Prostaglandin-H2 D-isomerase (PTGDS) is an enzyme that in humans is encoded by the PTGDS gene.
The protein encoded by this gene is a glutathione-independent prostaglandin-D synthase that catalyzes the conversion of prostaglandin H<sub>2</sub> (PGH<sub>2</sub>) to prostaglandin D<sub>2</sub> (PGD<sub>2</sub>). PGD<sub>2</sub> functions as a neuromodulator as well as a trophic factor in the central nervous system. PGD<sub>2</sub> is also involved in smooth muscle contraction/relaxation and is a potent inhibitor of platelet aggregation. This gene is preferentially expressed in brain. Studies with transgenic mice over-expressing this gene suggest that this gene may be also involved in the regulation of non-rapid eye movement sleep. Furthermore, PTGDS and its product PGD<sub>2</sub> are elevated in the bald-scalp areas of men with male pattern baldness (androgenetic alopecia).
Prostaglandin D<sub>2</sub> synthase is used clinically as a diagnostic marker for liquorrhea, that is, to check whether fluid leaking from the nose or ear contains cerebrospinal fluid. This is important in the assessment of head trauma severity. In a medical context, the older term "beta-trace protein" is frequently used to refer to PTGDS.