title: Properties and specificity of binding sites for the immunomodulator bestatin on the surface of mammalian cells.
abstract: The binding of the immunomodulator bestatin, an inhibitor of cell surface bound leucine aminopeptidase and aminopeptidase B, to mammalian cells of varying origin has been studied. The specific binding of [3H] bestatin was a rapid and saturable process exhibiting one affinity, characterized by an association constant of 0.8 x 10(5) M-1, as determined in the L5178y mouse lymphoma system. Optimal binding was observed at 37 degrees C. L-leucine and L-leucine-beta-naphthylamide prevented the binding, suggesting that the complex was formed between leucine aminopeptidase and bestatin. The protein nature of the bestatin-"receptor" was suggested by its susceptibility to trypsin. Under the conditions used here intracellular translocation of bestatin appeared to be negligible. A maximum of about 2.2 x 10(6) bestatin molecules could bind to L5178y mouse lymphoma cells. Under identical conditions by far the highest amount of bestatin was bound to macrophages from mice. Lower levels were measured with T-lymphocytes; very low binding capacity was observed with B-lymphocytes. Experiments with synchronized L5178y cells revealed a cell cycle dependent change of binding capacity for bestatin; the highest level was observed during the transition from S-to G2 phase and the lowest during G1- and early S phase. These data lend further support to the assumption that the immuno-potentiating activity of bestatin is due to a stimulation of T-lymphocyte proliferation probably mediated through the activation of macrophages.
mesh_terms: D000276:Adjuvants, Immunologic / Q000378:metabolism; D000626:Aminopeptidases / Q000037:antagonists & inhibitors; D000818:Animals; D001665:Binding Sites; D002453:Cell Cycle; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D007700:Kinetics; D007930:Leucine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D007940:Leukemia L5178 / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism; D008297:Male; D051379:Mice; D013601:T-Lymphocytes / Q000378:metabolism
chemical_list: D000276:Adjuvants, Immunologic; D000626:Aminopeptidases; D007930:Leucine; C012211:ubenimex