Margolis R K, Margolis R U, Preti C, Lai D
Biochemistry. 1975 Nov 4;14(22):4797-804. doi: 10.1021/bi00693a004.
The distribution, carbohydrate composition, and metabolism of glycoproteins have been studied in mitochondria, microsomes, axons, and whole rat brain, as well as in various synaptosomal subfractions, including the soluble protein, mitochondria, and synaptic membranes. Approximately 90% of the brain glycoproteins occur in the particulate fraction, and they are present in particularly high amounts in synaptic and microsomal membranes, where the concentration of glycoprotein carbohydrate is 2-3% of the lipid-free dry weight. Treatment of purified synaptic membranes with 0.2% Triton X-100 extracted 70% of the glycoprotein carbohydrate but only 35% of the lipid-free protein residue, and the resulting synaptic membrane subfractions differed significantly in carbohydrate composition. The glycoproteins which are not extracted by Triton X-100 also have a more rapid turnover, as indicated by the 80-155% higher specific activity of hexosamine and sialic acid 1 day after labeling with [3H]glucosamine in vivo. The specific activity of sialic acid in the synaptosomal soluble glycoproteins 2 hr after labeling was greater than 100 times that of the synaptosomal particulate fraction, whereas the difference in hexosamine specific activity in these two fractions was only twofold, and by 22 hr there was little or no difference in the specific activities of sialic acid and hexosamine in synaptosomal soluble as compared to membrane glycoproteins. These data indicate that sialic acid may be added locally to synaptosomal soluble glycoproteins before there is significant labeling of nerve ending glycoproteins by axoplasmic transport. Fifty to sixty percent of the hyaluronic acid and heparan sulfate of brain is located in the various membranes comprising the microsomal fraction, whereas half of the chondroitin sulfate is soluble and only one-third is in microsomal membranes. When microsomes are subfractionated on a discontinuous density gradient over half of the hyaluronic acid and chondroitin sulfate are found in membranes with a density less than that of 0.5 M sucrose (representing a six- to sevenfold enrichment over their concentrations in the membranes applied to the gradient), whereas half of the heparan sulfate is present in membranes with a density greater than that of 0.8 M.
人们已经对糖蛋白在线粒体、微粒体、轴突和整个大鼠脑中的分布、碳水化合物组成及代谢进行了研究，同时也研究了其在各种突触体亚组分中的情况，这些亚组分包括可溶性蛋白、线粒体和突触膜。大脑中约90%的糖蛋白存在于颗粒部分，在突触膜和微粒体膜中含量尤其高，其中糖蛋白碳水化合物的浓度为无脂干重的2%-3%。用0.2%的 Triton X-100处理纯化的突触膜，可提取70%的糖蛋白碳水化合物，但只能提取35%的无脂蛋白残余物，且所得的突触膜亚组分在碳水化合物组成上有显著差异。未被 Triton X-100提取的糖蛋白周转也更快，这一点可通过体内用[³H]葡糖胺标记1天后己糖胺和唾液酸的比活性高80%-155%来表明。标记后2小时，突触体可溶性糖蛋白中唾液酸的比活性比突触体颗粒部分的比活性高100倍以上，而这两个部分中己糖胺比活性的差异仅为两倍，到22小时时，与膜糖蛋白相比，突触体可溶性糖蛋白中唾液酸和己糖胺的比活性几乎没有差异。这些数据表明，在轴浆运输对神经末梢糖蛋白进行显著标记之前，唾液酸可能会局部添加到突触体可溶性糖蛋白中。大脑中50%-60%的透明质酸和硫酸乙酰肝素位于构成微粒体部分的各种膜中，而硫酸软骨素的一半是可溶的，只有三分之一存在于微粒体膜中。当微粒体在不连续密度梯度上进行亚分级分离时，超过一半的透明质酸和硫酸软骨素存在于密度低于0.5M蔗糖的膜中（相对于应用于梯度的膜中的浓度，富集了6-7倍），而一半的硫酸乙酰肝素存在于密度高于0.8M的膜中。
