Evidence for the synthesis of thymosin α1 by calf thymocytes and for the production of this peptide by natural processing

Abstract Thymus and thymocytes from calf were extracted under isotonic conditions in the presence of protease inhibitors or under severe denaturing conditions (after quick freezing and thawing in boiling 0.1 m NaCl). The extracts, as well as the medium in which the thymocytes were obtained from thymus fragments (thymocyte supernatants), were size-fractionated by ultrafiltration. As in whole thymus isotonic extracts, thymosin α 1 [ A. L. Goldstein, T. L. K. Low, M. McAdoo, J. McClure, G. B. Thurman, J. Rossio, C-Y. Lai, D. Chang, S-S. Wang, C. Harvey, A. H. Ramel, and J. Meienhofer (1977) Proc. Natl. Acad. Sci. USA 74 , 725–729 ] was contained in isotonic extracts from thymocytes and also in thymocyte supernatants, as determined by isoelectric focusing and reverse-phase HPLC analysis. The extraction under denaturing conditions mainly yielded products with molecular masses over 50,000, showing very similar isoelectric focusing patterns in both thymocytes and whole thymus extracts. As deduced by isoelectric focusing analysis of diverse size-fractionated products, a strong association capacity seems to be responsible for an apparently high molecular mass of the components of these extracts. According to the p I , two of these components were prothymosin α [ A. A. Haritos, G. J. Goodall, and B. L. Horecker (1984) Proc. Natl Acad. Sci. USA 81 , 1008–1011 ] and thymosin α 1 . Prothymosin α was not detected in any isotonic extracts or thymocyte supernatants. These data suggest that calf thymocytes are capable of producing thymosin α 1 , which would arise by natural processing of its precursor.