TBXAS1

TBXAS1 Full Name

thromboxane A synthase 1 (platelet)

TBXAS1 Introduction

Thromboxane A synthase 1 (TBXAS1) is a cytochrome P450 enzyme (CYP5A1) that catalyzes the conversion of prostaglandin H2 (PGH2) to thromboxane A2 (TXA2), a potent vasoconstrictor and platelet aggregator. Encoded by the TBXAS1 gene on chromosome 7q34, TBXAS1 is predominantly expressed in platelets, megakaryocytes, monocytes, and lung, kidney, and vascular endothelial cells. This enzyme occupies a pivotal position at the branch point of arachidonic acid metabolism, diverting PGH2 from prostaglandin and prostacyclin synthesis toward thromboxane production, thereby playing a central role in regulating hemostasis, vascular tone, and inflammatory responses.

Figure 1. Strcuture of TBXAS1.

Thromboxane A2 Synthesis and Signaling in Hemostasis
TBXAS1 is an integral membrane protein of the endoplasmic reticulum that catalyzes the isomerization of PGH2 to TXA2 through a mechanism involving the formation of an iron-oxo intermediate characteristic of cytochrome P450 enzymes, though uniquely, TBXAS1 functions as an isomerase rather than a monooxygenase. TXA2, the product of this reaction, is an extremely labile eicosanoid with a half-life of approximately 30 seconds in aqueous solution, undergoing spontaneous non-enzymatic hydrolysis to the stable but biologically inactive thromboxane B2 (TXB2). TXA2 exerts its biological effects through the thromboxane prostanoid receptor (TP receptor), a G protein-coupled receptor that activates Gq/phospholipase C signaling, leading to increased intracellular calcium and platelet shape change, granule secretion, and aggregation. TBXAS1 also produces 12-hydroxyheptadecatrienoic acid (12-HHT), a byproduct of the isomerization reaction that acts as a ligand for the leukotriene B4 receptor BLT2.

TBXAS1 in Cardiovascular Disease, Bleeding Disorders, and Therapeutic Targeting
The TBXAS1-TXA2-TP receptor axis is a major therapeutic target in cardiovascular medicine. Low-dose aspirin irreversibly inhibits cyclooxygenase-1 (COX-1) upstream of TBXAS1, reducing TXA2 production and providing cardioprotective antiplatelet effects. Conversely, TBXAS1 deficiency causes an autosomal recessive bleeding disorder characterized by impaired platelet aggregation, prolonged bleeding time, and mucocutaneous bleeding (platelet-type bleeding despite normal platelet counts). TBXAS1 inhibitors, including dazoxiben, have been evaluated for cardiovascular indications, though primarily the upstream COX-1 and downstream TP receptor have been targeted therapeutically. Beyond hemostasis, the TBXAS1 pathway is implicated in the pathogenesis of pulmonary hypertension, where excessive TXA2 production contributes to pulmonary vasoconstriction and vascular remodeling. Genetic polymorphisms in TBXAS1 have been associated with modified risk for myocardial infarction, ischemic stroke, and aspirin resistance, highlighting the pharmacogenetic importance of this enzyme.

Alternate Names for TBXAS1

TBXAS1; thromboxane A synthase 1 (platelet); TS; TXS; CYP5; THAS; TXAS; CYP5A1; GHOSAL; BDPLT14