Synthesis and secretion of cobalamin binding proteins by opossum kidney cells

Opossum kidney epithelial cells synthesize and secrete two Cobalamin (Cbl) binding proteins of Mr 66,000 and 43,000. When grown on culture inserts, the apical medium contained both these proteins while the basolateral medium contained only the 43 kDa Cbl binder. Colchicine, a microtubule disruptive drug, increased two fold the apical but not the basolateral secretion of the Cbl binding proteins. Although the opossum Cbl binders did not cross react with anti-serum raised to Cbl binders from other species, the identity based on Cbl binding and size suggest that the 66 kDa and 43 kDa proteins are haptocorrin and transcobalamin II.