Catalyzes the conversion of prostaglandin H2 (PGH2) to thromboxane A2 (TXA2), a potent inducer of blood vessel constriction and platelet aggregation (PubMed:3745158). 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 (By similarity). 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 in lung, kidney and thymus. Belongs to the cytochrome P450 family.