LIPOCALIN-TYPE PROSTAGLANDIN-D SYNTHASE (BETA-TRACE) IS LOCATED IN PIGMENT EPITHELIAL-CELLS OF RAT RETINA AND ACCUMULATES WITHIN INTERPHOTORECEPTOR MATRIX

Glutathione-independent prostaglandin D synthase, identical to beta-tr ace, (a major CSF protein), is localized in the CNS. This enzyme, lipo calin-type prostaglandin D synthase, is a member of the lipocalin fami ly of secretory proteins that transport small lipophilic substances. T his enzyme's activity in adult rat retina was enriched sixfold in reti nal pigment epithelium (RPE) and even more in interphotoreceptor matri x (IPM), all higher than brain. Western blots with anti-lipocalin-type prostaglandin D synthase showed three distinct immunoreactive bands. In the retinal cytosolic fraction, only one band was observed (M(r) 25 ,000); in IPM, the larger component occurred (M(r) 26,000). The RPE me mbrane-bound fraction showed two bands (M(r) 20,000 and 23,000), indic ating synthesis, and the cytosolic fraction contained two bands (M(r) 23,000 and 26,000), indicating modification for release into IPM. At l east two glycosylation sites occurred on the prostaglandin D synthase moiety, explaining the three immunoreactive bands in Western blots. Im munohistochemistry with polyclonal antibodies against this lipocalin-t ype enzyme showed intense localization in RPE, but less in photorecept or outer and inner segments. in situ hybridization showed mRNA specifi cally expressed in RPE. Thus, lipocalin-type prostaglandin D synthase is predominantly expressed in RPE and actively accumulated in IPM. Thi s may demonstrate gene sharing because, while catalyzing prostaglandin D, synthesis, it may perform an additional, unrelated role in IPM. Th is enzyme is secreted from the RPE into IPM from which it is then take n up by photoreceptors. However, the nature of its ligand(s) is not kn own; they may be retinoids and/or docosahexanoic acid.