Ostray F, Gams R A
Blood. 1977 Nov;50(5):877-88.
Incorporation of vitamin B12 into L1210 cells requires the protein binder transcobalamin II (TCII). The process is saturable, follows Michaelis-Menten kinetics (Km = 2.5 X 10(-9) M at 37 degrees C), is both temperature and calcium (K50 - 1 X 10(-6) M) dependent, and is inhibited by apo-TCII, indicating the presence of a TCII specific receptor on the cell membrane. B12 also leaves the cell by a calcium-independent pathway bound to either TCII or to a protein with chromatographic properties similar to those of TCIII. Since intact TCII-B12 can be found in the cytosol and can promote B12 uptake by mitochondria, it is proposed that the B12 released from the cell bound to the TCIII-like protein is derived by mitochondrial processing of incorporated TCII-B12. The slower time course of release of the latter B12 is consistent with this postulate.
维生素B12掺入L1210细胞需要蛋白质结合剂转钴胺素II（TCII）。该过程具有饱和性，遵循米氏动力学（37℃时Km = 2.5×10⁻⁹ M），既依赖温度又依赖钙（K50 = 1×10⁻⁶ M），且受脱辅基TCII抑制，表明细胞膜上存在TCII特异性受体。维生素B12也通过与TCII或与具有类似于TCIII色谱性质的蛋白质结合的钙非依赖性途径离开细胞。由于完整的TCII - B12可在胞质溶胶中发现并可促进线粒体对维生素B12的摄取，因此有人提出，与类TCIII蛋白结合从细胞释放的维生素B12是由掺入的TCII - B12经线粒体加工产生的。后一种维生素B12释放的较慢时间进程与这一假设一致。
