Fractional conversion of thromboxane A2 and B2 to urinary 2,3-dinor-thromboxane B2 and 11-dehydrothromboxane B2 in the cynomolgus monkey

Measurement of Thromboxane Biosynthesis in Health and Disease

Thromboxane (TX) A₂ is a chemically unstable lipid mediator involved in several pathophysiologic processes, including primary hemostasis, atherothrombosis, inflammation, and cancer. In human platelets, TXA₂ is the major arachidonic acid derivative via the cyclooxygenase (COX)-1 pathway. Assessment of platelet TXA₂ biosynthesis can be performed ex vivo through measurement of serum TXB₂, an index of platelet COX-1 activity, as well as in vivo through measurement of urinary enzymatic metabolites, a non-invasive index of platelet activation. This article reviews the main findings of four decades of clinical investigation based on these analytical approaches, focusing on the measurement of TXA₂ metabolites to characterize the pathophysiologic role of transiently or persistently enhanced platelet activation and to describe the clinical pharmacology of COX-1 inhibition in health and disease.

Thromboxane biosynthesis and metabolism in relation to cardiovascular risk factors

Enhanced platelet biosynthesis of thromboxane A(2) is associated with several cardiovascular risk factors, as a consequence of a direct effect on platelet biochemistry and/or some form of endothelial dysfunction. Moreover, episodic increases in thromboxane biosynthesis occur in acute coronary and cerebral ischemic syndromes. Thromboxane-dependent platelet activation represents an important mechanism that amplifies the consequences of acute vascular lesions as well as those of longstanding metabolic or hemodynamic disturbances, and results in increased risk of vascular occlusive events.

Targeting the urine and plasma determinants of thromboxane A2 metabolism in detection of aspirin effectiveness

Detection of reduced aspirin effectiveness has gained significant importance since clinical consequences of aspirin resistance were reported. Nevertheless, due to differentiated molecular basis of aspirin resistance, the conflicting choice of referential method for detection of acetylsalicylic acid ineffectiveness has become troublesome. This study, using a rat model of antiplatelet therapy, examines the aptitude of selected TXB2 metabolism-based methods in the detection of acetylsalicylic acid effectiveness. We hypothesized that ex-vivo whole blood spontaneous TXB2 generation assay could be, contrary to basal TXB2 and urine 11-dTXB2, a novel surrogate measure for impaired acetylsalicylic acid-dependent inhibition of thromboxane synthesis. To address this hypothesis, we evaluated the sensitivity of TXB2 generation assay in hirudinized whole blood to detect acetylsalicylic acid-mediated inhibition of cyclooxygenase activity in healthy rats and diabetic rats treated with acetylsalicylic acid. In diabetic and control animals, both acetylsalicylic acid drenches in the dose-independent manner contributed to significant attenuation of basal plasma TXB2 and urinary 11-dTXB2 formation. Urinary concentrations of 11-dTXB2 were, contrary to basal TXB2, significantly higher, regardless of acetylsalicylic acid dose, among all diabetic groups, compared with corresponding control groups. Determination of TXB2 generation in whole blood enabled sensitive detection of dose-related acetylsalicylic acid effect in both groups, as well as increased TXB2 formation in diabetes. We showed for the first time that evaluation of spontaneous generation of TXB2 in hirudinized whole blood enables, contrary to basal plasma TXB2 and urine 11-dTXB2 concentrations, to sensitively determine the acetylsalicylic acid effect in healthy and diabetic subjects.

Kinetic and structural evidence for the identification of 11-hydroxythromboxane B2 dehydrogenase as cytosolic aldehyde dehydrogenase

We have recently purified 11-hydroxythromboxane B2 dehydrogenase from porcine kidney and identified it as cytosolic aldehyde dehydrogenase (EC 1.2.1.3) based on amino acid analysis and other protein characteristics. In the present paper we have studied the catalytic interaction of thromboxane B2 (TXB2) with different aldehyde substrates and a potent aldehyde dehydrogenase inhibitor, disulfiram. TXB2 was a competitive inhibitor of the aldehyde dehydrogenase reaction in assays with 3,4-dihydroxyphenylacetaldehyde, a high affinity substrate. The conversion of TXB2 to 11-dehydro-TXB2 was also inhibited by propanal and disulfiram. The protein characteristics of the enzyme have also been further studied. The native enzyme is a tetramer and has an isoelectric point of 7.0 which is comparable with that of cytosolic aldehyde dehydrogenases from other species. Taken together the present data further indicate that 11-hydroxythromboxane B2 dehydrogenase is identical with cytosolic aldehyde dehydrogenase and that substrates and inhibitors of aldehyde dehydrogenase interact with thromboxane metabolism in vitro.

Effect of n-3 fatty acid ethyl ester supplementation on fatty acid composition of the single platelet phospholipids and on platelet functions

Twenty healthy male volunteers were randomly assigned to receive either four 1-g capsules of n-3 polyunsaturated fatty acids (PUFA) ethyl esters or four 1-g capsules of olive oil (as placebo) for a period of 4 months, followed by a 3-month wash-out period. Fatty acids of platelet phospholipid fractions, platelet aggregation, and thromboxane B2 (TXB2) formation were analyzed at 0, 2, and 4 months of treatment and at 1, 2, and 3 months of wash-out. During n-3 PUFA supplementation, accumulations of eicosapentaenoic (EPA), docosapentaenoic (DPA), and docosahexaenoic (DHA) acids were markedly increased after 2 months, with slight differences in further accumulation up to 4 months among the various phospholipid fractions. Significant decreases in platelet sensitivity to collagen, serum TXB2 levels, and urinary TXB2 metabolites were also observed following n-3 PUFA treatment. During the first and second month of wash-out, slight differences were observed in changes of various fatty acids among different phospholipid fractions, but after 3 months of wash-out, alterations were no longer detectable with respect to pretreatment values. After 3 months of wash-out, platelet function parameters also were returned to baseline. Thus, both platelet lipids and function are influenced by n-3 PUFA ethyl ester supplementation, and significant alterations are still detectable after 2 months of wash-out.

Evaluation of plasma 11-dehydro-thromboxane B2 as an indicator for thromboxane A2 synthesis in vivo in laboratory animals

Since thromboxane (TX) B2 is not a reliable indicator of TXA2 generation in vivo, because of artifactual TXA2 generation during blood collection, we tested the feasibility of replacing TXB2 with 11-dehydro (dh)-TXB2 as the indicator. Plasma levels of TXB2 and 11-dh-TXB2 were measured after i.v. administration of TXB2 to rabbits, guinea pigs, rats, dogs and a monkey, and after i.v. infusion of collagen in rabbits. In the rabbits and dogs, 2,3-dinor-TXB2 levels were also measured. After intravenous injection of TXB2 (10 micrograms/kg) in rabbits, the ratio of the area under the curve (AUC) of 11-dh-TXB2 to that of TXB2 (1.94) was far higher than the AUC ratio between 2,3-dinor-TXB2 and TXB2 (0.42). When endogenous TXA2 was generated by infusion of collagen (2 mg/kg/5 min), the plasma level of 11-dh-TXB2 was significantly increased, and had a longer half-life than TXB2. In the guinea pigs, rats and monkey, the peak plasma levels of 11-dh-TXB2 were significantly increased after injection of TXB2 (10 micrograms/kg, i.v.), whereas the AUC ratios of 11-dh-TXB2/TXB2 were less than one fourth of that in rabbits. No significant increase in 11-dh-TXB2 was observed after TXB2 injection (10 micrograms/kg) in dogs, but the 2,3-dinor-TXB2 level rose significantly, its AUC ratio to TXB2 being 0.29, comparable with that in rabbits. The order of the 11-dh-TXB2/TXB2 AUC ratios was: rabbits > guinea pigs > monkey > rats >> dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and some properties of 11-hydroxythromboxane B2 dehydrogenase from porcine kidney

A protein with NAD-dependent 11-hydroxythromboxane B2 dehydrogenase activity was purified to apparent homogeneity from porcine kidney using a relatively simple purification procedure, involving precipitation, anion-exchange chromatography (diethylaminoethyl-cellulose), affinity chromatography (5'-AMP-Sepharose) and gel-filtration chromatography (Protein Pak 125). The dehydrogenase was found to have a molecular mass of 50-55 kDa as determined by comparison with standards on SDS/PAGE. The molecular mass on gel-filtration chromatography was dependent on the ionic strength of the buffer. The apparent Km and Vmax values for thromboxane B2 were also dependent of the ionic strength with a Vmax of 214 nmol min-1 mg-1 using 250 mM Tris/HCl, pH 8.0, and a corresponding Km of 2.9 mM. The enzyme was NAD dependent and was clearly separated from the proteins with 15-hydroxyprostaglandin dehydrogenase activity also present in the kidney. Furthermore, it was found that 11-hydroxythromboxane B2 dehydrogenase did not utilize prostaglandin D2, prostaglandin E2, prostaglandin F2 alpha or cholic acid as substrate, and that the enzyme did not catalyse the reverse reaction, conversion of 11-dehydrothromboxane B2 to thromboxane B2.

Thromboxane biosynthesis and metabolism in relation to cardiovascular risk factors

Enhanced platelet biosynthesis of thromboxane A2 is associated with several cardiovascular risk factors, as a consequence of a direct effect on platelet biochemistry and/or some form of endothelial dysfunction. Moreover, episodic increases in thromboxane biosynthesis occur in acute coronary and cerebral ischemic syndromes. Thromboxane-dependent platelet activation represents an important mechanism that amplifies the consequences of acute vascular lesions as well as those of long-standing metabolic or hemodynamic disturbances, and results in increased risk of vascular occlusive events.

Dietary fish oil accelerates the response to coronary thrombolysis with tissue-type plasminogen activator. Evidence for a modest platelet inhibitory effect in vivo

To assess the platelet inhibitory effect of high doses of fish oils and relate it to alterations in eicosanoid synthesis, we used a canine model in which coronary thrombosis, the time to reperfusion induced by recombinant tissue-type plasminogen activator (rt-PA), and the rate of spontaneous reocclusion are sensitive to platelet inhibition. In the animals fed fish oil, the time to rt-PA induced thrombolysis was accelerated (mean, 63 vs. 27 minutes; p less than 0.003). The time to thrombotic occlusion and the rate of reocclusion were unaltered. The ratio of eicosapentaenoic acid (EPA) to arachidonic acid rose in platelet and endothelial cell membranes, whereas serum thromboxane (Tx) B levels fell a mean 86%, and basal excretion of 2,3-dinor-TxB2 (TxA2-M) declined. Basal prostaglandin (PG) I2 formation was unaltered, whereas biosynthesis of EPA-derived TxA3 and PGI3 increased. In control animals, TxA2 formation increased during thrombosis; there was a further, more marked rise during reperfusion. PGI2 formation also increased, probably as a response to platelet-vascular interactions. Stimulated production of both eicosanoids was strikingly suppressed in the animals fed fish oil. Fish oils significantly enhance the efficacy of rt-PA in vivo, albeit to a modest extent. Because the time to reperfusion is highly sensitive to Tx-dependent platelet activation, this effect is likely to reflect the demonstrated suppression of TxA2 biosynthesis by fish oils.