Binding and processing of I-125-ACTH by isolated rat splenic lymphocytes

The effect of incubation temperature and ligand competition was tested for I-125-ACTH binding to isolated rat lymphocytes. alpha MSH but not Agouti-li ke peptide was an effective competitive inhibitor for cell surface binding at 4 degrees C. Cells incubated with I-125-ACTH at 37 degrees C rapidly ass ociated ligand for 10 min and then gradually lost the radioactivity with ti me. Cells incubated with I-125-ACTH at 4 degrees C accumulated ligand to on ly about half the maximal amount when compared to cells incubated at 37 deg rees C for 10 min. Temperatures below 20 degrees C and toxins that block ly sosomal degradation blocked the loss of cell-associated radioactivity. Thes e results suggest the lymphocyte ACTH receptor is the Melanocortin 5 recept or and the receptor is internalized by endocytosis to deliver ligand to the lysosome. (C) 1999 Academic Press.