Receptor binding of asialoerythropoietin

The interaction of 125 I‐asialoeythropoietin (asialoepo) with receptors has been characterized both by binding assay and affinity cross‐linking. Purified spleen cells from mice infected with the anemia strain of Friend virus (FVA cells) have receptors for 123 I‐asialoepo with two classes of affinity constant: one with Kd = 0.02–0.03 nM and 300–400 per cell, the other with lower affinity (kd = 0.9–1.2 nM) and 1,000–1,200 per cell. The Kd value for the high affinity site is one third of that for the binding of native 125 I‐erythropoietin ( 125 I‐epo) to the same FVA cells (Kd = 0.08–0.1 nM). Using 125 I‐asialoepo or 125 I‐epo affinity cross‐linking methods, we find two components with apparent molecular weights of 88 kDa and 105 kDa in FVA cells, and in the transformed mouse cell lines, 201, IW32, and NN10, in agreement with earlier studies using 125 I‐epo. These results indicate that 125 I‐asialoepo binds to the same receptors as 125 I‐epo, but with greater affinity for the high affinity site. Since 201 cells contain only a single class of lower affinity receptors for erythropoietin (epo), finding the same two components as found for FVA cells by cross‐linking experiment indicates that the two components do not represent the two classes of receptor.