Demonstration of the specific binding of bovine transferrin to the human transferrin receptor in K562 cells: evidence for interspecies transferrin internalization

Specific binding of ferric bovine transferrin to the huyman transferrin receptor was investigated using K562 cells propagated in serum-free medium without transferrin supplemented with 10-5 elemental iron. Affinity chromatography of solubilized extracts of K562 cells surface-labeled with 125I was performed using bovine transferrin- and human transferrin-Sepharose 4B resins. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of resin eluates reveal that bovine transferrin specifically binds a M/sub r/ = 188,000 protein which dissociates into a M/sub r/ = 94,000 protein under reducing conditions, a finding identical to what is seen with human transferrin. The M/sub r/ = 94,000 reduced protein isolated by bovine transferrin resin shows an identical one-dimensional partial proteolytic digestion map with that of the human transferrin receptor. Unlabeled bovine transferrin was shown to specifically compete 125I-labeled human transferrin from the human transferrin receptor on the surface of K562 cells at 40C in a similar manner as unlabeled human transferrin; however, approximately a 2000-fold higher concentration of bovine ligand was required to achieve comparable competition (50% inhibition of binding). Indirect immunofluorescence cytolocalization of bovine transferrin in K562 cells grown in serum-free medium supplemented with ferric bovine transferrin reveal patterns similar to those seen for human transferrin. K652 cells grown in serum-free medium supplemented with either 300 μg/ml of ferric human or ferric bovine transferrin were found to demonstrate superimposable growth curve