Stimulation of microsomal prostaglandin synthesis by a vasoactive material isolated from blood plasma

Stimulation of prostaglandin synthesis by a material with coronary vasoconstrictor activity extracted from blood plasma was examined. The vasoactive material decreased the Km for arachidonate in the overall synthesis of prostaglandins by rabbit renal microsomal preparations but did not change Vmax. Increases in prostaglandin synthesis caused by the vasoactive material and L-tryptophan or L-epinephrine were additive or synergistic, whereas increases produced by the vasoactive material and hemin or hemoglobin were not. However, hemin and hemoglobin stimulated synthesis of all prostaglandins equally whereas the active material increased the synthesis of prostaglandin F_2α at the expense of other prostaglandins, both in the presence and absence of heme compounds. The increase in prostaglandin F_2α with respect to the other prostaglandins occurred in the presence of reduced glutathione. The vasoactive material attenuated inhibition of prostaglandin synthesis induced by indomethacin or aspirin but not that produced by 5,8,11,14-eicosatetraynoic acid. The interaction of the vasoactive material and indomethacin was competitive whereas hemin attenuated the effects of only low concentrations of indomethacin. Epinephrine enhanced indomethacin inhibition. These data indicate that mode of action of the vasoactive material in prostaglandin synthesis is unlike that of glutathione, aromatic amines, or heme containing compounds.     

Arachidonic acid level in cellular lipids determines the amount of prostaglandins synthesized during cell growth in tissue culture

Methylcholanthrene transformed mouse fibroblast cells can be induced to synthesize prostaglandins by a short term incubation with various vasoactive agents including serum, bradykinin and thrombin or in response to mechanical detachment from the culture dish. The ability of the cells to synthesize prostaglandins upon stimulation changes during growth of the culture on the dish; the response is maximal on the first day after inoculation and decreased sharply thereafter. Feeding of the cells with fresh growth medium enhances prostaglandin production induced by all stimuli. The difference in the cell response during growth is probably not due to change of prostaglandin synthetase activity since the specific enzyme activities assayed with microsomal preparations of cells harvested from the first and third day culture are similar. However, analysis of the cellular content of arachidonic acid after saponification of the total lipid extract of cells harvested at different days of growth reveals that the level of arachidonic acid per cell during growth is parallel to the response to stimuli. It is maximal on the first day and decreases sharply on the second day and stays low on the third day. Our study suggests that the level of arachidonic acid in the cell governs the extent of prostaglandin synthesis upon stimulation.     

Arachidonic acid level in cellular lipids determines the amount of prostaglandins synthesized during cell growth in tissue culture.

Methylcholanthrene transformed mouse fibroblast cells can be induced to synthesize prostaglandins by a short term incubation with various vasoactive agents including serum, bradykinin and thrombin or in response to mechanical detachment from the culture dish. The ability of the cells to synthesize prostaglandins upon stimulation changes during growth of the culture on the dish; the response is maximal on the first day after inoculation and decreased sharply thereafter. Feeding of the cells with fresh growth medium enhances prostaglandin production induced by all stimuli. The difference in the cell response during growth is probably not due to change of prostaglandin synthetase activity since the specific enzyme activities assayed with microsomal preparations of cells harvested from the first and third day culture are similar. However, analysis of the cellular content of arachidonic acid after saponification of the total lipid extract of cells harvested at different days of growth reveals that the level of arachidonic acid per cell during growth is parallel to the response to stimuli. It is maximal on the first day and decreases sharply on the second day and stays low on the third day. Our study suggests that the level of arachidonic acid in the cell governs the extent of prostaglandin synthesis upon stimulation.     

Biosynthesis and metabolism of prostaglandins in chick epiphyseal cartilage.

Microsomal fractions of cells isolated from chick epiphyseal cartilage catalyzed the synthesis of prostaglandins from radiolabeled delta8,11,14-eicosatrienoic and from arachidonic acids. In addition, the microsomal supernatants contained both 15-hydroxyprostaglandin dehydrogenase and prostaglandin 15-keto delta13,14-reductase activities. Two major classes of prostaglandins (E and F) were synthesized; however, a major product which chromatographically behaves as PGA was also isolated. Synthetase activities were analyzed for pH optima and response to known stimulators and inhibitors of prostaglandin systhesis. The different activators had varying stimulatory effects on prostaglandin synthesis; the anti-inflammatory drugs were all strongly inhibitory. Synthetase activity in the growth plate was highest in the zone of hypertrophy, declining substantially in the more heavily calcified regions. Degradative enzyme activities were highest in the zone of maturation and significantly lower in the adjacent hypertrophic zone. The net effect of these opposing activities would be to elevate prostaglandin levels at the zone of hypertrophy, a finding which suggests that prostaglandins may play a role in the modulation of epiphyseal cartilage metabolism.     

Biosynthesis of prostaglandins in rabbit kidney medulla. Properties of prostaglandin synthase.

A simple radioactive-substrate assay for prostaglandin synthase (EC 1.14.99.1), which uses t.l.c. to measure simultaneously different prostaglandins synthesized from one precursor substrate, was developed. Rabbit kidney-medulla prostaglandin synthase catalyses the formation of prostaglandin E2, prostaglandin F2alpha and prostaglandin D2 from arachidonic acid. Fractionation of crude homogenates indicated that the microsomal fraction possessed the highest specific activity of prostaglandin synthase, whereas the soluble fraction exhibited little enzyme activity but rather contained a heat-labile inhibitory macromolecular factor(s), which might be attributed to the serum albumin present in this fraction. The microsomal fraction possessed low intrinsic enzyme activity, but the actvity could be fully stimulated by the presence of both GSH (reduced glutathione) and a phenolic cofactor. Only cysteine could partially replace GSH, whereas other thiols were inactive and some were even inhibitory. A variety of phenolic compounds, including catecholamines, dopamine (3,4-dihydroxyphenethylamine), 5-hydroxytryptamine and quinol, were active in stimulating prostaglandin synthase. In all cases, the stimulation was reflected in the synthesis of all three prostaglandins with ratios not significantly altered by different phenolic cofactors. The synthesis of each of the different prostaglandins appeared to have similar pH optima. The enzyme system was not inhibited by thiol-group inhibitors or a variety of metal chelators except for cyanide and 8-hydroxyquinoline. Characterization of the kidney-medulla prostaglandin synthase system indicated that it exhibited properties similar to those of the enzyme system present in seminal vesicles.     

Biosynthesis and metabolism of prostaglandins in chick epiphyseal cartilage

Microsomal fractions of cells isolated from chick epiphyseal cartilage catalyzed the synthesis of prostaglandins from radiolabeled Δ^8,11,14-eicosatrienoic and from archidonic acids. In addition, the microsomal supernatants contained both 15-hydroxyprostaglandin dehydrogenase and prostaglandin 15-keto Δ^13,14-reductase activities. Two major classes of prostaglandins (E and F) were synthesized; however, a major product which chromatographically behaves as PGA was also isolated. Synthetase activities were analyzed for pH optima and response to known stimulators and inhibitors of prostaglandin synthesis. The different activators had varying stimulatory effects on prostaglandin synthesis; the anti-inflammatory drugs were all strongl inhibitory. Synthetase activity in the growth plate was highest in the zone of hypertrophy, declining substantially in the more heavily calcified regions. Degradative enzyme activities were highest in the zone of maturation and significantly lower in the adjacent hypertrophic zone. The net effect of these opposing activities would be to elevate prostaglandin levels at the zone of hypertrophy, a finding which suggests that prostaglandins may play a role in the modulation of epiphyseal cartillage metabolism.     

Conversion of arachidonic acid to prostanoids in the avian uterus

In an effort to obtain information on the possible source of prostaglandins which have been shown to play an important role in oviposition we examined the metabolism of arachidonic acid in microsomal preparations of both the muscular and the glandular tissue of the hen uterus. We found that adrenaline and tryptophan (but not hydroquinone) were effective stimulators of prostanoid synthesis. On incubation with [3H]arachidonic acid we identified, using TLC radiochromatography and several solvent systems, prostaglandins F2 alpha and E2 and, predominantly, thromboxane B2 which could not be attributed to platelet contamination. Addition of reduced glutathione increased prostaglandin E2 formation at the expense of thromboxane B2 and at 1 mM concentration suppressed adrenaline-promoted prostanoid synthesis. While the former effect has been documented in many other systems and could be ascribed to the activation of prostaglandin H2 to prostaglandin E2 isomerase, the latter effect is postulated to be due to an inhibition of cyclooxygenase. Interestingly, this inhibitory effect was shared by a number of reducing agents. Although the subcellular preparations were derived from structurally and functionally different tissues, there was no qualitative difference with respect to prostanoid synthesis. Our data support the role of locally produced primary prostaglandins in the regulation of oviposition and raise the question of a potential role for thromboxane in this process.     

Mechanism of prostaglandin biosynthesis in rabbit kidney medulla. A rate-limiting step and the differential stimulatory actions of L-adrenaline and glutathione.

Microsomal prostaglandin synthase (EC 1.14.99.1) from rabbit kidney medulla was assayed with [5,6,8,9,11,12,14,15-3H]-and [1-14C]-arachidonic acid as the substrate. The ratios of prostaglandin F2 alpha to prostaglandin E2 and to prostaglandin D2 were determined by both 3H and 14C labelling. When 3H was used as a label the ratios were much higher than with 14C labelling indicating that the removal of hydrogen at C-9 or C-11 was the rate-limiting step in the biosynthesis of prostaglandin E2 or prostaglandin D2. This finding shows that the octatritiated arachidonic acid is not the appropriate substrate marker for studying the regulation of the synthesis of different prostaglandins by various agents. When the enzyme assay was carried out in the presence of SnCL2, which was capable of accumulating exclusively prostaglandin F2alpha at the expenses of prostaglandin E2 and prostaglandin D2, the addition of L-adrenaline to the microsomal fraction either alone or with reduced glutathione equally stimulated the formation of prostaglandin F2alpha, whereas the addition of reduced glutathione to the microsomal fraction either alone or with L-adrenaline produced no additional effect. These results suggest that endoperoxide is formed as the common intermediate for the biosynthesis of three different prostaglandins in rabbit kidney medulla, and that L-adrenaline stimulates the synthesis of endoperoxide, whereas reduced glutathione facilitates the formation of prostaglandins from endoperoxide.     

Elevated prostaglandin synthetase activity in methylcholanthrene-transformed mouse BALB/3T3

Cell lines transformed from 3T3 spontaneously, by radiation, or by treatment with chemical carcinogens, polyoma and SV40 virus produce up to 5 times more prostaglandins than their untransformed parent line. Several aspects of prostaglandin biosynthesis by MC5-5 and 3T3 were compared. When stimulated by serum, bradykinin, or thrombin, MC5-5 produced 2-to 5-fold more prostaglandins than 3T3. With the use of cells labeled with radioactive arachidonic acid in their cellular lipids, these higher levels were shown not to be due to increased availability of the prostaglandin precursor, arachidonic acid. Prostaglandin synthetase activity in microsomal fractions prepared from MC5-5 was 6 times higher than that of microsomes of untransformed cells. The increased prostaglandin levels produced by transformed cells therefore appear to be the result of elevated prostaglandin synthetase activity.     

Presence of an intracellular endometrial inhibitor of prostaglandin synthesis during early pregnancy in the cow

Previous studies have detected reduced endometrial secretion of prostaglandins during pregnancy in cattle. The present experiment tested the hypothesis that reduced secretion of prostaglandins is caused by induction of an intracellular endometrial inhibitor of prostaglandin synthesis. The microsomal fraction of parturient bovine cotyledons was utilized as a source of enzymes for prostaglandin synthesis. Endometrial tissues collected at Day 17 of the estrous cycle (n = 12) and pregnancy (n = 12) were homogenized and subjected to differential centrifugation for preparation of microsomes and a high-speed (100,000 x g) cytosolic supernatant. Endometrial intracellular preparations were then examined for the ability to modulate prostaglandin synthesis by cotyledonary microsomes from parturient cows. Endometrial intracellular preparations from cyclic cows decreased (P less than 0.05) PGF synthesis by cotyledonary microsomes to a slight extent (supernatant, 21% reduction; microsomes, 11% reduction), while preparations from pregnant cows markedly decreased (P less than 0.01) PGF synthesis (supernatant, 63% reduction; microsomes, 28% reduction; supernatants vs microsomes, P less than 0.01). Regardless of the amount of arachidonic acid available as substrate (25-400 micrograms) endometrial supernatant from pregnant cows (pooled sample) caused a 50% inhibition (IC50) of prostaglandin synthesis at a tissue equivalent of 270 +/- 9.1 mg. The mechanism of inhibition by endometrial high-speed supernatant from pregnant cows appears to be non-competitive with respect to arachidonic acid. The inhibitor(s) may be proteinaceous (70-75 kDa and 25-35 kDa) and can be precipitated by 20% saturated ammonium sulfate. In conclusion, early pregnancy in cattle appears to be associated with increased amounts of an intracellular endometrial inhibitor of prostaglandin synthesis.     

Influence of chronic antigen exposure on the metabolism of PGH2 by microsomal preparations of airway and parenchymal tissues in the sheep

The effects of repeated antigen exposure on the synthesis of mediators by lung tissues are not well understood. To investigate the influence of antigen challenge on the synthesis of prostaglandins by central airway and peripheral lung tissues, fourteen sensitive sheep underwent biweekly exposure to aerosolized Ascaris suum antigen (7) or saline (7). Following the fifth exposure, microsomal and high speed supernatant fractions were prepared from trachealis muscle and lung parenchyma. Synthesis of thromboxane (TX) A2, prostaglandin (PG) D2 and PGI2 from the PG endoperoxide intermediate, PGH2, was assayed over a range of substrate concentrations from 3-200 microM. Synthesis of PGI2 by trachealis microsomes was approximately 5-fold greater than that of TXA2. PGI2 and TXA2 production was identical in tracheal preparations from Ascaris- and saline-exposed animals. In parenchymal tissues, where TXA2 production predominated over PGI2 by 9-fold, preparations from Ascaris-exposed animals synthesized 50% more TXA2 than controls at PGH2 concentrations of 25 microM and above, whereas synthesis of PGI2 and PGD2 were similar in preparations from both groups of animals. The density of pulmonary mast cells was decreased by 21% in the Ascaris group, whereas polymorphonuclear leukocyte density was unchanged. These results demonstrate the differential synthesis of TXA2 and PGI2 in central airways and peripheral lung regions of the sheep. They further indicate that repeated exposure of the airways to antigen selectively enhances TXA2 synthesis in the lung periphery of sensitized animals. The site of this increased enzymatic activity, whether in resident cells or newly-infiltrated cells, has not been determined.     

Increased level of the mitochondrial ND5 transcript in chemically induced rat hepatomas

We have constructed a cDNA library from a hepatoma cell line (HTC cells) and isolated the clones corresponding to mRNAs present at a much higher level in hepatomas than in normal hepatocytes. The characterization of one of these clones is described in this paper. This clone is homologous to part of the mitochondrial ND5 gene (a subunit of NADH-ubiquinone oxidoreductase). The level of this mRNA was found increased in HTC cells and in hepatocytes from diethylnitrosamine-treated rats long before the development of tumors and strongly increased in carcinoma nodules as compared to hepatocytes from nontreated rats. Southern blot analysis showed a mitochondrial DNA heterogeneity in hepatomas with an alteration of the structure of part of the molecules.     

Regulation of prostaglandin synthesis during differentiation of cultured mouse myeloid leukemia cells

Mouse myeloid leukemia cells (Ml) were induced to differentiate into mature macrophages and granulocytes by various inducers. The differentiated Ml cells synthesized and released prostaglandins, whereas untreated Ml cells did not. When the cells were prelabelled with [¹⁴C]arachidonate, the major prostaglandins released into the culture media were found to be prostaglandin E₂, D₂, and F_2α in an early stage of differentiation, but the mature cells produced predominantly prostaglandin E₂. The synthesis and release of prostaglandins were completely inhibited by indomethacin. Dexamethasone, a potent inducer of differentiation of Ml cells, did not induce production of prostaglandins in resistant Ml cells that could not differentiate even with a high concentration of dexamethasone. These results suggest that production of prostaglandins in Ml cells is closely associated with differentiation of the cells. Homogenates of dexamethasone-treated Ml cells converted arachidonate to prostaglandins, but this conversion was scarcely observed with homogenates of untreated Ml cells. Dexamethasone and the other inducers stimulated the release of arachidonate from phospholipids. Therefore, induction of prostaglandin synthesis during differentiation of Ml cells may result from induction of prostaglandin synthesis activity and stimulation of the release of arachidonate from cellular lipids. Lysozyme activity, which is a typical biochemical marker of macrophages, was induced in Ml cells by prostaglandin E₂ or D₂ alone, as well as by inducers of differentiation of the cells, but it was not induced by arachidonate or prostaglandin F_2α. These results suggest that prostaglandin synthesis is important in differentiation of myeloid leukemia cells.     

Enzymic coupling of acylhydrolase and prostaglandin synthase activities in subcellular fractions from rabbit renal medulla

We have recently shown that mitochondrial and plasma-membrane fractions from kidney medulla possess Ca²⁺-stimulated acylhydrolase and prostaglandin synthase activities. The nature of the enzymic coupling between the Ca²⁺-stimulated arachidonic acid release and its subsequent conversion into prostaglandins was investigated in subcellular fractions from rabbit kidney medulla. Plasma-membrane, mitochondrial and microsomal fractions were found to have similar apparent K_m values for conversion of added exogenous arachidonate into prostaglandins. The rate of prostaglandin biosynthesis (V_max.) from added arachidonic acid in the microsomal fraction was approx. 2-fold higher than in the other subcellular fractions. In contrast, prostaglandin E₂ synthesis from endogenous arachidonate in plasma-membrane and mitochondrial fractions was 3–4-fold higher than in microsomes. Furthermore, Ca²⁺ stimulated endogenous arachidonate deacylation and prostaglandin E₂ generation in the former two fractions but not in microsomes. In mitochondrial or crude plasma-membrane fractions, in which prostaglandin biosynthesis was inhibited with aspirin, arachidonate released from these fractions was converted into prostaglandins by the microsomal prostaglandin synthase. Thus an intracellular prostaglandin generation process that involves inter-fraction transfer of arachidonic acid can operate. Prostaglandin generation by such an inter-fraction process is, however, less efficient than by an intra-fraction process, where arachidonic acid released by mitochondria or crude plasma membranes is converted into prostaglandins by prostaglandin synthase present in the same fraction. This demonstrates the presence of a tight intra-fraction enzymic coupling between Ca²⁺-stimulated acylhydrolase and prostaglandin synthase enzyme systems in both mitochondrial and plasma-membrane fractions.     

Sertoli-cell prostaglandin synthesis. Effects of (follitropin) differentiation and dietary vitamin E.

The synthesis of prostanoids by the Sertoli cell was assessed as part of a study on the role of vitamin E in maintaining spermatogenesis. Analyses of eicosanoid synthesis from endogenous substrate were carried out using freshly isolated Sertoli-cell-enriched preparations from both pre-pubertal and adult rats fed purified diets with and without vitamin E, as well as cells carried in primary culture. Freshly isolated cells from both the immature and fully differentiated adult testes produced PGI2 (prostaglandin I2) and PGE2, but PGF2 alpha was produced only by cells of the adult vitamin E-deficient rat. Cells from adult controls synthesized PGF2 alpha after primary culture. In contrast with other hormone responses of this cell, which are refractory in the adult, FSH (follitropin) potentiated prostaglandin production by freshly isolated cells of both immature and adult rats. The FSH response of Sertoli cells from immature animals did not change after primary culture. Adult cells were refractory to the hormone after culture, but the total amounts of prostaglandins produced by these cells were 10-fold higher than by either freshly isolated or cells of the immature in culture. Analogues of cyclic AMP did not potentiate prostaglandin synthesis. However, mepacrine, a phospholipase inhibitor, blocked the FSH effect. The finding that Sertoli cells synthesize prostaglandins and FSH enhances prostaglandin production implicates a potential role for eicosanoids in spermatogenesis and suggests that vitamin E may affect intratesticular regulators.     

Sister-chromatid exchange induction by carcinogens in HTC cells An in vitro system which does not require addition of activating factors

The hepatic tumor cell line (HTC) was tested for the ability to produce sister chromatid exchanges (SCEs) in response to chemical carcinogens which require activation. Without the addition of exogenous microsomal enzyme preparations, cyclophosphamide, N-nitrosodiethylamine (DEN) and aflatoxin B₁ (AFB₁) induced significant levels of SCEs in these cells. Mitomycin C (MMC) and ultraviolet light, which do not require activation, also produced significant levels of SCEs. The induction of SCEs in HTC cells by AFB₁ was shown to be inhibited by estradiol, a known inhibitor of microsomal activating enzymes. For the carcinogens tested, the HTC cell SCE assay was quite sensitive and comparable to other mammalian test systems. Exceptional sensitivity was found in the case of AFB₁. SCE analysis of HTC cells offers a simplified system of detecting carcinogens requiring activation. This system also has the potential of investigating interactions between agents such as steroid hormones and carcinogens.     

Synthesis of prostaglandins and cyclic AMP by cultured embryonic palate mesenchyme at various population densities

During embryonic development, facial and palate mesenchymal cells exhibit differential growth rates. Normal palatal growth is regulated in part by hormones and growth factors. Because hormonal responsiveness of some cells correlates with their cell density, we have investigated the relationship between embryonic palate mesenchymal cell population density and their ability to synthesize prostaglandins (PGs) and cyclic AMP. Primary cultures of palate mesenchymal cells exhibited typical lag, log, and stationary phases of growth with a doubling time of 32-34 hrs. The ability of cells to produce PGE2 in response to a calcium ionophore (A23187), an activator of phospholipase A2 (melittin), arachidonic acid, or serum was maximal during the period of early exponential growth. Prostaglandin F2 alpha synthesis in response to A23187 or arachidonic acid showed a similar transient increase also corresponding temporally to the period of early exponential growth. The ability to synthesize PGF2 alpha in response to melittin, however, failed to diminish after early exponential growth. The pattern of cAMP synthesis in response to isoproterenol and PGE1 was different from that seen for induced prostaglandin synthesis. A transient increase in sensitivity to isoproterenol and PGE1 was seen that corresponded temporally to the period of late exponential growth just prior to attainment of confluency. Decreased sensitivity to stimulation of either prostaglandin or cAMP production as the cells became confluent was shown to be a density-dependent phenomenon; confluent cultures that were subcultured to reestablish logarithmic growth exhibited density-dependent hormonal responses identical to those seen in primary cultures. The ability of palate mesenchymal cells to synthesize both prostaglandins and cAMP, thought to be critical for proper palatal development, might thus be related to local differential craniofacial growth rates.     

Enhanced prostaglandin synthesis as a mechanism for inhibition of melanoma cell growth by ascorbic acid

Both ascorbic acid and the 1-series prostaglandins have been reported to be important regulators of cell growth and since ascorbic acid also increases the synthesis of the 1-series prostaglandins, it is possible that the effects of ascorbic acid on cell growth might be mediated by changes in 1-series prostaglandin synthesis induced by ascorbic acid. This study attempted to examine this possible relationship. The effects of ascorbic acid, prostaglandin E1 and the essential fatty acid precursors of the prostaglandins, linoleic acid and gamma-linolenic acid on the in vitro growth of transformed BL6 murine melanoma cells and untransformed monkey kidney (LLCMK) cells was determined. The effects of ascorbic acid addition on the growth inhibitory effect of the essential fatty acids and on the activity of delta-6-desaturase, a key enzyme in 1-series prostaglandin synthesis were also examined. Addition of ascorbic acid, prostaglandin E1 and both essential fatty acids was found to reduce BL6 growth while PGE1 and to a lesser extent the essential fatty acids reduced LLCMK cell growth. The growth inhibitory effect of the essential fatty acids was enhanced by ascorbic acid which was also found to stimulate delta-6-desaturase activity in BL6 cells. The growth inhibitory effect of ascorbic acid on BL6 cells may thus be mediated by changes in prostaglandin synthesis through an association with the metabolism of the essential fatty acid precursors of the prostaglandins.     

Characterization of prostaglandin synthesis in the housefly,Musca domestica (L.)

The in vitro formation of prostaglandins (PG) was examined in the housefly Musca domestica. PG synthetase activity was detected in homogenates of whole insects and in head and thorax, abdomen, ovary and male reproductive tissues. Studies to determine the sub-cellular localization of PG synthetase indicated that the microsomal fraction contained the highest activity. Products obtained from radiolabeled arachidonic acid (20:4) and 8,11,14-eicosatrienoic acid (20:3, n-6) were PGE₂ and PGE₁, respectively, with lower amounts of the PGF series also present. In microsomal preparations from whole insects and reproductive tissues from both males and females, 20:3(n-6) was 2–2.5 times more efficiently converted to PG than was 20:4. Non-steroidal anti-inflammatory drugs (NSAID) fed to houseflies did not inhibit PG production from 20:4, whereas when they were included in microsomal preparations at high levels, they inhibited PG synthesis.     

Superoxide dismutase content and microsomal lipid composition of tumours with different growth rates

The content of cytosolic superoxide dismutase has been determined in Morris hepatomas 3924A (fast-growing) and 44 (slow-growing) and in ascites tumour cells (Novikoff hepatoma and Ehrlich-Lettré). The enzyme is decreased in all the tumours examined. The lowest amounts were found in the tumours with the fastest growth rates. Measurements of the lipid composition and fluidity of microsomal membranes isolated from Morris hepatomas show that also these parameters are changed in relation to the growth rate. The lipid to protein ratio and the degree of fatty acid unsaturation decrease gradually from rat liver to hepatoma 44 and 3924A microsomes. The different lipid composition is reflected also by differences in the physical environment of the bilayer, as indicated by data obtained with spin-labeled fatty acids. It is proposed that the changes in the membrane lipid composition and organization are consequent to the decrease in the protective effect of cytosolic superoxide dismutase against the O2- induced lipid peroxidation.