An enzymatic mechanism for the antithrombotic and antihemostatic actions of aspirin

Identification of an enzyme in platelet microsomes which generates thromboxane A2 from prostaglandin endoperoxides

The microsomal fraction of horse and human platelets contains an enzyme which converts prostaglandin cyclic endoperoxides (PGG2 or PGH2) to a substance which is much more potent in contracting strips of rabbit aorta. This substance has the same characteristics as thromboxane A2, and can be distinguished from other products of arachidonic acid metabolism by differential bioassay.

Effects of feeding ethyl-dihomo-gamma-linolenate on prostaglandin biosynthesis and platelet aggregation in the rabbit

1. The ethyl ester of dihomo-gamma-linolenic acid (20:3omega6) (1 g/kg/day) was fed to rabbits for 25 days. Plasma lipids and platelet aggregation were analyzed on day 1, 11, 16, 21 and 26. 2. All plasma lipid classes were greatly enriched with 20:3omega6. Arachidonic acid levels were elevated to a smaller extent. The different platelet phospholipid fractions analyzed were also highly enriched with 20:3omega6, whereas the arachidonic acid content in platelet phospholipids was significantly lower than in control animals. 3. The excretion of 7 alpha-hydroxy-5,11-diketotetranorprostane-1,16-dioic acid, the major urinary metabolite of prostaglandin E1 and E2 was increased 4.6 fold by the treatment. 4. Platelet aggregation in response to ADP, collagen and arachidonic acid did not differ at any time betweeen 20:3omega6 treated rabbits and controls. 5. It is concluded that prostaglandin E biosynthesis can be increased by enriching the prostaglandin precursor pool. Platelet aggregation in vitro is not altered by feeding ethyl 20:3omega6.

Vitamin E. An inhibitor of the platelet release reaction

Formation of lipid peroxides rises sharply when platelets undergo the release reaction. In this study the in vitro effect of vitamin E on platelet aggregation was investigated. alpha-Tocopherol, an anitoxidant of known inhibitory action on lipid peroxidation, was added to platelet suspensions in concentrations up to 1.5 mM. A dose-dependent reduction in platelet aggregation was observed, with complete inhibition of the secondary wave of aggregation at greater than or equal to 0.9 mM alpha-tocopherol. The inhibitory effect of alpha-tocopherol on the platelet release reaction was further documented by the decrease in aggregation-induced release of [14C]5-hydroxytryptamine from prelabeled platelets and by the reduction of N-acetylglucosaminidase activity released into the medium. The sharp rise in lipid peroxides normally associated with platelet aggregation was markedly reduced by alpha-tocopherol and also by acetylsalicylic acid, a known inhibitor of the platelet release reaction. In vivo studies examined the effect of oral vitamin E administration (1,200-2,400 IU daily) on plasma and platelet levels of alpha-tocopherol. Up to 1,800 IU daily, increasing dosages of vitamin E resulted in increasing concentrations of alpha-tocopherol in plasma and platelets, but intake of vitamin E in excess of this dosage failed to show any further increase in plasma or platelet levels.

Prostaglandin E2 potentiation of platelet aggregation induced by LASS endoperoxide: absent in storage pool disease, normal after aspirin ingestion

Patients with storage pool disease and normal subjects who ingest aspirin show diminished collagen-induced platelet aggregation and an absent second wave of aggregation with ADP or adrenaline. These 'second-phase' aggregation responses are thought to be mediated by cyclic endoperoxide ('labile aggregation stimulating substance', LASS) that is derived from arachidonic acid and is the precursor of prostaglandin E2 (PGE2) and PGE2alpha. Furthermore, although PGE2 does not directly aggregate platelets, it markedly potentiates LASS-induced aggregation. The platelets of six patients with storage pool disease were capable of converting arachidonic acid to LASS, but the potentiation of LASS-induced aggregation by PGE2 was markedly diminished. In contrast, PGE2-potentiation of LASS aggregation was not reduced after aspirin ingestion. The effects of aspirin can be attributed entirely to its ability to block the enzymatic conversion of arachidonic acid to LASS and PGE2. These findings explain why a mutual correction of the aggregation defects is often seen when aspirin-treated platelets are mixed with storage pool-deficient platelets. This is because 'aspirin platelets' aggregate to the mixture of LASS and PGE2 produced by the storage pool-deficient platelets, which are themselves unresponsive. The findings in storage pool disease support previous conclusions that prostaglandin sensitization of platelets to the pro-aggregatory effects of LASS is an important factor in irreversible aggregation, and could be clinically important.

Prostaglandins as mediators of hypercalcemia associated with certain types of cancer

We investigated the role of prostaglandins in the hypercalcemia associated with neoplasia. In patients with hypercalcemia and solid tumors the excretion of the major urinary metabolite of the E prostaglandins, 7 alpha-hydroxy-5, 11-diketotetranorprostane-1, 16-dioic acid (PGE-M), was significantly greater than normal, P LESS THAN 0.01 (median of 58.4 and 7.1 ng per milligram of creatinine respectively). Slightly elevated values were seen in normocalcemic patients with solid tumors (14.3 ng per milligram). The levels of the metabolite were normal in hypercalcemic patients with either hematologic neoplasia or primary hyperparathyroidism. Immunoreactive parathyroid hormone was undetectable in the plasma of all hypercalcemic patients with solid tumors. Inhibition of prostaglandin synthesis by aspirin or indomethacin reduced excretion of both the urinary metabolite and serum calcium in six hypercalcemic patients with solid tumors and elevated excretion of the metabolite. These findings support the concept that prostaglandins are mediators of the hypercalcemia caused by certain solid tumors.

The mechanism of the effect of aspirin on human platelets. I. Acetylation of a particulate fraction protein

Aspirin (acetylsalicylic acid) inhibits platelet prostaglandin synthesis and the ADP- and collagen-induced platelet release reaction. The mechanism of the inhibitory effect is unknown but may involve protein acetylation, since aspirin acetylates a variety of substrates, including platelet protein. We have examined the relationship between protein acetylation and aspirin's physiologic effect on platelets. Suspensions of washed human platelets were incubated at 37 degrees C with (3H)aspirin, and incorporation of radioactivity into protein was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Exposure to (acetyl-3H)aspirin but not (aromatic ring-3H)aspirin resulted in radioactive labeling of three platelet proteins, suggesting that the drug acetylates these three proteins. The acetylation of two of the proteins (located in the supernatant fraction) was not saturable, implying that these reactions may not be physiologically significant. Acetylation of the third protein, approximate mol wt 85,000 (located in the particulate fraction), saturated at an aspirin concentration of 30 muM and was complete within 20 min. Platelets prepared from aspirin-treated donors did not incorporate any (acetyl-3H)aspirin radioactivity into the particulate protein for 2 days after drug treatment and did not show full pretreatment uptake of radioactivity for 12 days thereafter. The course of increasing incorporation of (acetyl-3H)aspirin radioactivity parralleled that of platelet turnover. Therefore, in addition to its saturability, acetylation of the particulate fraction protein by aspirin was permanent. In two respects, the inhibition of platelet function by aspirin correlates well with the aspirin-mediated acetylation of the particulate fraction protein. Both persist for the life-span of the aspirin-treated platelet, and both occur at a similar saturating aspirin concentration. The evidence suggests that the physiologic effect of aspirin on human platelets is produced by acetylation of a single protein located in the particulate fraction. The acetylated protein may be related to cyclo-oxygenase, the prostaglandin G2 biosynthetic enzyme.

Acetylation of prostaglandin synthase by aspirin

When microsomes of sheep or bovine seminal vesicles are incubated with [acetyl-3H]aspirin (acetyl salicylic acid), 200 Ci/mol, we observe acetylation of a single protein, as measured by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The protein has a molecular weight of 85,000 and corresponds to a similar acetylated protein found in the particulate fraction of aspirin-treated human platelets. The aspirin-mediated acetylation reaction proceeds with the same time course and at the same concentration as does the inhibition of prostaglandin synthase (cyclo-oxygenase) (EC 1.14.99.1; 8,11,14-eicosatrienoate, hydrogen-donor:oxygen oxidoreductase) by the drug. At 100 muM aspirin, 50% inhibition of prostaglandin synthase and 50% of maximal acetylation are observed after 15 min at 37 degrees. Furthermore, the substrate for cyclo-oxygenase, arachidonic acid, inhibits protein acetylation by aspirin at concentrations (50% inhibition at 10-30 muM) which correlate with the Michaelis constant of arachidonic acid as a substrate for cyclooxygenase. Arachidonic acid analogues and indomethacin inhibit the acetylation reaction in proportion to their effectiveness as cyclo-oxygenase inhibitors. The results suggest that aspirin acts as an active-site acetylating agent for the enzyme cyclo-oxygenase. This action of aspirin may account for its anti-inflammatory and anti-platelet action.

Inhibition of platelet aggregation caused by estrogen treatment in patients with carcinoma of the prostate

Platelet aggregation is increased in patients with carcinoma of the prostate treated with estrogens. Hence, these patients have a high incidence of cardiovascular and thromboembolic diseases. Platelet aggregation has been tested with the platelet aggregation test. It was inhibited by administration of 500 mg. acetylsalicylic acid twice daily. An aggregation inhibiting effect has been found in all 38 patients. To reduce the excess hazards of cardiovascular complications of estrogens in treating carcinoma of the prostate acetylsalicylic acid is recommended as an adjunct therapy.

A simple nonradioisotope technic for the determination of platelet life-span

Acetylsalicylic acid was shown both in vivo and in vitro to prevent the platelet lipid peroxidation normally induced by the aggregating agents thrombin and epinephrine, and the sulfhydryl inhibitor N-ethylmaleimide. After aspirin ingestion, there was a significant reduction (p smaller than 0.005) in platelet lipid peroxidation, with a gradual return to base-line values over a 10-day period. With these values, a normal platelet survival curve was constructed with a mean half-life of 4.4 days (range of 2.9 to 5.9 days). These values agree with the standard 51-Cr survivals in three patients with chronic idiopathic thrombocytopenic purpura. Half-lives of 1.0, 2.5, and 4.1 days by lipid peroxide technic compared with 1.9, 2.5, and 3.9 days by the concurrent use of 51-Cr. Thus, the technic may be used to measure platelet survival.

Labile aggregation stimulating substance (LASS): the factor from storage pool deficient platelets correcting defective aggregation and release of aspirin treated normal platelets

Mixtures of equal volumes of Hermansky-Pudlak (HP) syndrome (adenine nucleotide storage pool deficient) and aspirin treated (AT) platelets (inhibited release of adenine nucleotides) undergo irreversible aggregation when exposed to collagen or adrenaline, whereas neither alone will do so. The present investigation has explored the basis for the mutual correction. Correction in the mixed system was accompanied by release of significant amounts of [14C]5-hydroxy-tryptamine from the AT platelets, whereas very little isotope was released when AT platelets alone were exposed to collagen. The correction of aggregation and the release of isotope could both be suppressed by pre-treating HP platelets with aspirin. Labile aggregation stimulating substance (LASS) composed of two closely linked intermediates of prostaglandin (PG) E2 and PGF2alpha biosynthesis, produced using a microsomal fraction of HP platelets, could correct the aggregation and secretion defect of AT platelets exposed to collagen. These findings indicate that HP platelets when mixed with AT platelets and exposed to collagen secrete a substance which is responsible for the correction. LASS, identified as the factor involved, acted as an intercellular messenger which mediated the correction by overcoming the influence of aspirin on normal cells.

Physiological role of an endoperoxide in human platelets: hemostatic defect due to platelet cyclo-oxygenase deficiency

The endoperoxide prostaglandin G2 (PGG2) induced platelet aggregation as well as the platelet release reaction (release of ADP and serotonin) when added to human platelet-rich plasma. Formation of a metabolite of PGG2 [8-(l-hydroxy-3-oxopropyl)-9,12L-dihydroxy-5,10-heptadecadienoic acid] and a lipoxygenase product [12L-hydroxy-5,8,10,14-eicosatetraenoic acid] accompanied the release reaction caused by aggregating agents such as collagen, ADP, epinephrine, and thrombin. Indomethacin inhibited the release reaction and PGG2 formation induced by these agents but had no effect on PGG2-induced release reaction. The aggregating effect of PGG2 was abolished by furosemide, which is a competitive inhibitor of ADP-induced primary aggregation. These data indicate that the aggregating effect of PGG2 is due to release of ADP and that PGG2 synthesis is required for induction of the release reaction by various aggregating agents. A subject with a hemostatic defect due to abnormal release mechanism [decreased aggregation with epinephrine (second wave) and collagen and normal platelet ADP] had a deficiency of the cyclo-oxygenase that catalyzes formation of PGG2. Normal aggregation and release reaction were obtained with added PGG2. Ii is concluded that the endoperoxide (PGG2) is essential in normal hemostasis because of its role in initiating the release reaction required for aggregation by collagen and the second wave of aggregation caused by, e.g., ADP.

Release of prostaglandin F1alpha and F2alpha from superfused platelets: quantitative evaluation of the inhibitory effects of some aspirin-like drugs

The release of PGF1alpha and PGF2alpha from superfused blood platelets was studied by the combined use of two radioimmunoassay systems with different specificities. PGF1alpha only accounted for approximately 30% of the total immunoreactivity. A substantially similar pattern of release was obtained with platelets of rat and human origin, although the latter released considerably lower amount of both compounds. Indomethacin, Femoprofen, Ditazole and Aspirin all inhibited PGF2alpha release from rat platelets in descending order of potency. Hydrocortisone was partically inactive. The release of PGF1alpha and PGF2alpha was inhibited to the same extent by both Indomethacin and Fenoprofen. Moreover, a quite similar inhibitory effect by the same drug on rat and human platelets was found in preliminary experiments. In agreement with a previous similar finding, Aspirin displayed a higher inhibitory activity than that reported in other tissues. The use of superfused platelets seems to provide a simple and reproducible model for studying pharmacologic influences upon PG formation.

Stimulus-secretion coupling in platelets. Effects of drugs on secretion on adenosine 5'-triphosphate

The mechanism of stimulus-secretion coupling in platelets was investigated by observing the effects of drugs on the kinetics on ATP secretion induced by either thrombin or the divalent cation ionophore A23187. The actual secretion is the same with either of these agents, since the rate constants and activation energies of secretion are the same and since drugs that affect the final, enzyme-independent steps of thrombin-induced secretion have the same effect on ionophore-induced secretion. Drugs that affect early steps of thrombin-induced secretion have no effect on ionophore-induced secretion. Drugs that act through cAMP (PGE1, theophylline, dibutyryl-cAMP) slow an early step in the mechanism of thrombin-induced secretion and completely block at higher levels, with the required concentration of inhibitor dependent on thrombin concentration. The inhibition of rate appears to be all-or-none, with no intermediate rates observed. By replacing thrombin with trypsin, which makes it possible to observe a complete change in rate-determining step from an enzyme-dependent to an enzyme-independent platelet step, it was found that these drugs slow the rate only when the enzyme-independent step is rate determining. These drugs have no effect on A23187-induced secretion. It was concluded that cAMP inhibits at a step after the enzyme step but before the final step by interfering with transmission of the stimulus-secretion coupling signal. Disruption of microfilament function by cytochalasin B (10 muM) accelerates the rate of secretion induced by either thrombin or ionophore. The microtubule agents colchicine, vinblastine, and vincristine had effects only at concentrations above those usually considered necessary for the specific inhibition of microtubule function. Drugs that inhibit prostaglandin synthesis (aspirin, indomethacin, eicosatetraynoic acid), drugs that block ATP production (antimycin A, deoxyglucose), or several other drugs previously reported to inhibit platelet function had no effect on secretion.

2,3-diphosphoglycerate: a physiological inhibitor of platelet aggregation

2,3-Diphosphoglycerate (2,3-DPG) may inhibit the platelet release reaction and the irreversible aggregation of human blood platelets induced by adenosine diphosphate, epinephrine, or norepinephrine. The effects of 2,3-DPG on platelet aggregation were more pronounced in cases with low hematocrit (less than 30 percent). Dipyridamole and vincaminor potentiated the antiaggregating effect of 2,3-DPG. Erythocytes (10-3 to 10-4 per microliter) exhibited a similar antiaggregating effect, especially when secured from anemic patients.

Prostaglandin endoperoxides. A new concept concerning the mode of action and release of prostaglandins

Methods were developed for quantitative determination of the three major metabolites of arachidonic acid in human platelets, i.e., 12L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE), 12L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and 8-(1-hydroxy-3-oxopropyl)-9,12L-dihydroxy-5,10-heptadecadienoic acid (PHD). Aggregation of washed platelets by thrombin was accompanied by release of 1163-2175 ng/ml of HETE, 1129-2430 ng/ml of HHT, and 998-2299 ng/ml of PHD. The amount of PGG(2) (prostaglandin G(2)) produced as calculated from the sum of the amounts of its metabolites (HHT and PHD) was 2477-5480 ng/ml. In contrast, the amounts of PGE(2) (prostaglandin E(2)) and PGF(2alpha) (prostaglandin F(2alpha)) released were approximately two orders of magnitude lower. In this system, the prostaglandins thus exert their biological action through the endoperoxides, which are almost exclusively metabolized to nonprostanoate structures and only to a small extent to the classical prostaglandins. Platelets from subjects given aspirin produced less than 5% of the above mentioned amounts of HHT and PHD, whereas the production of HETE was stimulated about 3-fold. This provides additional evidence for our earlier proposal [Hamberg, M., Svensson, J., Wakabayashi, T. & Samuelsson, B. (1974) Proc. Nat. Acad. Sci. USA 71, 345-349] that the anti-aggregating effect of aspirin is through inhibition of PGG(2) formation.

Prostaglandin endoperoxides. Novel transformations of arachidonic acid in human platelets

Arachidonic acid incubated with human platelets was converted into three compounds, 12L-hydroxy-5,8,10,14-eicosatetraenoic acid, 12L-hydroxy-5,8,10-heptadecatrienoic acid, and the hemiacetal derivative of 8-(1-hydroxy-3-oxopropyl)-9,12L-dihydroxy-5,10-heptadecadienoic acid. The formation of the two latter compounds from arachidonic acid proceeded by pathways involving the enzyme, fatty acid cyclo-oxygenase, in the initial step and with the prostaglandin endoperoxide, PGG(2), as an intermediate. The first mentioned compound was formed from 12L-hydroperoxy-5,8,10,14-eicosatetraenoic acid, which in turn was formed from arachidonic acid by the action of a novel lipoxygenase. Aspirin and indomethacin inhibited the fatty acid cyclo-oxygenase but not the lipoxygenase, whereas 5,8,11,14-eicosatetraynoic acid inhibited both enzymes. The almost exclusive transformation of the endoperoxide structure into non-prostaglandin derivatives supports the hypothesis that the endoperoxides can participate directly and not by way of the classical prostaglandins in regulation of cell functions. The observed transformations of arachidonic acid in platelets also explain the aggregating effect of this acid.

Acetylenic analog of arachidonate that acts like aspirin on platelets

Development of irreversible platelet aggregation and the accompanying release of platelet-bound serotonin and production of prostaglandins is suppressed by 5,8,11,14-eicosatetraynoic acid (TYA). These findings may be explained by an ability of TYA to inhibit the enzymatic conversion of arachidonate to a newly recognized factor, labile aggregation-stimulating substance, which induces platelet aggregation, and to prostaglandins E(2) and F(2alpha).