Allende M L, Schwendiman R M, Young W W
Department of Biological and Biophysical Sciences, School of Dentistry, University of Louisville 40292, USA.
Curr Eye Res. 1997 Dec;16(12):1263-9. doi: 10.1076/ceyr.16.12.1263.5032.
beta 1,4 N-Acetylgalactosaminyltransferase (GalNAcT) is a type II integral membrane protein of the Golgi apparatus that catalyzes the synthesis of the glycosphingolipids GM2, GD2, and GA2. The activity of GalNAcT in chick retinal cells increases 6-fold between embryonic days 7 and 14. Because GalNAcT, like many Golgi glycosyltransferases, is proteolytically cleaved from Golgi membranes to release a soluble form into the culture medium of cells transfected with the cloned human enzyme, we tested whether GalNAcT might be released from embryonic retinal cells into the vitreous humor.
Samples of vitreous humor and plasma and extracts of retinal cells were assayed for GalNAcT activity.
The activity of a soluble form of GalNAcT in embryonic 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 based 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.
GalNAcT in embryonic chicken vitreous humor is likely due either to a specific release from neural retinal cells or due to non-specific lysis of these cells during apoptosis associated with the development of the retina. Regardless of the source, GalNAcT in the vitreous humor has the potential to function as a lectin by binding to gangliosides GD3 and GM3 on the surface of retinal cells and, thereby, to influence neuronal development.
β1,4 - N - 乙酰半乳糖胺基转移酶（GalNAcT）是高尔基体的一种II型整合膜蛋白，可催化糖鞘脂GM2、GD2和GA2的合成。在鸡视网膜细胞中，GalNAcT的活性在胚胎第7天到第14天之间增加了6倍。由于GalNAcT与许多高尔基体糖基转移酶一样，会从高尔基体膜上被蛋白水解切割，从而释放出可溶性形式进入用克隆的人类酶转染的细胞培养基中，我们测试了GalNAcT是否可能从胚胎视网膜细胞释放到玻璃体液中。
对玻璃体液、血浆样本以及视网膜细胞提取物进行GalNAcT活性检测。
胚胎期鸡玻璃体液中可溶性GalNAcT的活性在胚胎第10天之前几乎检测不到，然后在第16天之前增加了6倍多，并在出生前一直保持在该水平。基于对Mn++的需求、糖鞘脂底物特异性和产物特性，该活性被鉴定为真正的GalNAcT。胚胎血浆中的GalNAcT活性约为相应玻璃体液的10%，这表明血浆不是玻璃体液中该活性的来源。
胚胎鸡玻璃体液中的GalNAcT可能是由于从神经视网膜细胞特异性释放，或者是由于在与视网膜发育相关的细胞凋亡过程中这些细胞的非特异性裂解。无论来源如何，玻璃体液中的GalNAcT都有可能通过与视网膜细胞表面的神经节苷脂GD3和GM3结合而作为一种凝集素发挥作用，从而影响神经元发育。
