the pericardium could influence the tone of surface blood vessels on the myocardium.     

The conversion of 5α-lanost-24-ene-3β,9α-diol and parkeol into poriferasterol by the alga Ochromon as malhamensis

The 4,4-dimethylsterols 4α-lanost-24-ene-3β,9α-diol-[2-³H₂] and parkeol-[2-³H₂] were synthesized from lanosterol and subsequently incubated with cultures of Ochromonas malhamensis. 5α-Lanost-24-ene-3β,9α-diol was converted into poriferasterol with three times the efficiency of parkeol. Clionasterol was also found to be labelled from both parkeol and 5α-lanost-24-ene-3β,9α-diol. No significant incorporation of radioactivity into sterols was obtained after feeding 5α-lanost-24-ene-3β,9α-diol to higher plants, the chlorophyte alga Trebouxia, yeast or a cell free homogenate of rat liver.     

Parathyroid hormone is not involved in 1,25-dihydroxyvitamin D regulation of 25-hydroxyvitamin D metabolism in vivo

1,25-Dihydroxyvitamin D₃ administration to vitamin D-deficient rats suppresses accumulation of 1,25-dihydroxy-[3α-³H]vitamin D₃ and stimulates accumulation of 24,25-dihydroxy-[3α-³3H]vitamin D₃ from 25-hydroxy-[3α-³H]vitamin D₃ equally well in the presence and absence of parathyroid glands. These results demonstrate that this regulatory action is not mediated by the parathyroid glands and support conclusions from $\text{in}\text{vitro}$ studies that this represents a direct action of 1,25-dihydroxyvitamin D₃.     

Effects of endometrium on metabolismof arachidonic acid by bovine blastocysts in vitro

The effects of endometrium on metabolism of [³H]-arachidonic acid ([³H]-AA) by bovine blastocysts recovered on day 19 postmating were studied $\text{in vitro}$. Blastocysts (n = 12) and endometrial slices were assigned to four incubation groups. In group 1, blastocysts were incubated alone; group 2, endometrial slices were incubated alone; group 3, blastocysts were incubated with endometrial slices; group 4, blastocysts were incubated in 7.5 ml fresh incubation medium plus 7.5 ml frozen-thawed medium from endometrial incubations. In all groups, tissues were incubated in 15 ml modified minimum essential medium (MEM) containing 5 μCi of [³H]-AA and 200 μg radioinert arachidonic acid for 24 h at 37°C in an atmosphere of 50% N₂:45% O₂:5% CO₂. For incubation controls, 5 μCi of [³H]-AA were added to 15 ml MEM and incubated at the same time as tissues from each cow. To evaluate metabolism of [³H]-AA, [³H]-AA and its metabolites were extracted from aliquots of MEM and separated on columns of Sephadex LH-20. Most (78.3 ± 3.2%) of the radioactivity (dpm) in the incubation controls was recovered as [³H]-AA, indicating that there was little breakdown of [³H]-AA in the absence of tissue. Blastocysts produced compounds that migrated with [³H]-13,14-dihydro-15-keto-PGF_2^α ([²H]-PGFM), [³H]-PGE₂ and [³H]-PGF_2^α. Endometrial slices metabolized very little of the [³H]-AA. Data from groups 1 and 4 were combined (group $\text{1}\text{4}$) for analysis because the distribution of dpm did not differ between the two groups. In group 3, blastocysts and endometrial slices incubated together tended(P<.10) to produced more [³H]-PGE₂ than did group $\text{1}\text{4}$, there tended to be less (P<.10)_[³H]-PGF_2^α, and there was more (P<.05) [³H]-PGFM than in group $\text{1}\text{4}$. Neither endometrial secretions nor endometrial slices altered the proportion of [³H]-AA metabolized by blastocysts. Endometrial slices appear capable of metabolizing [³H]-PGF_2^α synthesized by blastocysts, and capable of directing blastocyst metabolism of [³H]-AA away from synthesis of [³H]-PGF_2^α and toward synthesis of [³H]-PGE₂. It is postulated that the endometrium has an important role in regulating the amounts and ratios of prostaglandins in th uterine lumen during early prenancy in cows.     

Concentrative accumulation of 3H-prostaglandins by some rabbit tissues in vitro: The chemical nature of the accumulated 3H-labelled substances

Following incubation with [³H]-PGF_2α, 73–91% of the ³H activity accumulated by rabbit uterus, choroid plexus or anterior uvea was shown to remain associated with PGF_2α on two different chromatographic systems. The tissue to medium ratios, calculated on the basis of chromatographically identified [³H]-PGF_2α, were greater than unity (2.3–10.4) for all three tissues and the extracted ³H activity could be effectively accumulated by these tissues for a second time. Under conditions when 85% of authentic [³H]-PGF_2α and only 8% of [³H]-15-keto-PGF_2α was adsorbed on rabbit anti-PGF_2α serum, 60–75% of the extracted ³H was adsorbed onto the antiserum. Following incubation with a mixture of 5,6-[³H]-PGE₁ and 2-[¹⁴C]-PGE₁, the anterior uvea and the uterus showed similar $\text{T}\text{M}$ ratios for ³H and ¹⁴C and the ${}^3\text{H}{}^{14}\text{C}$ ratios were essentially constant in their respective homogenates, extracts and chromatographic fractions, indicating insignificant β-oxidation of the accumulated PGE₁. In the case of the kidney cortex, a substantial fraction of the accumulated ¹⁴C did not extract as a PG presumably as a result of β-oxidation. It is concluded that metabolic alteration of the accumulated PG molecule does occur in some tissues, but such chemical alterations are not an integral part of the PG accumulative process. These results are consistent with the concept that some vertebrate tissues can accumulate PGs against a concentration gradient by an active transport mechanism.     

Measurement of prostaglandin F2α metabolites by radioimmunoassay

Antibodies against 15 keto PGF_2α and 13,14 dihydro 15 keto PGF_2α were produced in goats and rabbits using the appropriate prostaglandin protein conjugate. Tritium labeled 15-keto, and 13,14 dihydro 15-keto PGF_2α were prepared from ³H-PGF_2α. These antibodies and ³H-labeled compounds were used to develop radioimmunoassays for the respective F_2α metabolites. The antibodies had relatively little cross-reactivity (≤0.1%) with the parent F_2α molecule. Infusion of PGF_2α in monkeys increased 15-keto-h₂ levels 10–20 fold higher than PGF_2α in peripheral plasma. The levels of this metabolite were not altered detectably during clotting, indicating relatively slow rates of PGF_2α metabolism in vitro. These assays should be useful to follow release rates of exogenous prostaglandins from various formulations and delivery systems, and in vivo tissue synthesis of PGF_2α, where low levels preclude measuring the parent compound.     

Inositol Trisphosphate Metabolism in Carrot (Daucus carota L.) Cells

The metabolism of exogenously added d-myo-[1-³H]inositol 1,4,5-trisphosphate (IP₃) has been examined in microsomal membrane and soluble fractions of carrot (Daucus carota L.) cells grown in suspension culture. When [³H]IP₃ was added to a microsomal membrane fraction, [³H]IP₂ was the primary metabolite consisting of approximately 83% of the total recovered [³H] by paper electrophoresis. [³H]IP was only 6% of the [³H] recovered, and 10% of the [³H]IP₃ was not further metabolized. In contrast, when [³H]IP₃ was added to the soluble fraction, approximately equal amounts of [³H]IP₂ and [³H]IP were recovered. Ca²⁺ (100 micromolar) tended to enhance IP₃ dephosphorylation but inhibited the IP₂ dephosphorylation in the soluble fraction by about 20%. MoO₄²⁻ (1 millimolar) inhibited the dephosphorylation of IP₃ by the microsomal fraction and the dephosphorylation of IP₂ by the soluble fraction. MoO₄²⁻, however, did not inhibit the dephosphorylation of IP₃ by the soluble fraction. Li⁺ (10 and 50 millimolar) had no effect on IP₃ metabolism in either the soluble or membrane fraction; however, Li⁺ (50 millimolar) inhibited IP₂ dephosphorylation in the soluble fraction about 25%.     

Cyclization of squalene-2,3-epoxide to 10α-cucurbita-5,24-dien-3β-ol by microsomes from Cucurbita maxima seedlings

Labelled 10α-cucurbita-5,24-dien-3β-ol was obtained from [3-³H]squalene-2,3-epoxide incubated with microsomes of Cucurbita maxima seedlings. By contrast the lanostane triterpenoids [2-³H]cycloartenol, [2-³H]parkeol and their corresponding derivatives [2,12-³H]11-ketocycloartenol and [2-³H]24,25-dihydro-90α,11a-epoxyparkeol, incubated under the same conditions, gave no rearranged products with a cucurbitane skeleton. These results suggest that biosynthesis of cucurbitane triterpenoids from squalene-2,3-epoxide occurs through direct cyclization without the intermediacy of lanostane-type triterpenoids such as cycloartenol or parkeol. Labelled cycloartenol, α-amyrin, β-amyrin and 24-methylenecycloartanol were also obtained from [3-³H]squalene-2,3-epoxide in the same enzymatic system yielding labelled 10α-cucurbitadienol.