Human interphotoreceptor matrix contains serum albumin and retinol-bindingprotein

II is usually assumed that IRBP (interphotoreceptor retinoid-binding protei n) is the only protein present in the interphotoreceptor matrix (IPM) capab le of shuttling visual-cycle retinoids between photoreceptors and the retin al pigment epithelium. However, this laboratory previously presented qualit ative evidence (Western blots) that serum albumin is present in human IPM. Furthermore, Ong and coworkers (1994) found that cultured RPE cells synthes ize serum retinol-binding protein (RBP) and secrete it, mainly into the api cal culture medium, which would correspond to the IPM in intact eyes. As bo th of these proteins can bind all-trans-retinol and 11-cis-retinal, it was of interest to quantify the amounts of albumin and REP in human IPM, We use d radial immunodiffusion to accomplish this. The average molar ratio of ser um albumin to IRBP in these samples was 1.9: that of REP to IRBP was 0.015. The presence of a high concentration of serum albumin in the IPM in situ w as confirmed by the intense immunohistochemical staining seen in sections o f fresh human eyes, The human case is not unique: various concentrations of albumin were found in the IPM of all vertebrate species examined (by gel e lectrophoresis). These results indicate that both serum albumin, because of its very high concentration in the IPM, and REP, because of its comparativ ely light binding to retinoids, need to be considered, along with IRBP, as proteins that may participate in visual-cycle transport. The accompanying p aper addresses this concern. (C) 2000 Academic Press.