Effects of divalent cations on vitamin B12 adsorption to brush borders of rat intestine.

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A brush border preparation from rat intestine was incubated with rat intrinsic factor-vitamin B12 complex in 0.01 M Tris-HCl buffer, pH 7.4. The 57Co-B12 uptake to brush borders was proportional to the amount of protein or to alkaline phosphatase activity in the preparations. The uptake increased with time of incubation. At 37°C, the uptake after incubation for 15 min was 80-85% of that for one hr. The uptake at 4°C was approximately 70% of that at 37°C. There was no difference as a result of adding glucose to the incubation medium. The uptake was observed in the alkaline environment above pH 6.3. Maximum uptake occurred at pH 8.0. Brush borders washed with Krebs-Ringer bicarbonate buffer (pH 7.4) exhibited no difference in B12 uptake, whether in the presence or absence of calcium ion. But brush borders washed with ethylenediaminetetraacetate exhibited no uptake when incubated in calcium-free medium. The uptake reached a maximum by addition of calcium ion at a concentration of 0.3 mM, and was not alter up to 10 mM. Addition of magnesium ion exhibited no uptake. Calcium-dependent B12 uptake was markedly inhibited by manganese ion. Magnesium ion seemed to slightly inhibit the calcium-dependent uptake.
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