IRBP enhances removal of 11-cis-retinaldehyde from isolated RPE membranes

Interphotoreceptor retinoid-binding protein (IRBP) greatly enhances the con version of all-trans-retinol to 11-cis-retinal by the retinal pigment epith elium (RPE) and facilitates 11-cis-retinal release from the RPE, However, t he mechanisms by which IRBP exerts these effects are not clear, Using a mod el system of purified bovine IRBP and isolated bovine RPE membranes, we inv estigated the possibility that IRBP may favor the delivery of all-trans-ret inol to, or the release of 11-cis-retinal from, RPE membranes. As the inter photoreceptor space contains serum retinol-binding protein (RBP) and serum albumin in addition to IRBP, we similarly examined the exchange of retinoid s between RPE membranes and human REP or bovine serum albumin (BSA). Isolat ed RPE membranes were loaded with radioactive 11-cis-retinal and incubated with solutions of IRBP, RBP, BSA or with buffer alone. Membranes (pellet) a nd retinoid-binding protein or buffer (supernatant) were separated by centr ifugation and analysed for radioactive 11-cis-retinal. Membranes incubated with buffer alone released only 4-5 % of their 11-cis-retinal, while 25 mu M IRBP removed 18-35%,. More retinal was released as the membrane concentra tion was reduced. In contrast, REP and BSA removed little retinal, even tho ugh both proteins are capable of binding this retinoid, Similar results wer e obtained with bovine liner membranes, consistent with the idea that the e ffects of IRBP do not depend on an RPE surface receptor for IRBP. IRBP was also markedly superior to REP and BSA in removing all-trans-retinol from RP E membranes. In addition, IRBP efficiently delivered bound all-trans-retino l to membranes: however, in contrast to their differential removal of retin oids, all three binding proteins delivered comparable amounts of retinol to membranes. (This result supports the practice of using BSA as a retinoid c arrier in in vitro experimental systems), We conclude that, whereas IRBP sh ares with other retinoid-binding proteins the ability to deliver retinol to membranes, IRBP is unique in its capacity to remove 11-cis-retinal from me mbranes. This may be the feature of IRBP that drives the vitamin A cycle to efficiently produce 11-cis-retinal, (C) 2000 Academic Press.