Proteins of the bovine interphotoreceptor matrix: retinoid binding and other functions.

Bovine interphotoreceptor matrix (IPM) is rich in protein, containing a limited and specific set of these macromolecules. The matrix proteins are synthesized by adjacent tissues (neural retina and pigment epithelium), but are not, themselves, major constituents of these tissues. Of possible functional importance are the presence of some lysosomal hydrolases and of glycoprotein components that foster the aggregation of RPE cells. The IPM contains also a unique retinoid-binding protein - IRBP, a large glycoprotein located only in the IPM. IRBP carries more all-trans-retinol in light- than in dark-adapted eyes; this finding, coupled with IRBP's strategic position, indicates that IRBP may be a shuttle for vitamin A in the visual cycle. Furthermore, IRBP may mediate exchange of retinal among opsin molecules in the dark. The molecular weight of bovine IRBP is 133,000; the molecular radius is 56A. These measurements show that IRBP is probably an elongated molecule. Retinol binds relatively loosely to IRBP (with dissociation constant 1.3 X 10(-6) M), implying that IRBP should be able to transfer its vitamin A to cellular retinoid carriers in the retina and RPE. The absorbance, fluorescence, and circular dichroic characteristics of IRBP are compared to those of other retinol-binding proteins.