Regulation of agonist-induced prostaglandin E1 versus prostaglandin E2 production. A mass analysis.

Prostaglandin E1 (PGE1) and prostaglandin E2 (PGE2), derived by enzymatic oxidation of cellular dihomogammalinolenic acid (DHLA) and arachidonic acid (AA), respectively, have diverse and, at times, distinct biological actions. It has been suggested that PGE1 specifically inhibits a variety of inflammatory processes, and, in light of the potential therapeutic benefit of PGE1 and its fatty acid precursor in inflammatory disorders, there is growing interest in the biochemical mechanisms which determine the balance between PGE1 and PGE2 synthesis. Metabolic studies in this area have been hampered by the difficulties in measuring the extremely small masses of these prostaglandins which are generated in cell culture systems. We studied the regulation of PGE1 versus PGE2 synthesis using an essential fatty acid-deficient, PGE-producing, mouse fibrosarcoma cell line, EFD-1. Because EFD-1 cells contain no endogenous AA or DHLA, we were able to replete the cells with AA and DHLA of known specific activities; thus, the mass of both cellular AA and DHLA, and synthesized PGE1 and PGE2, could be accurately determined. The major finding of this study is that production of PGE2 was highly favored over production of PGE1 due to preferential incorporation of AA versus DHLA into, and release from, the total cellular phospholipid pool. Further, we correlated the selective release of AA versus DHLA from total cellular phospholipids with the selective incorporation of AA versus DHLA into specific phospholipid pools. In addition, we showed that conversion of DHLA to AA by delta 5 desaturase was enhanced by increasing the cellular mass of n-6 fatty acids and by increasing the cell proliferative activity. Together, these results indicate that the relative abundance of PGE2 versus PGE1 in vivo is not merely a function of the relative abundance of AA versus DHLA in tissues, but also relates to markedly different cellular metabolism of these two fatty acids.     

Kinetics of prostaglandin H2 degradation. A method of determining prostaglandin H-convertase activity

The quantitative study of the processes that accompany nonenzymatic degradation of prostaglandin H2 has been carried out. The thiobarbituric acid test which shows the content of malonic dialdehyde in the reaction mixture has been used to study kinetics of the degradation. The apparent rate constants of this process have been pH-independent over the pH-range 5,5-9,5, and the calculated conversion halftime changes from 5,8 to 3,6 min at these pH values. Thromboxane synthetase from human platelets has been chosen to demonstrate the possibility of application of thiobarbituric acid test for determination of the activity of prostaglandin endoperoxide convertase. It has been shown that the apparent rate constant of the reaction in the presence of the enzyme is the linear function of its concentration.     

Molecular basis of prostaglandin potency. II. Proton NMR studies of the conformation of prostaglandin F2alpha.

The proton nmr of PGF2alpha (dissolved in aqueous tBuOH) in the presence of PrC13 and limited amounts of base shows LIS shifts for the terminal methyl indicating an average effective chain length (C-1 to C-20) like that of octanoic acid, further evidence that the alpha and omega carbons of prostaglandins are closer in space than would be expected based on a random distribution of confermers with alternate zig-zag sections of hydrocarbon chains. Assessing the exact nature of the sidechain alignment will require further experiments.     

Reactions of prostaglandin endoperoxides with prostaglandin I synthetase solubilized from rabbit aorta microsomes.

It has been shown that subunit I of cytochrome oxidase (～ MWt. 40,000) can be resolved into a number of smaller polypeptides. This resolution apparently occurs through two stages with the generation of polypeptides of approximate molecular weights of 20,000 and 8,500.     

Metabolism of prostaglandin glycerol esters and prostaglandin ethanolamides in vitro and in vivo.

Prostaglandin glycerol esters (PG-Gs) and prostaglandin ethanolamides (PG-EAs) are generated by the action of cyclooxygenase-2 on the endocannabinoids 2-arachidonylglycerol (2-AG) and arachidonylethanolamide, respectively. These novel eicosanoids may have unique pharmacological properties and/or serve as latent sources of prostaglandins at sites remote from their tissue of origin. Therefore, we investigated the metabolism of PG-Gs and PG-EAs in vitro and in vivo. PGE(2)-G was rapidly hydrolyzed in rat plasma to generate PGE(2) (t(1/2) = 14 s) but was only slowly metabolized in human plasma (t(1/2) > 10 min). An intermediate extent of metabolism of PGE(2)-G was observed in human whole blood (t(1/2) approximately 7 min). The parent arachidonylglycerol, 2-AG, and the more stable regioisomer, 1-AG, also were much more rapidly metabolized in rat plasma compared with human plasma. PGE(2)-EA was not significantly hydrolyzed in plasma, undergoing slow dehydration/isomerization to PGB(2)-EA. Both PGE(2)-G and PGE(2)-EA were stable in canine, bovine, and human cerebrospinal fluid. Human 15-hydroxyprostaglandin dehydrogenase, the enzyme responsible for the initial step in PG inactivation in vivo, oxidized both PGE(2)-G and PGE(2)-EA less efficiently than the free acid. The sterically hindered glyceryl prostaglandin was the poorest substrate examined in the E series. Minimal 15-hydroxyprostaglandin dehydrogenase oxidation of PGF(2 alpha)-G was observed. PGE(2)-G and PGE(2)-EA pharmacokinetics were assessed in rats. PGE(2)-G was not detected in plasma 5 min following an intravenous dose of 2 mg/kg. However, PGE(2)-EA was detectable up to 2 h following an identical dose, displaying a large apparent volume of distribution and a half-life of over 6 min. The results suggest that endocannabinoid-derived PG-like compounds may be sufficiently stable in humans to exert actions systemically. Furthermore, these results suggest that the rat is not an adequate model for investigating the biological activities of 2-arachidonylglycerol or glyceryl prostaglandins in humans.     

Sex differences in prostaglandin metabolism.

Prostaglandin E2 (PGE2), PGF2alfa and PGD2 were synthetized from [1-¹⁴C]-arachidonic acid by rat kidney medulla microsomal fraction. The formation of each prostaglandin was significantly less in female animals than in males. The rate of inactivation of [³H]-PGF2alfa by kidney cortex cytosol was almost linear with the time of incubation during the first 30 min. The production of PGF2alfa metabolite (13,14-dihydro-15-keto PGF2alfa) was higher in male rats than in females.     

Isomerization of prostaglandin H2 into prostaglandin D2 in the presence of serum albumin.

The prostaglandin endoperoxide, prostaglandin H2, decomposes in aqueous media mainly into prostaglandin E2. This paper shows that in the presence of serum albumin from a number of species prostaglandin H2 decomposes mainly into prostaglandin D2, an isomer of prostaglandin E2. The effect on endoperoxide decomposition exerted by serum albumin may well have physiological significance since intace endoperoxides can be released from tissues and since the biological properties of prostaglandins E2 and D2 are quite different.     

Endogenous inhibitors of human placental prostaglandin dehydrogenase.

The presence of endogenous inhibitors of NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase (PGDH) has been indicated by increasing total activity after the initial purification step of PGDH in human placenta. Based on this observation, we tried to characterize and analyze endogenous inhibitors of PGDH in human placenta in this study. The inhibitors were extracted from the supernatant by precipitation at pH 5.2 and partially purified by acetone precipitation and by thin layer chromatography. The inhibitors were stable to heating at 100 degrees C for 10 min, and to trypsin digestion. The pattern of inhibition was competitive with regard to PGE2 and uncompetitive with regard to NAD at pH 8.0. The Ki value for PGE2 was 18.9 microM. Analysis by gas chromatography and mass spectrometry indicated that the inhibitors consisted of fatty acids which were palmitic, stearic, oleic and linoleic acids. Myristic, palmitic and stearic acids were confirmed to exert an inhibitory action on PGDH and showed a competitive inhibition pattern. Stearic acid was less potent in inhibition than other fatty acids. These findings suggest that intracellular fatty acids may play a unique role in the control of PGDH activity.     

Stimulation of prostaglandin biosynthesis by lipoic acid.

Enzyme preparations from sheep seminal vesicles display an enhanced ability to synthesize prostaglandins, particularly prostaglandin F from polyunsaturated fatty acids if alpha-lipoic acid is present in the incubation mixture prior to the addition of fatty acid. The stimulation by lipoate is reversible, time dependent, and involves modifications of V and Km for oxygenase activity. Product studies, structure vs. activity studies, and purification data indicate that lipoate exerts it effect by a mechanism distinct from a glutathione-like metabolism of the endoperoxide linkage in prostaglandin G and prostaglandin H. In addition, product studies suggest that lipoate is not a cofactor for the endoperoxide isomerase component of prostaglandin synthetase. Purification of the endoperoxide synthesizing activity by ion-exchange chromatography and isoelectric focusing yields preparations which are more responsive to lipoate than microsomal preparations.     

Cyclic AMP and platelet prostaglandin synthesis.

The present study has investigated the influence of agents which elevate intracellular levels of endogenous platelet adenosine 3'5'-cyclic monophosphate (cyclic AMP), and the effect of the exogenous cyclic AMP analog, dibutyryl cyclic AMP, on the conversion of 14C-arachidonic acid by washed platelets. Prostaglandin E1 (PGE1), PGE1 with theophylline, or dibutyryl cyclic AMP incubated with washed platelets prevented arachidonic acid induced platelet aggregation, but had no effect on the conversion of arachidonic acid to 12L-hydroxy-5,8,10, 14-eicosatetraenoic acid (HETE), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT), or thromboxane B2. Ultrastructural studies of the platelet response revealed that agents acting directly or indirectly to increase the level of cyclic AMP inhibited the action of arachidonic acid on washed platelets and prevented internal platelet contraction as well as aggregation. The influence of PGE1 with theophylline, and dibutyryl cyclic AMP on the thrombin induced release of 14C-arachidonic acid from platelet membrane phospholipids was also investigated. These agents were found to be potent inhibitors of the thrombin stimulated release of arachidonic acid from platelet phospholipids, due most likely to an inhibition of platelet phospholipase A activity. The results show that dibutyryl cyclic AMP and agents which elevate intracellular cyclic AMP levels act to inhibit platelet activation at two steps 1) internal contraction and 2) release of arachidonic acid from platelet phospholipids.     

Effects of nitric oxide and nitric oxide-derived species on prostaglandin endoperoxide synthase and prostaglandin biosynthesis.

Prostaglandins and NO. are important mediators of inflammation and other physiological and pathophysiological processes. Continuous production of these molecules in chronic inflammatory conditions has been linked to development of autoimmune disorders, coronary artery disease, and cancer. There is mounting evidence for a biological relationship between prostanoid biosynthesis and NO. biosynthesis. Upon stimulation, many cells express high levels of nitric oxide synthase (NOS) and prostaglandin endoperoxide synthase (PGHS). There are reports of stimulation of prostaglandin biosynthesis in these cells by direct interaction between NO. and PGHS, but this is not universally observed. Clarification of the role of NO. in PGHS catalysis has been attempted by examining NO. interactions with purified PGHS, including binding to its heme prosthetic group, cysteines, and tyrosyl radicals. However, a clear picture of the mechanism of PGHS stimulation by NO. has not yet emerged. Available studies suggest that NO. may only be a precursor to the molecule that interacts with PGHS. Peroxynitrite (from O2.-+NO.) reacts directly with PGHS to activate prostaglandin synthesis. Furthermore, removal of O2.- from RAW 267.4 cells that produce NO. and PGHS inhibits prostaglandin biosynthesis to the same extent as NOS inhibitors. This interaction between reactive nitrogen species and PGHS may provide new approaches to the control of inflammation in acute and chronic settings.     

Some biological effects of prostaglandin endoperoxide analogs.

The effects of two methano-epoxy analogs of the prostaglandin endoperoxides PGG₂ and PGH₂ were tested on human platelets and rabbit aorta strips. One of these analogs, 9α, 11α-methano-epoxy-15- hydroxy-prosta-5, 13-dienoic acid, was 3.7 times more potent than the endoperoxide, PGG₂, as aggregating agent and was 6.2 times more active than PGH₂ in eliciting contractions of the isolated rabbit aorta. The analog initiated the platelet release reaction, but was less active than the endoperoxide in this respect. Furthermore, the release of ¹⁴C-serotonin induced by this analog was inhibited by indomethacin, which indicated that generation of endoperoxide was required.The corresponding 9α, 11α, epoxy-methano-analog was less active than the 9α, 11α, methano-epoxy analog in the test systems employed.     

Novel prostaglandin dehydrogenase in rat skin.

Present evidence suggests that skin is an important organ of prostaglandin metabolism. To clarify its role, the basic kinetics of 15-hydroxyprostaglandin dehydrogenase (PGDH) from rat skin were investigated with either NAD+ of NADP+ as co-substrate. Prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E2 (PGE2) were used as substrates and preliminary studies were made of the inhibitory effects of the reduced co-substrates NADH and NADPH. A radiochemical assay was used in which [3H]PGF2 alpha or [14C]PGE2 were incubated with high-speed supernatant of rat skin homogenates. The substrate and products were then extracted by solvent partition, separated by t.l.c. and quantified by liquid-scintillation counting. At linear reaction rates and at an NAD+ concentration of 10 mM the mean apparent Km for PGF2 alpha was 24 microM with a mean apparent Vmax. of 9.8 nmol/s per litre of reaction mixture. For PGE2 the mean apparent Km was 8 microM, with a mean apparent Vmax, of 2.7 nmol/s per litre of reaction mixture. With NADP+ as a co-substrate at a concentration of 5 mM a mean apparent Km of 23 microM was obtained for PGF2 alpha with a mean apparent Vmax. of 5.2 nmol/s per litre. For PGE2 values of 7.5 microM and 3.0 nmol/s per litre were obtained respectively. These results show that skin contains NAD+- and NADP+-dependent PGDH. An important finding was that the NADP+-linked enzyme gave Km values for PGE2 that were considerably lower than those reported for NADP+-linked PGDH from other tissues. Furthermore, preliminary inhibition studies with the NAD+-linked PGDH system indicate that this enzyme is not only inhibited by NADH, but also by NADPH, a property not previously reported for NAD+-linked PGDH derived from other tissues.