Fe(II)-induced decomposition of epidioxides. A chemical model for prostaglandin E, prostacyclin and thromboxane biosynthesis

Density functional studies on thromboxane biosynthesis: mechanism and role of the heme-thiolate system

Reaction mechanisms for the isomerization of prostaglandin H(2) to thromboxane A(2), and degradation to 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and malondialdehyde (MDA), catalyzed by thromboxane synthase, were investigated using the unrestricted Becke-three-parameter plus Lee-Yang-Parr (UB3LYP) density functional level theory. In addition to the reaction pathway through Fe(IV)-porphyrin intermediates, a new reaction pathway through Fe(III)-porphyrin pi-cation radical intermediates was found. Both reactions proceed with the homolytic cleavage of endoperoxide O-O to give an alkoxy radical. This intermediate converts into an allyl radical intermediate by a C-C homolytic cleavage, followed by the formation of thromboxane A(2) having a 6-membered ring through a one electron transfer, or the degradation into HHT and MDA. The proposed mechanism shows that an iron(III)-containing system having electron acceptor ability is essential for the 6-membered ring formation leading to thromboxane A(2). Our results suggest that the step of the endoperoxide O-O homolytic bond cleavage has the highest activation energy following the binding of prostaglandin H(2) to thromboxane synthase.

Characterization of heme environment and mechanism of peroxide bond cleavage in human prostacyclin synthase

Prostacyclin is a potent mediator of vasodilation and anti-platelet aggregation. It is synthesized from prostaglandin H(2) by prostacyclin synthase (PGIS), a member of Family 8 in the cytochrome P450 superfamily. Unlike most P450s, which require exogenous reducing equivalents and an oxygen molecule for mono-oxygenation, PGIS catalyzes an isomerization with an initial step of endoperoxide bond cleavage of prostaglandin H(2) (PGH(2)). The low abundance of PGIS in natural tissues necessitates heterologous expression for studies of structure/function relationships and reaction mechanism. We report here a high-yield prokaryotic system for expression of enzymatically active human PGIS. The PGIS cDNA is modified by replacing the hydrophobic amino-terminal sequence with the more hydrophilic amino-terminal sequence from P450 2C5 and by adding a four-histidine tag at the carboxyl terminus. The resulting recombinant PGIS associates with host cell membranes and was purified to electrophoretic homogeneity by nickel affinity, hydroxyapatite and CM Sepharose column chromatography. The recombinant PGIS, with a heme:protein ratio of 0.9:1, catalyzes prostacyclin formation at a K(m) of 13.3 muM PGH(2) and a V(max) of 980 per min. The dithionite-reduced PGIS binds CO with an on-rate of 5.6 x 10(5) M(-1) s(-1) and an off-rate of 15 s(-1). The ferrous-CO complex of PGIS is very short-lived and decays at a rate of 0.7 s(-1). Spectral binding assays showed that imidazole binds weakly to PGIS (K(d) approximately 0.5 mM,) but clotrimazole, a bulky and rigid imidazole derivative, binds strongly (K(d) approximately 1 microM). The transient nature of the CO complex and the weak imidazole binding seem to support an earlier proposal that PGIS active site has a limited space, but the tight binding of clotrimazole argues against this view. It appears that the heme distal pocket of PGIS is fairly adaptable to ligands of various structures. UV-visible absorption, magnetic circular dichroism and electron paramagnetic resonance spectra indicate that PGIS has a typical low-spin heme with a hydrophobic active site. PGIS catalyzes homolytic scission of the peroxide bond of a test substrate, 10-hydroperoxyoctadeca-8,12-dienoic acid, accompanied by formation of a heme intermediate with a Compound II-like optical spectrum.

3-(1-imidazolylmethyl) indoles: potent and selective inhibitors of human blood platelet thromboxane synthetase

Several 3-(1-imidazolylmethyl) indoles were tested for inhibition of the microsomal enzymes which catalyse the biosynthesis of thromboxane A2, prostaglandin I2, and prostaglandin endoperoxides. These products were measured by bioassay to assess levels of enzyme activity. The highest activity against human blood platelet thromboxane A2-synthetase was obtained with 2-cyclopropyl-3(1-imidazolylmethyl) indole (IC50 1 x 10(-10) M). This compound also exhibited the highest activity against pig aorta prostaglandin I2-synthetase (IC50 8.4 x 10(-7) M). Of much more potential therapeutic interest, 2-isopropyl-3-(1-imidazolylmethyl) indole showed almost complete selectivity against thromboxane A2-synthetase. Both compounds exhibited IC50's of 2 x 10(-8) M against the latter enzyme but showed only weak effects (IC50's greater than 10(-4) M) against prostaglandin I2-synthetase and ram seminal vesicle PGH2-synthetase.