Murine splenocytes release an inhibitory peptide of lymphocyte DNA synthesis into serum-free culture medium.

Murine splenocytes release a novel repressing activity of Concanavalin A-induced lymphocyte DNA synthesis. This activity was purified by gel filtration, ion exchange and thin layer chromatography. It was characterized to be a heat-stable peptide whose molecular weight was estimated to be about 1 000 daltons. The strong inhibition by this peptide was observed when added at early periods of the cell culture. This factor does not affect early RNA synthesis in lymphocytes, but nucleotide (UTP) incorporation at early periods was considerably decreased. In addition, although the factor inhibits drastically lymphocyte DNA synthesis, the cell viability is not changed significantly. These results suggest that the peptide suppresses the cell membrane function at early periods and specifically blocks the concomitant DNA synthesis.