Isolation from fetal bovine serum of an apolipoprotein-H-like protein which inhibits thymidine incorporation in fetal calf erythroid cells.

A 46 kDa heparin-binding protein which inhibits thymidine incorporation in cultures of fetal calf liver erythroid cells was isolated from fetal bovine serum by affinity chromatography on heparin-Sepharose, ion-exchange chromatography, gel filtration and reversed-phase h.p.l.c. The N-terminal sequence of the first 22 amino acids showed 81% identity with the published sequence of human apolipoprotein H. The isolated protein inhibited thymidine incorporation with an ED50 (concn. producing 50% of maximal effect) of 36 nM. A 100% inhibition of thymidine incorporation and a 40% decrease in cell numbers in cultures of fetal calf erythroid cells were observed at a protein concentration of 840 nM. No effects could be seen in cultures of 3T3 cells used as controls. Human apolipoprotein H had no inhibitory activity in any of the cell cultures tested, suggesting a species-specificity or a different structure or function for the bovine heparin-binding protein.