1. Binding of cobalamin (Cbl) was compared in liver and kidney plasma membranes prepared from rat and human tissues.

2. Single, high-affinity, saturable (200 pmol/l), binding sites for TC II-Cbl were found in all tissues; by contrast no receptors were present for free cobalamin, for which only non-specific adsorption occurred.

3. Binding constants for TC II-CNCbl determined for liver and kidney plasma membranes were of a similar magnitude. Mean values for Ka (litre/nmol) were 16.7 (rat liver), 18.8 (rat kidney), 8.0 (human liver) and 7.5 (human kidney).

4. Results for binding TC II-OHCbl instead of TC II-CNCbl showed no difference in Ka and Bmax. values, although the non-specific adsorption was decreased to a third.

5. Competitive inhibition results showed that the receptors are specific for the TC II molecule and that binding is unaffected either by the cobalamin moiety or by the presence of free cobalamin. Degradation of the receptor protein molecules by trypsin (10 μg/ml) resulted in 90% inhibition of binding.

6. It is concluded that differences between liver and kidney in cobalamin uptake and accumulation cannot be attributed to differences in their TC II receptors.

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