Distribution of interstitial retinol‐binding protein (IRBP) in the vertebrates

Immunoblots of interphotoreceptor matrix preparations from 20 species belonging to six vertebrate classes were probed with antibodies against bovine interstitial retinol‐binding protein (b‐IRBP). Each preparation displayed an immunoreactive protein band. In the Osteichthyes, the apparent M_r of this band was 67,600 ± 2,700 (x̄ ± SD, n = 8). In two of the Osteichthyes, the band was resolved into a closely spaced doublet. Including previously published data for five mammals and one amphibian, species from the other classes (Chondrichthyes, one species; Amphibia, four species; Reptilia, one species; Aves, one species; Mammalia, nine species) had IRBPs with M_r that averaged 2.0 times that of the Osteichthyes, namely 134,200 ± 8,600 (x̄ ± SD, n = 17). Frog IRBP was very similar to mammalian IRBP in terms of its immunohistochemical distribution (determined with rabbit anti‐frog IRBP antibodies), its molecular weight (sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel‐filtration chromatography), retinol‐ and concanavalin A‐binding ability, and because it was synthesized and secreted in vitro by the isolated retina but not by the pigmented layers of eye. Goldfish IRBP apparently binds exogenous (³H)‐retinol but does not bind concanavalin A and has about half the M_r of frog IRBP. The occurrence of IRBP‐like proteins crossreacting with anti b‐IRBP antibodies in the interphotoreceptor matrix of all six major vertebrate classes is consistent with the hypothesis that IRBP is an important element in the vertebrate visual cycle.