The thromboxane synthetase system of the microsomes of bovine blood platelets was solubilized by the treatment with Triton X-100. The solubilized preparation was separated into two enzyme fractions by DEAE-cellulose chromatography. One catalyzed the formation of prostaglandin H2 from arachidonic acid in the presence of heme and tryptophan. The other fraction converted prostaglandin H2 to thromboxane B2 and 12L-hydroxy-5,8,10-heptadecatrienoic acid. However, incubation of the latter fraction

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... prostaglandin H2 at lower temperature produced an unstable compound with platelet-aggregating activity, which was presumably thromboxane A2 and which decomposed readily to thromboxane B2 and 12L-hydroxy-5,8,10-heptadecatrienoic acid.