We have isolated a protein from media conditioned by a murine marrow-derived cell line (PB6) and from mouse marrow supernatants that antagonizes interleukin 3-dependent proliferation of cells in culture and reversibly inhibits DNA synthesis of erythroid progenitor cells (BFU-E) in vitro. This protein, p16 (monomer Mr = 16 kD on SDS-PAGE), was purified to homogeneity and amino acid sequencing of a polypeptide fragment yielded a sequence identical to that of murine cytosolic Cu,Zn-containing superoxide dismutase (SOD). The identification of p16 as SOD was confirmed by the detection of SOD enzymatic activity in pure p16 fractions, and when a commercial human erythrocytic SOD preparation was tested it showed the same cell inhibitory activities as p16. These observations show that superoxide dismutase is able to affect the cycling and growth factor responses of hematopoietic cells, activities that have not previously been associated with this enzyme.

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