Specific binding of angiogenin (ANG) to calf pulmonary artery endothelial cells 
was demonstrated. Cellular binding at 4 degrees C of 125I-labeled human 
recombinant ANG was time and concentration dependent, reversible, and saturable 
in the presence of increasing amounts of the unlabeled molecules. The 
interaction was shown to be specific since a large excess of unlabeled ANG 
reduced labeled ANG binding by 80%, whereas similar doses of RNase A, a 
structurally related protein, had no effect. Scatchard analyses of binding data 
revealed two apparent components. High-affinity sites with an apparent 
dissociation constant of 5 x 10(-9) M were shown to represent cell-specific 
interactions. The second component, comprising low-affinity/high-capacity sites 
with an apparent dissociation constant of 0.2 x 10(-6) M, was essentially 
associated with pericellular components. High-affinity ANG binding sites varied 
with cell density and were found on other endothelial cells from bovine aorta, 
cornea, and adrenal cortex capillary but not on Chinese hamster lung 
fibroblasts. Divalent copper, a modulator of angiogenesis, was found to induce a 
severalfold increase in specific cell-bound radioactivity. Placental 
ribonuclease inhibitor, a tight-binding inhibitor of both ribonucleolytic and 
angiogenic activities of ANG, abolished 125I-labeled human recombinant ANG 
binding only in the absence of copper.