Partial characterization of lectin binding sites of retinal photoreceptor outer segments and interphotoreceptor matrix

We have used cytochemistry together with exoglycosidase digestion and polyacrylamide gel electrophoresis to partially characterize lectin binding sites of the interphotoreceptor matrix and of photoreceptor outer segments. In order to obtain uniform access of reagents to all regions of the preparation, we have used a procedure in which plastic sections are etched with sodium ethoxide prior to cytochemical analysis. Neuraminidase pretreatment of plastic-embedded sections of Xenopus laevis eyecups leads to a loss of wheat germ agglutinin binding and a concomitant appearance of Ricinus communis agglutinin binding to the interphotoreceptor matrix. In contrast, wheat germ agglutinin binding to the outer segments is not altered by the neuraminidase pretreatment. These results suggest that wheat germ agglutinin binding sites of interphotoreceptor matrix are sialoglyconjugates and that outer segment binding sites are not sialoglycoconjugates. Enzyme digestions followed by lectin cytochemistry of matrix polypeptides separated by polyacrylamide gel electrophoresis do not identify a likely candidate to give rise to the cytochemical staining patterns. Lectin cytochemistry of retinas from which the interphotoreceptor matrix has been extracted do not show a loss of wheat germ agglutinin binding sites to the matrix. These results suggest that the major wheat germ agglutinin binding sites in the interphotoreceptor matrix are to as yet unidentified sialoglycoconjugates.