Fourth module of Xenopus interphotoreceptor retinoid-binding protein: Activity in retinoid transfer between the retinal pigment epithelium and rod photoreceptors

Purpose. Interphotoreceptor retinoid-binding protein (IRBP), an extracellular protein believed to support the exchange of retinoids between the neural retina and retinal pigment epithelium (RPE) in the vertebrate eye, exhibits a modular, i.e., repeat, structure. The present study was undertaken to determine whether an individual module of IRBP has activity in retinoid transfer between the RPE and rod photoreceptors. Methods. The retinoid transfer activity of a recombinant protein corresponding to the fourth module of Xenopus laevis IRBP (X4IRBP) was examined in two ways. First, X4IRBP was tested for its ability to support the regeneration of porphyropsin in detached/reattached Xenopus retina/RPE-eyecups. Following illumination and removal of native IRBP, Xenopus eyecups supplemented with 42 μM X4IRBP or (as a control) Ringer's solution were incubated in darkness and then analyzed for regenerated porphyropsin. Second, toad (Bufo marinus) RPE-eyecup preparations were used to evaluate X4IRBP's ability to promote the release of 11-cis retinal from the RPE. Results. The regeneration of porphyropsin in X4IRBP-supplemented Xenopus retina/RPE-eyecups (0.45 ± 0.04 nmol; mean ± SEM, n = 11) exceeded that in controls (0.13 ± 0.02 nmol, n = 11). For promoting the release of 11-cis retinal from the toad RPE, 42 μM X4IRBP was more effective than equimolar bovine serum albumin although considerably less than that of 26 μM native bovine IRBP. Conclusions. The results indicate a low but significant activity of IRBP's fourth module in reactions relevant to retinoid exchange.