Increased thromboxane B₂ biosynthesis in platelets

Abstract

The synthesis of thromboxane B₂ is increased in platelets from rabbits with experimental hypercholesterolemia, but the increase is not due to increased phospholipids hydrolysis. We have clarified the mechanism for the increased thromboxane synthesis. The biosyntheses of prostaglandin H₂ and thromboxane B₂ were unaffected by supperoxide dismutase, xanthine oxidase, mannitol, or benzoate in other experiments designed to study the possible involvement of reactive oxygen species. These results suggest that O₂ ^.− and OH^. were not likely to be involved as intermediates in the synthesis of prostaglandin H₂ and thromboxane B₂ in platelets. The rate of prostaglandin H₂ biosynthesis was promoted in deuterium oxide, and this deuterium oxide enhancement effect was reversed by 2,5-diphenylfuran, suggesting that singlet oxygen may be involved in prostaglandin H₂ biosynthesis. The biosynthesis of prostaglandin H₂ was promoted by ADP-Fe³⁺ but inhibited by EDTA and EDTA-Fe³⁺. The effect of ADP-Fe³⁺ could not be replaced by EDTA-Fe³⁺. The effects of glutathione, glutathione peroxidase and H₂O₂ on cyclooxygenase and thromboxane synthetase were studied by using partially purified enzymes and platelet microsomes. Glutathione and glutathione peroxidase inhibited the activity of cyclooxygenase but did not inhibit that of thromboxane synthetase. H₂O₂ caused the inactivation of cyclooxygenase, but the addition of H₂O₂ did not inhibit the formation of thromboxane B₂ from prostaglandin H₂. An examination of glutathione concentration and glutathione peroxidase activity in platelets from normal and experimentally hypercholesterolemic rabbits demonstrated that both were decreased in platelets from later group. The observed alterations in glutathione levels and glutathione peroxidase activity are large enough to cause increased thromboxane B₂ synthesis in platelets but the possibility that other unidentified factors may also contribute cannot be excluded.