Studies on the transcobalamin receptor in hog kidney

The binding of the cobalamin-transcobalamin complex by its solubilized receptor from hog kidney membrane was studied. The receptor bound the complex in a system containing bivalent cations, and the affinity was dependent on the NaCl concentration but not on temperature. The binding of cobalamin-transcobalamin to the receptor had an association constant of approximately 4.6 x 10(9) liter/mol and it was saturable and highly specific as competition by other proteins was not observed. The receptor had higher affinity for the cobalamin-transcobalamin complex (holo-TC) than for transcobalamin (apo-TC). Basic amino compounds known to interfere with tubular reabsorption of proteins did not inhibit the binding. Studies on subcellular fractions supported the view that the receptor was located on the brush border membrane of the kidney.