Intestinal uptake and release of cobalamin complexed with rat intrinsic factor.
Intestinal uptake and release of cobalamin complexed with rat intrinsic factor.
复制标题
与大鼠内因子复合的钴胺素的肠道摄取和释放。
DOI：
10.1152/ajpgi.1985.248.3.g326
复制
发表时间：
1985
期刊：
影响因子：
--
通讯作者：
Alpers,DH
中科院分区：
文献类型：
--
作者：
Seetharam,B;Presti,M;Frank,B;Tiruppathi,C;Alpers,DH
肠细胞摄取内因子（IF）和钴胺素（Cbl）的机制及其随后的命运尚不确定。为了研究这个问题，将双标记 IF X Cbl 添加到小肠器官培养物中。当 125I-IF X [57Co]Cbl 添加到兔回肠外植体中时，结合和内化线性增加 24 小时。使用未标记的 IF X Cbl 追踪 18 小时后，没有 125I-IF 返回细胞表面。发现 35-45% 的内化 Cbl 是游离的，未与 IF 或任何其他蛋白质结合。大约 60% 的两种内化配体与膜结合，但通过非 Ca2+ 依赖性键结合，表明与刷状缘受体以外的蛋白质结合。在 pH 5.0 时，IF 中钴胺素的释放程度 (30%) 与细胞内游离 Cbl (35-45%) 的释放程度相同。胰腺蛋白酶和回肠匀浆均不影响 IF 中 Cbl 的释放。当添加组织蛋白酶时，释放的 Cbl 不大于单独归因于 pH 5.0 的 Cbl。添加到组织外植体中的氯喹不会改变游离细胞内 Cbl 的百分比。根据这些结果，我们表明 IF X Cbl 被内化并与肠细胞内的受体分离。释放机制尚不清楚，但似乎需要酸性 pH 值 (5.0)。也许当 IF X Cbl 复合物进入具有酸性环境的非溶酶体细胞区室时，Cbl 在粘膜中释放。 IF 没有大量再循环至刷状缘膜。
The mechanism of uptake of intrinsic factor (IF) and cobalamin (Cbl) by enterocytes and their subsequent fate have been uncertain. To examine this problem double-labeled IF X Cbl was added to small intestinal organ cultures. When 125I-IF X [57Co]Cbl was added to rabbit ileal explants, binding and internalization increased linearly for 24 h. After an 18-h chase with nonlabeled IF X Cbl, no 125I-IF returned to the cell surface. An amount of 35-45% of the internalized Cbl was found free, not bound to IF or any other protein. About 60% of both internalized ligands was bound to membranes but by a non-Ca2+-dependent bond, suggesting binding to a protein other than the brush-border receptor. Cobalamin was released from IF at pH 5.0 to the same degree (30%) as free Cbl was found inside the cell (35-45%). Neither pancreatic proteases nor ileal homogenates effected release of Cbl from IF. When cathepsins were added, the Cbl released was no greater than could be attributed to pH 5.0 alone. Chloroquine added to tissue explants did not alter the percentage of free intracellular Cbl. From these results we suggest that IF X Cbl is internalized and detached from the receptor within the enterocyte. The mechanism of release is not known but seems to require an acid pH (5.0). The Cbl is released in the mucosa, perhaps when the IF X Cbl complex enters a nonlysosomal cellular compartment with an acidic environment. There is no substantial recycling of IF to the brush-border membrane.