Selective inhibition of DNA synthesis by a protein released from spleen cells

Spleen cells cultured release proteins into the culture medium, some of which shows both binding activity to single stranded DNA (ss-DNA) and inhibitory activity to 3H-thymidine incorporation into cells. The inhibitory protein of DNA synthesis was purified to near homogeneity by ammonium sulfate precipitation, ss-DNA-cellulose affinity chromatography, and gel filtration chromatography. The molecular weight of the major band of this protein, as estimated by sodium dodecyl-sulfate gel electrophoresis, was approximately 14,000. The effect of the protein was compared with various cell lines and primary cell culture, and it was found that the protein strongly inhibited DNA synthesis of HeLa, L, and HEp2 cells, but only inhibited to a minor extent synthesis of Ehrlich cells and primary culture of mouse embryo.