Beesley R C, Bacheller C D
Am J Physiol. 1980 Dec;239(6):G452-6. doi: 10.1152/ajpgi.1980.239.6.G452.
Brush-border membrane vesicles from hamster intestine were employed to investigate uptake (binding) of vitamin B12 (B12). Ileal vesicles took up 25 times more B12 than did jejunal vesicles. Uptake of B12 by ileal vesicles was dependent on intrinsic factor (IF) and required Ca2+. Increasing the Ca2+ concentration caused an increase in uptake of B12 reaching a maximum at approximately 8 mM Ca2+. At high Ca2+ concentrations, 6-8 mM, Mg2+ had little effect on uptake of B12. At low Ca2+ concentrations, up to 2 mM, Mg2+ stimulated B12 uptake. Mg2+, Mn2+, and, to a lesser extent, Sr2+ stimulated Ca2+-dependent B12 uptake, but Zn2+, Ba2+, Na+, K+, and La3+ did not. B12 was apparently not metabolized and was bound as IF-B12 complex, which could be removed with (ethylenedinitrilo)tetraacetic acid (EDTA). Our results suggest that two types of divalent cation reactive sites are involved in binding of IF-B12. One is Ca2+ specific. The other is less specific reacting with Mg2+, Mn2+, Sr2+, and perhaps Ca2+ itself, thereby stimulating Ca2+-dependent binding of IF-B12 to its ileal receptor.
采用来自仓鼠肠道的刷状缘膜囊泡来研究维生素B12（B12）的摄取（结合）情况。回肠囊泡摄取的B12比空肠囊泡多25倍。回肠囊泡对B12的摄取依赖于内因子（IF）且需要Ca2+。增加Ca2+浓度会导致B12摄取量增加，在约8 mM Ca2+时达到最大值。在高Ca2+浓度（6 - 8 mM）下，Mg2+对B12摄取几乎没有影响。在低Ca2+浓度（高达2 mM）下，Mg2+会刺激B12摄取。Mg2+、Mn2+以及在较小程度上的Sr2+会刺激依赖Ca2+的B12摄取，但Zn2+、Ba2+、Na+、K+和La3+则不会。B12显然没有被代谢，而是以IF - B12复合物的形式结合，该复合物可用乙二胺四乙酸（EDTA）去除。我们的结果表明，两种二价阳离子反应位点参与了IF - B12的结合。一种是Ca2+特异性的。另一种特异性较低，能与Mg2+、Mn2+、Sr2+以及可能还有Ca2+本身发生反应，从而刺激IF - B12与其回肠受体的依赖Ca2+的结合。
