Catalyzes the conversion of prostaglandin H2 (PGH2) to thromboxane A2 (TXA2), a potent inducer of blood vessel constriction and platelet aggregation. Cleaves also PGH2 to 12-hydroxy-heptadecatrienoicacid (12-HHT) and malondialdehyde, which is known to act as a mediator of DNA damage. 12-HHT and malondialdehyde are formed stoichiometrically in the same amounts as TXA2. Additionally, displays dehydratase activity, toward (15S)-hydroperoxy-(5Z,8Z,11Z,13E)-eicosatetraenoate (15(S)-HPETE) producing 15-KETE and 15-HETE. prostaglandin H2 = thromboxane A2 prostaglandin H2 = (12S)-hydroxy-(5Z,8E,10E)-heptadecatrienoate + malonaldehyde a hydroperoxyeicosatetraenoate = an oxoeicosatetraenoate + H2O (15S)-hydroperoxy-(5Z,8Z,11Z,13E)-eicosatetraenoate = 15-oxo-(5Z,8Z,11Z,13E)-eicosatetraenoate + H2O (15S)-hydroperoxy-(5Z,8Z,11Z,13E)-eicosatetraenoate + AH2 = (5Z,8Z,11Z,13E,15S)-hydroxyeicosatetraenoate + A + H2O Monomer. Expressed primarily in lung, kidney, and spleen. Deficient mice are viable, fertile and exhibited no gross abnormalities in size, body weight, and anatomical features of major organs. However deficency causes a mild hemostatic defect and protects mice against arachidonate-induced shock and death. Belongs to the cytochrome P450 family.