Characterization of a galactosyltransferase in purified bovine rod outer segments

Purified bovine rod outer segments (ROS) were used to study the transfer of labeled galactose from UDP-[3H]galactose to endogenous ROS glycoproteins, exogenous glycoproteins and N-acetylglucosamine (GlcNAc). The ROS reaction was also compared with that of the retinal microsomal fraction and milk galactosyltransferase. The results indicate that the ROS reaction was enhanced by exposure to light. Illumination, however, had no effect on the transfer of labeled galactose to either endogenous microsomal glycoproteins by retinal microsomal galactosyltransferase or the transfer of the sugar to ROS glycoproteins by milk galactosyltransferase. Manganese was most effective, followed by cobalt, as cofactor for the ROS enzyme. Calcium and magnesium produced about 60% of the activity observed with manganese. The ROS enzyme transferred minimal amounts of labeled galactose to asialo-agalactotransferrin or ovalbumin but readily transferred the sugar to GlcNAc. The latter reaction had an optimum pH of 6.3 and was linear for at least 90 min. It reached a maximum at about 30 mM GlcNAc and was inhibited by higher concentrations of the aminosugar and by low concentrations of alpha-lactalbumin. On the other hand, the transfer of galactose to ROS glycoproteins was not affected by low concentrations of alpha-lactalbumin. Our data suggest that the ROS galactosyltransferase may have a certain specificity towards its acceptor in the ROS. Its activation by light may indicate a role in the light-activated processes of the photoreceptor cell.