Interleukin-4 and IL-10 bind covalently to activated human alpha(2)-Macroglobulin by a mechanism that requires Cys(949)

alpha(2)-Macroglobulin (alpha(2)M) functions as an extracellular carrier of diverse cytokines, including transforming growth factor-beta 1 (TGF-beta 1 ), that expresses anti-inflammatory activities, The results presented here demonstrate that interleukin-10 (IL-10) and IL-4, which also regulate the i nflammatory response, bind to alpha(2)M, Unlike TGF-beta, IL-4 and IL-10 bi nd almost exclusively to the receptor-recognized, or activated, form of alp ha(2)M, Purified IL-4-alpha(2)M complexes were predominantly covalent due t o thiol disulfide exchange involving Cys(949) in the alpha(2)M subunit, Blo cking Cys(949) with iodoacetamide significantly inhibited IL-4- and IL-10 b inding. Bovine serum albumin (BSA), which possesses a free Cys residue and undergoes thiol disulfide exchange reactions, did not compete with alpha(2) M for the binding of IL-4 or IL-10, These results suggest a model in which IL-4 and IL-10 associate with activated alpha(2)M to form complexes that ar e initially noncovalent but unstable. In these complexes, Cys(949) is prope rly aligned to undergo thiol disulfide exchange and generate stable, covale nt IL-4-alpha(2)M and IL-10-alpha(2)M complexes.