42] Cyanoborohydride reduction of rhodopsin

Abstract

Publisher Summary This chapter discusses the cyanoborohydride reduction of rhodopsin. The dark-adapted form of rhodopsin resists reduction, apparently because the binding site is deep in a hydrophobic pocket of the molecule, inaccessible to small hydrophilic molecules. This contrasts with the more accessible retinal binding sites of cone pigments, which can be directly reduced by sodium borohydride. Cyanoborohydride is a more hydrophobic borohydride derivative. It serves to penetrate into the hydrophobic core of the rhodopsin molecule and reduce the retinal linkage to lysine. The usefulness of the reaction, beyond confirming that the dark-adapted form of rhodopsin has a retinal Schiff base, is that it chemically binds the retinal to its original binding site in a secondary amino linkage that is not subject to hydrolysis or transaminization to other amino groups in the process of fragmentation and purification. Two additional advantages of cyanoborohydride are, first, that it has a greater specificity for Schiff base reduction relative to aldehyde reduction and, second, that it has a greater stability at neutral and acid pH values.