| Abstract: | Radiation inactivation was used to determine the functional molecular size of the rat liver membrane protein which binds desialylated glycoproteins. Purified plasma membranes from rat liver were irradiated with high energy electrons from a linear accelerator and then assayed for 125I-asialo-orosomucoid binding activity. Target size analysis of the data revealed that increasing doses of ionizing radiation from 1-48 megarads resulted in a monoexponential decay in binding activity due to a decrease in the number of available binding sites; dissociation and binding affinity were unaffected. The molecular weight of the rat binding protein, determined in situ by target analysis, was 104,000 +/- 17,000; that of the rabbit binding protein was 109,000 +/- 5,000. Comparison of the value obtained by irradiation of the intact rat plasma membrane with that of the purified receptor revealed the latter to have an apparent molecular weight of 148,000 +/- 16,000. Evidence is presented to indicate that the observed increase in target size was a response to the presence of Triton X-100 used in the solubilization and assay procedure. In contrast to the size of the ligand binding functional unit, the antireceptor antibody binding site was estimated to be 30,000 +/- 2,000. |
