APPEARANCE OF BETA-1,4 N-ACETYLGALACTOSAMINYLTRANSFERASE (GLYCOSPHINGOLIPIDS GA2 GM2/GD2 SYNTHASE) IN EMBRYONIC CHICKEN VITREOUS-HUMOR DURING DEVELOPMENT/

Purpose. beta 1,4 N-Acetylgalactosaminyltransferase (GalNAcT) is a typ e II integral membrane protein of the Golgi apparatus that catalyzes t he synthesis of the glycosphingolipids GM2, GD2, and GA2. The activity of GalNAcT in chick retinal cells increases B-fold between embryonic days 7 and 14. Because GalNAcT, like many Golgi glycosyltransferases, is proteolytically cleaved from Golgi membranes to release a soluble f orm into the culture medium of cells transfected with the cloned human enzyme, we tested whether GalNAcT might be released from embryonic re tinal cells into the vitreous humor. Methods. Samples of vitreous humo r and plasma and extracts of retinal cells were assayed for GalNAcT ac tivity. Results. The activity of a soluble form of GalNAcT in embryoni c chick vitreous humor was nearly undetectable until embryonic day 10, then increased more than six fold until day 16, and remained at that level until birth. The activity was identified as authentic GalNAcT ba sed on a requirement for Mn++, GSL substrate specificity, and product characterization. GalNAcT activity in embryonic plasma was roughly 10% that of the corresponding vitreous humor, suggesting that the plasma was not the source of the activity in the vitreous. Conclusions. GalNA cT in embryonic chicken vitreous humor is likely due either to a speci fic release from neural retinal cells or due to non-specific lysis of these cells during apoptosis associated with the development of the re tina. Regardless of the source, GalNAcT in the vitreous humor has the potential to function as a lectin by binding to gangliosides GD3 and G M3 on the surface of retinal cells and, thereby, to influence neuronal development.